Designing Weed-Resistant Plant Communities by Maximizing Niche Occupation and Resource
Capture
by MICHAEL F CARPINELLI
A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of
Philosophy in Land Resources and Environmental Sciences
Montana State University
© Copyright by MICHAEL F CARPINELLI (2000)
Abstract:
To develop an ecological basis for designing weed-resistant plant communities, I tested the hypothesis
that susceptibility to weed invasion is determined by temporal and spatial resource availability and by
the ability of weeds to capture those resources. In two competition experiments, three desirable species
with differing spatial and temporal growth patterns (crested wheatgrass, intermediate wheatgrass, and
alfalfa), and one weed (spotted knapweed) were used to determine the potential for minimizing weed
invasion by maximizing niche occupation and resource capture. I tested this potential in two types of
plant communities: well-established, uninfested plant communities (invasion) and newly establishing
plant communities following revegetation of a previously weed-infested site (revegetation). In these
experiments, desirable species richness varied, while the total number of desirable plants was held
constant.
Comparative growth of isolated individuals was used to test the hypothesis that a species’ growth
characteristics, when grown in isolation, may be useful in predicting its relative growth in mixtures.
These results were variable and did not aid in interpreting results from the competition experiments. In
the invasion experiment, seedlings of the desirable species were planted in spring 1995. Spotted
knapweed was sown in fall 1996, simulating invasion of a well-established plant community. Sampling
occurred in fall 1997 and fall 1998. Spotted knapweed recruitment was positively related to soil water
content and negatively related to desirable species richness. These results suggest that soil water plays
a role in spotted knapweed germination and establishment, and that by maximizing niche occupation,
soil water may be preempted from invading weeds.
In the revegetation experiment, all species were sown simultaneously in spring 1996. This simulated
revegetation of a site containing spotted knapweed seeds in the seed bank because of prior infestation.
Sampling occurred in fall 1996 and fall 1997. Spotted knapweed recruitment was not related to
desirable species richness in either year. In 1997, spotted knapweed dominated the desirable species,
These results suggest that efforts to revegetate weed-infested rangeland must address weed
reestablishment from seeds or propagules in the soil. Only then can other strategies, such as
maximizing niche occupation by desirable species, be expected to provide long-term success. DESIGNING WEED-RESISTANT PLANT COMMUNITIES B Y MAXIMIZING
NICHE OCCUPATION AND RESOURCE CAPTURE
by
Michael Francis Carpinelli
A dissertation submitted in partial fulfillment
o f the requirements for the degree
of
Doctor o f Philosophy
in
Land Resources and Environmental Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
May 2000
11
3 )3 ^
(^X X ^
APPROVAL
o f a dissertation submitted by
Michael Francis Carpinelli
This dissertation has been read by each member o f the dissertation committee and has
been found to be satisfactory regarding content, English usage, format, citations,
bibliographic style, and consistency, and is ready for submission to the College o f
Graduate Studies.
? 2 i z#K>
Roger L. Sheley, Ph D.
Dater
Bruce D . Maxwell, Ph D.
Date
Approved for the Department o f Land Resources and Environmental Sciences
Jeffrey S. Jacobsen, Ph D.
Date
Approved for the College o f Graduate Studies
Bruce McLeod, Ph D.
tj&t
c5
Date
Ill
STATEMENT OF PERM ISSION TO USE
In presenting this dissertation in partial fulfillment o f the requirements for a doctoral
degree at Montana State University, I agree that the Library shall make it available to
borrowers under rule o f the Library. I further agree that copying o f this dissertation is
allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U S.
Copyright Law. Requests for extensive copying or reproduction o f this dissertation
should be referred to Bell & Howell Information and Learning, 300 N orth Zeeb Road,
Ann Arbor, Michigan 48106, to whom I have granted “the exclusive right to reproduce
and distribute my dissertation in and from microform along with the non-exclusive right to
reproduce and distribute my abstract in any format in whole or in part.”
Signature
Date
ZZy
ACKNOWLEDGMENTS
I thank my advisors Dr. Roger Sheley and Dr. Bruce Maxwell for their guidance and
support. I thank the members o f my graduate committee, D rs. Jon Wraith, Doug
Dollhopf, and Gerry Anderson for their assistance. I thank Dr. Jim Jacobs, my fellow
graduate students, and the lab and field assistants for their enthusiastic cooperation. I
thank Dr. Jeffrey Jacobsen for his support. Thank you Peggy Humphrey, Kathy Jennings,
Patty Shea, Dianne Brokke, Marie Rippy, Robin Adams, Paula Bielenberg, and Kim
Goodwin for your assistance.
I thank my family for a lifetime o f support in all o f my endeavors. Thank you Sandi
Rourke for your love, understanding, and encouragement.
Funding for this project was provided by the United States Department o f Agriculture
National Research Initiative Competitive Grants Program and the M ontana Noxious Weed
Trust Fund.
V
TABLE OF CONTENTS
K> to
I . INTRODUCTION AND LITERATURE REVIEW ......................................................... I
D iversity In d ic es ................................................................................................................
Invasibility and Species Ric hn ess ................................................................................
The N iche Concept ............................................................................................................ ..3 '
Current R angeland Weed M anagem ent .................................................................. .5
Relevant Research .................... ...................... :.................................................... r........ . . 7
A M odel S y ste m .................. .............................................................................................. ..8
Plant M aterials ......................................... ...... ................................................................ . 9
Spotted knapw eed............................................................................................................ . 9
Crested w heatgrass.............................. .............,........................................................... 11
Intermediate wheatgrass ................................................................. ................................ 13
A lfalfa........................■...............:................................................................................... 14
Growth Analysis of I solated Individuals ............................................................... 15
Quantification of Interspecific Interference and N iche D ifferentiation ... 16
Additive design................................................................................................................. 16
Substitutive design ........................................................................................................... 17
Addition series design...................................................................................................... 17
LITERATURE CITED
23
2. COMPARATIVE GROWTH AMONG INTERMEDIATE WHEATGRASS
CRESTED WHEATGRASS, ALFALFA, AND SPOTTED K N A PW E E D .............. :31
Introduction ..........................................................
31
M aterials and Met h o d s ...............................................:............................. :................... 34
Plant M aterials ............................................................................................................ 34
E xperimental D e s ig n .... ...............................................................................................35
Sampling .................................
35
D ata An a l y s is ......................................'............... ....................... ................................. 36
Resu l ts ..............:......................................................................................................... ......... 36
Growth Ra t e s ................................................................................................................. 36
Total Weight ..............................................;...................... ,................... ;..................... 39
Leaf -Related Mea su r es ...............................................................................................39 ,
R oot Weight .....................................................................................................................40
Shoot Weight ..................................................................
47
R oot-Related Mea su r es ..............................................................................................47
D iscussion ...............................-....................................................................... ■...............:....'48
LITERATURE CITED
41
Vl
TABLE OF CONTENTS - CONTINUED
3. DESIGNING W EED-RESISTANT PLANT COMMUNITIES BY MAXIMIZING
NICHE OCCUPATION BY DESIRABLE SP E C IE S ..................................................... 55
I n tr o d u c tio n .........................:...... ;.................................................................................... 55
M aterials and M e th o d s ........................................... '..................................■................... 56
Study s ite s..............................................
56
Plant m aterials.................................................................................................................... 57
Competition experim ent................................................................................................... 57
Experimental design..................................................................................................... 57
Planting schedule.......................................................................................................... 58
1997 Sam pling..................................................... '................ ..................................... 58
1998 Sam pling.........................
60
Soil water m onitoring.................................................................................................. 61
D ata analysis................. .............................................. :.................................................... 61
Niche differentiation.....................................
61
Spotted knapweed recruitm ent......................
63
•Soil w a te r......................................................................
64
Growth analysis experim ent.........................................................................
64
Experimental design and procedure...............
64
Sam pling........................................................................................................................ 65
Data, analysis.......................................................................................................................65
R e s u l t s ............... ........................... ;...................................................................... .'........................... 66
Competition experim ent................................................................................................... 66
Niche differentiation ..................................................................................................... 66
Spotted knapweed recruitm ent...................................................................................66
Soil w a te r.......................................................................................... ’................... .
74
Growth analysis experim ent...........................................
82
Shoot g ro w th ..................
82
R oot mass density........................................................................................................ 82
D iscussion .:........................................................................................................................... 82
LITERATURE C IT E D ...............................
...90
4. REVEGETATING WEED-INFESTED RANGELAND BY M AXIMIZING NICHE
OCCUPATION BY DESIRABLE SPECIES..................................................................... 92
I n tro d u c tio n .... •...................................................................................................................92
M aterials and M e t h o d s ................... •.
..............................................................'...... 94
Vll
TABLE OF CONTENTS - CONTINUED
Study site s ......................................................................... ................................... , ...........94
■Plant m aterials...................
95
Competition experim ent...................
95
Experimental design...............................
95
1996 Sam pling..................................................................................
97
1997 Sam pling........................................ :........'...........................................................98
Soil water m onitoring....................................................
98
D ata analysis.......... '........................................................... :.......................... ...................98
" ,Niche differentiation.....................................................................................................98
Spotted knapweed recruitm ent.................................................................................100
100
Spotted knapweed biomass and density...................................................
Soil w a te r.....................................................................................................................100
Community dynamics m o d el.....................................................................................101
Growth analysis experim ent...............................
102
Experimental design and procedure.........................................................................102
Sampling ...................................................................................................................... 103
Data analysis......................................
103
R esults .... .................................................... ....... :............................................................. 103
Competition experim ent.......................................................................... A ..................103
Niche differentiation.......................................................................................
103
Spotted knapweed recruitm ent.................................................................................104
Spotted knapweed biomass and density........... ..................................... :............. 104
Soil w a te r......................
Ill
Community dynamics m odel.................................................................................... I l l
Growth analysis experim ent........................................................................................... 113
Shoot g ro w th ................................................. .'.......................................................... 113
R oot mass density................................. :............ ..................................................... 115
D is c u s s io n .......................................................................................... :.............................. 120
LITERATURE C IT E D ................................................................................................ ....... :.. 126
APPENDIX
129
viii
LIST OF TABLES
Table
1. Mean total dry weight per p la n t.....................................................................................39
2. Mean leaf area per p la n t.................................................................................................. 40
3. Mean leaf area ra tio .......:...... ........... ................................................................... ...........40
'4. Mean dry root weight per p la n t
..............................................................................42
5. Mean dry shoot weight per p la n t......................................:........................-................... 42
6. Mean root-to-shoot ratio per p la n t................................................................. .............. 42
7. Mean maximum rooting depth per plant........................................................................ 42
8. R oot weight by depth increment and harvest date....................................................... 45
9. R oot length by depth increment and harvest d a te ....................................................... 46
10. Linear regression o f biomass on volume for 1998 harvest ....................................60
11. Linear regression o f density on biomass for 1997 h arv est
................................ 64
12. Relative competitive abilities and niche differentiation among
desirable species for the Invasion Experiment at ■
the Post Farm in 1997 ................’. .............................................................................. 67
13. Relative competitive abilities and niche differentiation among
desirable species for the Invasion Experiment at
the Post Farm in 1998 ................. ............................................................!..................68
14. Relative competitive abilities and niche differentiation among
desirable species for the Invasion Experiment at
R edB luffin 1997 , .................................................. '...................................................69
15. Relative competitive abilities and niche differentiation among
desirable species for the Invasion Experiment at
Red B luffin 1998 ............................ ,......................................................................... 70
16. D ata and sources for calculation o f seeds produced per p la n t..............................102
ix
LIST OF TABLES-CONTINUED
Table
17. Relative competitive abilities and niche differentiation among
desirable species for the Revegetation Experiment at
the Post Farm in 1996 ............................................................................................... 105
18. Relative competitive abilities and niche differentiation among
desirable species for the Revegetation Experiment at
the Post Farm in 1997 ...............................
106
19. Relative competitive abilities and niche differentiation among
desirable species for the Revegetation Experiment at
R edB luffin 1997 ........................................................................................
107
20. Linear regression o f spotted knapweed measured density
on its sowing density.................................................... ............................... ........... HO
21. Linear regression o f spotted knapweed biomass and density
on desirable species richness...................................................................................... HO
22. Linear regressions of spotted knapweed biomass production
on season-average available soil water and season-long
soil water depletion on total biomass production by
all species for the Post Farm in 1 9 9 7 ......................................................................113
23. Post Farm 1996 biomass by species and sowing density.......................................116
24. Post Farm 1997 biomass by species and sowing density
.......................'............117
25. Paired t-test comparisons o f observed vs. predicted biomass
for the Post F a rm .......................................................................................................118
26. Spitters models predicting average weight per plant
from density o f all species.......................................................................... ............. 118
X
LIST OF FIGURES
Figure •
1. Relative growth ra te s ......................................................... ............................................. 37
2. Unit leaf ra te s ................................................................................................................... 38
3. Total dry weight, leaf area, and leaf area ra tio .................. ..........................................41
4. R oot weight, shoot weight, root-to-shoot ratio,
and maximum rooting depth..................................................................
43
5. Mean dry root weight and root length by depth increment
and harvest interval........................................................
44
6. Plot plan....................................................................
59
7. Regressions of total shoot biomass o f desirable species on
on desirable species richness and diversity for
the Post Farm in 1998 ...........................................
71
8. Regressions o f spotted knapweed recruitment on desirable species
richness and diversity..................................................................
72
9. Effect o f desirable species richness on spotted knapweed
recruitm ent....................
73
10. Effect o f desirable species mixture on spotted knapweed
recruitm ent.............................................................
75
11. Regression o f spotted knapweed recruitment on total shoot
biomass o f desirable species.....................................................
76
12. Regression o f spotted knapweed recruitment on soil water
c o n ten t....................................................................................................:..................... 77
13. Effect o f desirable species mixture on soil water depletion
at the Post Farm (depths 10 cm - 90 c m ).... .................................................... ........78
14. Effect o f desirable species mixture on soil water depletion
at the Post Farm (depths 90 cm - 150 cm) ............................................................... 79
LIST OF FIGURES-CONTINUED
Figure
15. Effect o f desirable species mixture on soil water depletion
at Red Bluff (depths 10 cm - 90 c m )........................................................................ 80
16. Effect o f desirable species mixture on soil water depletion
at Red B luff (depths 90 cm - 150 c m )..................................................................... SI
17. Regression o f shoot weight over tim e ..................... .•................................................. 83
18. ANOVA for shoot weight at final h arv est................................................................. 84
19. R oot mass density plotted over tim e ................................................... :..................... 85
20. ANOVA for root mass density at fourth and fifth harvests.....................................86
21. R oot mass density by depth increment and species
for harvests I through 5 ................... :........................................... !............................ 87
22. Plot p la n ...........................................................................................................
96
23. Effect o f desirable species mixture and richness on spotted
knapweed recruitment at the Post Farm in 1996...................................................108
24. Effect o f desirable species mixture and richness on spotted
knapweed recruitment at the Post Farm and Red Bluff
in 1997 .............................................................................................
25..Mean shoot biomass by site and year
109
..............................................................HO
26. Mean shoot biomass for the Post Farm in 1997....................................................... 112
27. Regression o f spotted knapweed biomass over
desirable species biom ass...........................
112
28. Effects o f desirable species mixture and richness on soil water
depletion for the Post Farm in 1997 ................. ..................... .................................114
29. Diagrammatic representation o f community dynamics m o d el...............................115
30. Linear regressions o f shoot weight over tim e .................................. ...................... 119
xii
LIST OF FIGURES-CONTINUED
Figure
31. Mean shoot biomass at final harvest......................... .................................................120
32. Regressions o f root mass density over tim e ................................................. ...........121
33. Mean root mass density at fourth and fifth harvests...............................................122
34. M ean root mass density by depth increment and harvest
interval................. ....................................................................................................... 123
X lll
ABSTRACT
To develop an ecological basis for designing weed-resistant plant communities, I
tested the hypothesis that susceptibility to weed invasion is determined by temporal and
spatial resource availability and by the ability o f weeds to capture those resources. In two
competition experiments, three desirable species with differing spatial and temporal
growth patterns (crested wheatgrass, intermediate wheatgrass, and alfalfa), and one weed
(spotted knapweed) were used to determine the potential for minimizing weed invasion by
maximizing niche occupation and resource capture. I tested this potential in two types o f
plant Communities: well-established, uninfested plant communities (invasion) and newly
establishing plant communities following revegetation o f a previously weed-infested site
(revegetation). In these experiments, desirable species richness varied, while the total
number o f desirable plants was held constant.
Comparative growth o f isolated individuals was used to test the hypothesis that a
species’ growth characteristics, when grown in isolation, may be useful in predicting its
relative growth in mixtures. These results were variable and did not aid in interpreting
results from the competition experiments.
'
In the invasion experiment, seedlings o f the desirable species were planted in spring
1995. Spotted knapweed was sown in fall 1996, simulating invasion o f a well-established
plant community. Sampling occurred in fall 1997 and fall 1998. Spotted knapweed
recruitment was positively related to soil water content and negatively related to desirable
species richness. These results suggest that soil water plays a role in spotted knapweed
germination and establishment, and that by maximizing niche occupation, soil water may
be preempted from invading weeds.
In the revegetation experiment, all species were sown simultaneously in spring 1996.
This simulated revegetation o f a site containing spotted knapweed seeds in the seed bank
because o f prior infestation. Sampling occurred in fall 1996 and fall 1997. Spotted
knapweed recruitment was not related to desirable species richness in either year. In
1997, spotted knapweed dominated the desirable species. These results suggest that
efforts to revegetate, weed-infested rangeland must address weed reestablishment from
seeds or propagules in the soil. Only then can other strategies, such as maximizing niche
occupation by desirable species, be expected to provide long-term success.
I
INTRODUCTION AND LITERATURE REVIEW
The loss o f native perennial vegetation from North American rangeland has been
accompanied by invasions o f aggressive alien weeds. Grassland ecosystems o f the West,
once dominated by native perennial bunchgrasses, now contain extensive areas dominated
by exotic undesirable species (Roche and Talbott 1986). Invasive weedy species have
been associated with reduced biodiversity (Tyser and Key 1988, Belcher and Wilson 1989,
Kedzie-Webb 2000), reduced wildlife and livestock forage production (Bucher 1984,
Spoon et al. 1983), and altered integrity and function o f the ecosystem (Hooper and
Vitousek 1997, Vinton and Burke 1995). N ot all successful invasions alter large-scale
ecosystem properties (Simberloff 1981, Baker 1986); however, where an individual
species is capable o f altering processes such as productivity, soil development, hydrology,
or nutrient cycling, that species may affect ecosystem function (Vitousek 1990, Baker
1986, M ack 1986).
The purpose o f this chapter is to review the principles and mechanisms involved in the
invasion o f plant communities by undesirable plant species and show how this information
may be used to reduce the potential impact o f these invasive species. I discuss (I) species
diversity indices, (2) the relationship between invasibility and species richness o f plant
communities, (3) the niche concept, (4) current rangeland weed management and
rangeland revegetation, (5) relevant research that may contribute to future revegetation
strategies, (6) a model system for studying the potential to use the niche concept to design
weed-resistant plant communities, (7) the quantification o f growth, interference, and niche
2
differentiation, and (8) how the niche concept may be used to design weed-resistant plant
communities, including the objectives and hypotheses o f this study.
Diversity Indices
Ecologists use species diversity indices to describe the heterogeneity o f a community.
Diversity has two components: (I) species richness (the number o f species) and (2)
evenness (the relative abundances o f each species). Diversity indices combine these two
measures mathematically into a single numerical expression. For example, the Shannon
function uses natural logarithms (In) to estimate diversity ( H ) as
E ( # kiA )
where Yj is the summation symbol and p, is the proportion o f species i in the
community (Shannon and Weaver 1949). While these indices most often are based on
density, the functional importance o f a species in a community may be more appropriately
estimated using biomass or productivity (Hurlbert 1971, Lyons 1981). Though species
richness and diversity are related, these terms are not interchangeable. Both, however, are
used in the ecological literature to describe community heterogeneity.
Invasibilitv and Species Richness
Elton (1958) proposed that species-rich plant communities are less susceptible to
invasion than are species-poor ones. Although this belief has achieved paradigm status
among ecologists, it has not always been borne out by empirical evidence (Law and
M orton 1996). In a biome-scale investigation, Lonsdale (1999) found invasibility was
3
positively correlated with native plant species richness. Stohlgren et al. (1999) also found
such a relationship at landscape and biome scales. From these two studies, it appears that
species richness was probably a surrogate for habitat diversity. That is, as scale increased,
so did the number o f different habitats. As expected, exotic species richness increased
with increasing habitat diversity. However, at the I-m2 scale, Stohlgren et al. (1999)
found invasibility o f four prairie types in the Central Grasslands to be negatively related to
native plant species richness. In that same study, exotic species richness was negatively
correlated with native species cover and positively correlated with soil total % N at the
1000-m2 scale. In the early stages o f a long-term experiment investigating plant
community invasibility, Burke and Grime (1996) found that a plant community was most
invasible if it was disturbed and eutrophic (nitrogen fertilized). However, Tilman (1997)
found community invasibility to be negatively correlated with extractable soil nitrogen and
initial species richness. While the relationship between invasibility and soil N is unclear,
existing evidence indicates a negative relationship between invasibility and species richness
at the community level.
The Niche Concept
The niche concept has been used to describe the relationship between an organism
and its environment. This includes the “habitat” aspect (Grinnell 1917) emphasizing the
,
f
,
environmental requirements o f a species. Later, Hutchinson (1957) described the
fundamental niche as a multidimensional “hyper-volume.” The hyper-volume represents
the conditions where an organism’s expected absolute fitness is at least zero, a conceptual
4
space whose axes include all o f the environmental variables affecting that species.
Hutchinson (1957) believed this completely defined the ecological properties o f that
species. In contrast, Elton (1927) and MacArthur and Levins (1967) used the niche
concept to describe the effect a species has on its environment based primarily on its
trophic level, specifically, short-term impacts o f species on resource use. Despite their
differences, these niche concepts are consistent with Cause’s (1934) competitive exclusion
principle. This principle states that where two species overlap sufficiently in niche, the
weaker competitor will be eliminated.
To completely define a species’ niche, all environmental variables that affect, and are
affected by, a species would have to be measured. While it may be impossible to define a
species’ niche, the niche is a useful concept in assessing potential spatial and temporal
niche overlap among species (Vandermeer 1972, Begon et al. 1996). In practice,
monitoring a manageable number o f environmental variables thought to be most important
to a particular group o f species may aid in increasing our understanding o f the
relationships among those species.
The niche concept may help elucidate the relationship between invasibility and species
richness. 'As initial species richness increases, so does the likelihood o f niche occupation
and the preemption o f resources available to a potential invader. Since revegetafion is a
community-level process, maximizing species richness may be one tool to reduce
invasibility. .
5
Current Rangeland Weed Management
To a large extent, weed management strategies are technology- or tool-based and
rarely meet long-term land management objectives. In many cases, land managers rely on
repeated herbicide applications to control weeds, or wait for the future promise o f
biological controls. Sole reliance on conventional herbicides provides only short-term
control o f weeds. Picloram, the most widely used rangeland herbicide, applied at 0.28 to
0.56 kg/ha, provides control o f many broadleaf plants for 2 to 5 years (Davis 1990).
However, most weeds reinvade because repeated applications are necessary for long-term
control, and repeated applications are often omitted because o f their expense (Griffith
1999). At this time, biocontrol o f most nonindigenous plants is ineffective in reducing
weed populations (Wilson and McCaffirey 1999). The nature and magnitude o f the
invasive plant problem suggest that the establishment o f natural enemies alone offers no
guarantee o f successful biocontrol in the future (Cuda et al. 1989). Grazing management
offers some potential for managing a few invasive species' but little is known about
sustainable grazing strategies for most weeds (Olson 1999). It is becoming increasingly
clear that effective weed management must focus on developing and maintaining desired
plant communities rather than simply controlling weeds (Sheley et al. 1996).
Revegetation o f weed-dominated rangeland that has focused on establishment of
grass monocultures and the use o f broadleaf herbicides has produced variable and
unpredictable results (Hubbard 1975, Berube and Myers 1982, H uston et al. 1984, Larson
and McInnis 1989). The goal o f establishing persistent, biologically diverse plant
6
communities has often been sacrificed to accommodate the short-term goals o f plant
establishment and soil stabilization (Call and Roundy 1991). Existing revegetation
technology often fails to address spatial and temporal diversity within the community.
Some monocultures, such as crested wheatgrass [Agropyron desertorum (Fisch. ex Link)
Schult..] that are tolerant o f resource limitations and, therefore, highly competitive in
semiarid habitats, have remained fairly stable and resisted invasion for decades (Pyke and
Archer 1991). In most cases monocultures have not provided resistence to reinvasion
(Hull and Klomp 1966, Nichols et al. 1993).
In addition to weed-resistance, another goal o f many land managers is consistent
forage production. Often the most productive species when resources are plentiful may be
the least persistent under limiting conditions. At the community level, year-to-year
productivity may be higher and less variable in species-rich communities than in speciespoor communities as predicted by May (1973) and shown by Tilman (1996). In the
future, our objective must be to establish and maintain diverse plant communities that
resist reinvasion and meet other land-use objectives (Sheley et al. 1996).
It has been suggested that long-term, sustainable weed management must focus on
establishing desirable species with plant traits and population strategies that promote
continuous resource capture and enhance niche occupation (Larson et al. 1994, Chapin et
al. 1992, Chapin et al. 1998, Sheley et al. 1996). Plant communities that lack the structure
(i.e. niche occupation) necessary to capture available resources may be functioning below
their productive capacity and have an increased risk o f weed invasion (Larson et al. 1994).
Borman et al. (1991) found that established perennial grasses that initiate growth early and
7
maintain some growth through the winter can limit reinvasion by alien weeds.
Furthermore, establishment o f desirable species having contrasting above- and below­
ground growth patterns may minimize interspecific competition, maximize community
structure, and enhance resource capture (Pyke and Archer 1991, Tilman 1986).
Relevant Research
Austin (1982) and Grace (1988).showed that plant performance in monocultures can
often be used to predict plant performance in mixtures. Brown (1998) used growth
analysis o f individual species grown in low-density monocultures a priori to predict
relative productivity and resource use o f mixtures. Brown (1998) found that variability in
species traits such as root distributions, phenologies, and final plant size was higher among
functional groups than within functional groups (annual vs. perennial, grass vs. forb,
shallow-rooted vs. deep-rooted, etc.). Communities with varying degrees o f niche
occupation, based on the number o f functional groups represented, were established in the
field. Community productivity and water use increased as community niche occupation
increased. In a field study by Symstad (2000), invasibility by native species and functional
group diversity were negatively related. Neither Brown (1998) nor Symstad (2000)
quantified niche occupation by member species using relative competitive abilities o f
member species (Spitters 1983); rather, they assumed community niche occupation was
positively related to functional group diversity.
Jacobs and Sheley (1997) studied the effect o f defoliation o f Idaho fescue (Festuca
idahoensis Elmer) on water use and the subsequent performance o f spotted knapweed.
8
Defoliation o f Idaho fescue growing in mixtures with spotted knapweed increased soil
water content, resulting in an increase in spotted knapweed emergence and growth.
Perhaps growth analysis o f desirable species may be used a priori to predict community
resistance to weed invasion based on patterns o f resource use by desirable species.
A Model System
This study quantified the interference among one weed and three desirable species.
The weed species chosen for this study was spotted knapweed (Centaurea maculosa
Lam.), a deeply taprooted Eurasian perennial. This species was selected because o f its
ecological significance and wide distribution. With the potential o f spotted knapweed to
alter forage production, it has the potential to affect community function (Spoon et al.
1983, Tyser and Key 1989). The three desirable species chosen for this study were
crested wheatgrass [Agropyron cristatum (L ) Gaertn., var. Hycrest], intermediate
wheatgrass [Elytrigia intermedia (Host) Nevski, var. Rush], and alfalfa QAedicago saliva
L., var. Arrow). These four species evolved in the Old World under centuries o f heavy
pressure from humans. The likelihood that these species are niche-differentiated is higher
than that o f spotted knapweed and three desirable species that evolved in North America
under different environmental pressures (Mack 1986). Since the goal o f this study was to
examine the effects o f niche occupation on invasibility, I chose these three desirable
species to increase niche occupation by increasing desirable species richness. Crested
wheatgrass, intermediate wheatgrass, and alfalfa are commonly used in revegetation in the
9
West. There is also experimental evidence that they are niche-differentiated based on their
root distributions and phenologies (Holzworth and Lacey 1991).
Plant Materials
Spotted knapweed
Spotted knapweed is native to the grassland steppe o f central Europe, and east to
central Russia, Caucasia, and western Siberia (Rees et al. 1996, Sheley et al. 1999). In
Europe, spotted knapweed is most aggressive in the forest steppe on Typic Argiborolls,
but can form dense stands in more moist areas on well-drained soils including gravel, and
in drier sites where summer precipitation is supplemented by runoff. It does not compete
well with vigorously growing grass in moist areas. Lacey et al. (1995) reported that
spotted knapweed has been observed at elevations ranging from 578 to over 3040 m and
in precipitation zones ranging from 200 to 2000 mm annually.
Spotted knapweed was introduced to N orth America from Eurasia as a contaminant
in alfalfa (Muller et al. 1988, Roche and Talbott 1986). Spotted knapweed was also
apparently introduced through discarded soil used.as ship ballast (Roche and Talbott
1986). Spotted knapweed was first recorded in Victoria, British Columbia, in 1883 (Groh
1944) and spread further in domestic alfalfa seeds and hay before it was recognized as a
serious problem. (Roche and Talbott 1986).
I have documented the geographic spread o f spotted knapweed in the western United
States up to 1980 using herbarium records compiled by Forcella and Harvey (1980), and
current distribution through interviews with .weed authorities in the region. In the United
10
States,'spotted knapweed was limited to the San Juan Islands, Washington, until 1920. It
had spread to 20 counties in the Pacific Northwest by 1960 and to 48 counties by 1980.
Between 1980 and the present, the range o f spotted knapweed rapidly increased to include
326 counties in the western United States, including every county in Washington, Idaho,
Montana, and Wyoming. It spread at a rate o f 27% per year since 1920 (Chicoine et al.
1985) and by 1989 had infested over 2 million hectares o f rangeland throughout the
western United States (Lacey 1989). Although spotted knapweed spreads most rapidly on
disturbed areas, even rangeland in excellent condition is susceptible to invasion (Morris
and B edunah 1984).
. In general, knapweed (Centaurea spp.) individuals possess many traits that make
them superior to native grasses in capturing resources. Intense competitiveness (Powell
1990, Kennett et al. 1992, Sheley and Larson 1994b), rapid growth rates (Sheley et al.
1993), large seed output and longevity (Schirman 1981, Davis 1990, Sheley and Larson
1994b), and extended growing periods (Sheley and Larson 1994b) all contribute to the
successful domination o f rangeland by knapweeds.
Population characteristics may explain the monotypic domination o f grasslands by
knapweeds. M ost knapweeds have mechanisms that control seed development and
release, providing continuous seed rain (Callihan et al. 1989, Davis 1990). Moreover,
knapweeds exhibit germination polymorphism that distributes seed germination over time
(Nolan and Upadhyaya 1988), Life history models o f knapweeds suggest a conspecific
hierarchy o f size classes result from continuous seedling emergence (Sheley and Larson
11
1994b)! The hierarchy o f size classes within the population maximizes resource capture
and community structure (Sheley and Larson 1995).
Spotted knapweed is a member o f the Asteraceae family. A short-lived perennial, it
typically forms a basal rosette its first year and flowers in subsequent years. Basal rosette
leaves are borne on short pedicels and grow up to 200 mm long and 50 mm wide.
Rosettes are deeply divided, once or twice, into lobes on both sides o f the center vein.
Lobes are oblong with the broadest part above the middle. Flowering stems are erect, 2 to
12 dm tall, and branched above the middle. Stem leaves are alternate, sessile, and have
few lobes; or are linear and entire, and are reduced toward the stem apex. The uppermost
leaves are small and simple. Flowerheads are ovate to oblong, 6 mm wide and 12 mm
long, and are solitary or borne in clusters o f two or three at the branch ends. Involucral
bracts are foliaceous, ovate, yellow-green to brown below. Margins have a soft spine-like
fringe with the center spine shorter than the lateral spines. Spotted knapweed is so named
because o f the obvious black margin on the bract tips. Its flowers are purple to pink,
rarely white, with 25 to 35 flowers per head. Spotted knapweed blooms from June to
October. The flowerheads usually remain on the plant. The achenes are 2.5 to 3.5 mm
long, oval,- brown to black, with pale longitudinal lines. Seeds bear a persistent pappus7o f
simple bristles that are I to 2 mm long.
Crested wheaterass
- '
Crested wheatgrass is. a vigorous perennial bunchgrass with an extensive root system.
Native to Russia, it is well-suited to the Northern Great Plains, the central and northern
12
intermountain region, the subhumid regions o f the Pacific Northwest, and higher
elevations in the Southwest. It grows at elevations as low as 975 m in the northern
Rockies to elevations as high as 2750 m in the Southwest. It does best on deep, neutral to
slightly alkaline, well-drained soils, but does well on heavier textured soils. It prefers 200
to 400 mm o f annual precipitation, tolerates drought very well, is fairly tolerant o f salinity,
and is moderately tolerant o f flooding or high watertable. It establishes well from seed
and performs well on nutrient-poor sites. It is capable o f early spring growth, preceding
native grasses in the Great Plains. Crested wheatgrass also provides good fall grazing if
precipitation is adequate (Wheeler and Hill 1957, Munshower 1991).
Crested wheatgrass stands 3.5 to 7 dm tall. Its culms are erect and range from having
the pith hollow to filled. Its blades are flat, pubescent to glabrous above, 20 to l7 0 mm
long, 1.5 to 7 mm wide, with auricles and ligules 0.1 to I mm long. The spikes o f crested
wheatgrass are dense, 20 to 80 mm long, with closely overlapping spikelets that are
several times longer than the internodes and strongly divergent. There are 3 to 6 flowers
per spikelet with glumes 2 to 6 mm long. The awns, if present, are up to 3.5 mm long.
The lemmas are awnless or with awns up 5 mm long, and the lowest lemma is 4 to 8 mm
long. The anthers are 2.4 to 4 mm long. Crested wheatgrass flowers from June to August
(McGregor and Barkley 1986).
. ‘Hycrest,’ a cultivar o f hybrid crested wheatgrass, was used in this study. It is a cross
o f Agropyron desertorum (Fisch. ex Link) Schult. and A. cristatum (L.) Gaertn., and it
tends to be more robust than its parental species (Asay et al. 1985).
13
Intermediate wheatgrass
Intermediate wheatgrass, a moderately deeply rooted, cool-season, sod-forming grass,
emerges slightly later than crested wheatgrass. Native to Russia, in N orth America it is
best-adapted to the eastern Great Plains and western Midwest, but also does well in the
Intermountain Region. In the Rocky Mountains, it grows at elevations as low as 1550 m
in the north to elevations as high as 2200 m in the south. It does best on slightly acidic to
slightly alkaline, well-drained soils, but does well on sandy loam to clay loam textures. It
requires at least 400 mm o f annual precipitation, tolerates both drought and flooding or
high watertable, is quite tolerant o f salinity, and requires fairly fertile sites. Its main
virtues are its ease o f establishment and high production (Wheeler and Hill 1957,
Munshower 1991).
Intermediate wheatgrass stands 6 to 12 dm tall. Its culms are erect and solid. Its
blades are stiff; involute to flat, with numerous narrow ridges above, glabrous to scabrous
(rarely pilose), 50 to 280 mm long, and 2 to 8 mm wide. The sheaths are glabrous to
scabrous and often ciliate-margined. The ligules are 0.2 to I mm long, and the auricles are
usually pronounced. The spikes o f intermediate wheatgrass are fairly open, 130 to 220
mm long, with spikelets that are 4- to 7-flowered. The glumes are blunt-tipped, glabrous .
i
to pubescent. The first and second glumes are 5 to 8 mm long and 6 to 9 mm long,
respectively. The lemmas are usually blunt, but are sometimes acute or mucronate,
especially the upper ones. The lowest glume is 8 to 11 mm long. The lemmas and glumes
are awnless. ■The anthers are 2.5 to 5 mm long. Flowering occurs from June to
September (McGregor and Barkley 1986).
14
The variety ‘Rush’ was used in this study. It was developed through selection and
direct increase from field plots. ‘Rush’ is noted for its high seedling emergence and
drought tolerance. It is an excellent forage producer on sites with at least 30 cm o f
average annual precipitation (USDA-Natural Resources Conservation Service 1994).
Alfalfa
Alfalfa may be the only forage crop cultivated before recorded history. It most likely
originated in southwestern Asia, probably in Iran (Bolton 1962, Ivanov 1988). It is a very
deeply taprooted forb with an extended growing period, capable o f utilizing groundwater
in semiarid environments. The maximum rooting depth o f alfalfa exceeds 6 m (Weaver
1926, Taylor and Terrell 1982). It is common throughout North America, requiring at
least 400 mm o f annual precipitation. In the Rocky Mountains, it grows into the subalpine
zone. It does well on all deep soils, preferring slightly acidic to slightly alkaline sandy
loams to loams. It requires at least 400 mm o f annual precipitation, is intolerant o f a
watertable within 1.5 m o f the soil surface, and is moderately tolerant o f drought and
salinity (Munshower 1991). Water-soluble compounds from the aboveground portions o f
alfalfa are autotoxic and allelopathic, primarily affecting germination, although allelopathic
effects tend to be greater than autotoxic effects (Hegde and Miller 1990).
Alfalfa is a perennial herb with a knobby or shortly branching crown. Its multiple
stems are glabrous to finely hairy, erect to decumbent, and 3 to 6 dm tall. Its leaves are
alternate and pinnately trifoliate. The petioles o f principal leaves are 10 to 50 mm long.
The stipules are lanceolate, slightly toothed, 5 to 20 mm long, and united at the base with
15
the petiole. The racemose inflorescences are subglobose to short-cylindric, 5- to 40flowered. The flowers are 5 to 11 .mm long and are usually purple, but are occasionally
yellowish-green or brownish, yellow, or rarely white. The pods are several-seeded,
glabrous to sparsely hairy, usually spiraled I to 3 turns or nearly straight or falcate.
Flowering occurs from May through September (McGregor and Barkley 1986).
The alfalfa variety ‘A rrow ’ used in this study is highly resistant to Phytophthora root
rot, bacterial wilt, and Fusarium wilt. Its genetic origin includes, the varieties Endure,
Apollo II, Trident, WL-318, Anchor, Answer, Apollo, and Saranac AR (Certified Alfalfa
Seed Council 1992).
Growth Analysis o f Isolated Individuals
To be successful designing weed-resistant plant communities, we must understand the
phenology and growth o f the weed o f concern and o f the potential desirable species used
J
in revegetation. Growth analysis o f isolated individuals describes the phenology, root
distribution, and growth rate o f plants (Radford 1967, Evans 1972, Grime and Hunt 1975,
Erickson 1976, Hunt 1978, Hunt 1982, Hunt 1988). In this study, growth analysis was
performed on isolated individuals o f spotted knapweed, crested wheatgrass, intermediate .
wheatgrass, and alfalfa to characterize the inherent, growth potential o f each species. This
characterization may be used, in part, to explain differences in the outcomes of
interactions among these species (Radosevich and Rousch 1990).
One objective o f the growth analysis experiments in this study was to quantify the
growth rate and root distribution o f these four species during different stages o f growth.
16
Another objective was to determine the potential to use growth analysis o f isolated
individuals to predict the interference and resource use among one weed and three
desirable species. One o f the growth analysis experiments was conducted to study the
early stages o f establishment and growth in a controlled environment. Two additional
growth analysis experiments were conducted to study the growth o f these species in a field
environment during their second and third growing seasons.'
Quantification o f Interspecific Interference and Niche Differentiation
Several experimental designs attempt to quantify interspecific interference. Although
they were initially developed to assess the effect o f weeds on crops, they can be used to
quantify interference among any species. Each design has its own unique assumptions,
analysis, and conclusions.
Additive design
In this design, the “crop” species is grown at a constant background density and a
“weed” species is introduced at a range o f densities (Donald 1951). This design
confounds effects o f total density and species proportion.
The Index o f Competition (IC) is used to estimate the crop loss caused by weeds
when the population density o f the weed and the expected weed-free crop yield are known
(Cousens 1991). In the regression model, Bc - ac - bNw1/2, Bc is the crop biomass
produced per unit area, ac is the maximum weed-free crop biomass, b is the regression
coefficient o f B0 on the square root o f the population density o f the weed species, Nw.
17
The Index o f Competition is defined by the formula IC = b/ac. The predicted yield loss is
then calculated as L0 = ac • IC • Nw, where Lc is the crop biomass loss caused by the
presence o f the weed.
Substitutive design
-
The Replacement Series (de Wit 1960) design maintains constant total density while
the proportions o f each species vary from 0 to 100% (monoculture). This design avoids
the confounding effects o f an additive design; however, in a multiple replacement series,
an assumption must be made that the influence o f a particular species is independent, o f its
proportion in the community. Velagala et al. (1997) have shown that the competitive
relationship between intermediate wheatgrass and spotted knapweed was densitydependent.
Addition series design
Also called an additive series, this design systematically varies the density and
proportion o f each species. Usually two species are used, but it has also included a
constant background density o f a third species, or even a fourth species in a “plaid” design
|
(Radosevich 1988). Spitters (1983) expanded the reciprocal yield law equation to model
yield-density responses o f two-species systems:
Wx"1 = Pox + PxxNx + PxzN z
where wx is the mean biomass per plant, Nx and,N z are the neighbor densities o f species x
and z, respectively, and P0x, Pxx, and Pxz are regression coefficients. The regression
coefficients are interpreted as: l/p ox = the yield or biomass o f an isolated plant, Pxx is the
18
intraspecific competition coefficient, and Pxz is the interspecific competition coefficient.
Assuming that competitive effects are additive, the model can be expanded to include
more species:
wX-1 = Pox + PxxNx + PxzNz +
PxiNj
These models are fit with multiple linear regression and the parameter values are used to
interpret interference relationships and calculate niche differentiation between species. For
example, in a two-species system where
■ wx 1 = p0x + PxxNx + PxzNz and Wz"1 = Pz+ PzzNz + PzxNx
The relative competitive ability (RCx) o f each species is
RCx =
PxxZpxz and RCz = Pzz Zpzx.
RCx is 1Z3 if I plant o f species x and 3 plants o f species z have an equal influence on the
average weight per plant o f species x (Spitters 1983). Niche differentiation (ND) can also
be calculated from the relative competitive abilities o f each species:
ND = (Pm/PJZ#= ZPJ = PLCx' RCz = (PxxZPxz) - (PzzZPzx).
Niche differentiation increases as ND departs from unity; that is, species x and z are
decreasingly limited by the same resources (Spitters 1983). This interpretation assumes
that both species have equal resource utilization efficiency. Other assumptions include:
. (I) the relationship between density and biomass is rectangularly hyperbolic,
(2) the effects o f interference on the reciprocal o f plant biomass o f each species are .
additive,
19
(3) the effects o f interference are independent o f the total population density and o f
species proportions, and .
.
.
(4) the site is homogenous.
The overall goal o f this study was to determine the potential for designing a weedresistant community by maximizing structure and resource capture. The first supporting
objective was to determine the potential for manipulating species richness and diversity to
facilitate the establishment o f a structurally diverse community. I tested the hypothesis
that as species richness and diversity increase (niche differentiated species), community
structure increases. Community structure was measured as total aboveground biomass.
The second supporting objective was to determine the potential for using knowledge
o f species’ growth rates and root distributions to design plant communities with a high
degree o f structure and, thus, resource capture. I tested the hypothesis that plants shown
«
to differ when grown in isolation will also differ when grown in mixtures.
.
The third.supporting objective was to determine the potential to predict weed
establishment and invasion based on soil water use by desirable species. I tested the
hypothesis that as soil water content decreases, weed establishment and invasion decrease.
Sustainable weed management requires understanding complex ecosystem processes
that influence the invasion o f weeds into plant communities. In this study, I hypothesized
that susceptibility to weed invasion and dominance is determined by the temporal and
spatial availability o f resources within the community and the ability o f a particular weed
species to capture those resources (Whittaker 1975, Larson et al. 1994, Sheley and Larson
1994a). Sheley and Larson (1995) proposed that maximizing community structure and
20
resource capture limits weed invasion through resource preemption. However, the theory
is largely untested, especially with nonindigenous species. I propose to enhance resource
preemption by increasing species richness and diversity. Species richness and diversity
affect community structure and resource capture, and hence resource preemption, in the
following ways: community structure increases as species richness and diversity increase,
as long as member species differ in niche (Whittaker 1975); and as species richness and
diversity increase, niche occupation and resource capture increase (Tilman 1986).
Included in this study was a generalized model that attempts to predict, plant
community dynamics based strictly on intra- and interspecific interference and niche
partitioning. The model was used to predict the long-term dynamics o f plant communities
o f crested wheatgrass, intermediate wheatgrass, alfalfa, and spotted knapweed in varying
proportions and under different conditions. The model was parameterized using
demographic data and competition coefficients from a multiple replacement series
experiment (Carpinelli et al. Submitted). The simulations started by assigning an initial
seed bank size for each species, setting species demographic and interaction parameters,
and selecting the number o f generations to run the simulation.
The specific objectives were to:
(I) Quantify the effects o f species richness and diversity on soil w ater use.
;
Null hypothesis: Species richness and diversity do not affect soil water use.
Alternative hypothesis: As species richness and diversity increase, soil water use
increases.
21
Rationale: The different root distributions and seasons o f growth o f crested
wheatgrass, intermediate wheatgrass, and alfalfa will allow for more continual
water uptake, both temporally and spatially, as species richness and diversity
increase.
(2) Determine the effect o f desirable species richness and diversity on the degree o f
interference between spotted knapweed and three desirable species.
Null hypothesis: Desirable species richness and diversity do not affect spotted
knapweed recruitment and biomass production.
Alternative hypothesis: As desirable species richness and diversity increase,
spotted knapweed recruitment and biomass production decrease.
Rationale: As increased desirable species richness and diversity increase soil
water depletion, less water will be available for the germination, establishment,
and growth o f spotted knapweed.
(3) Determine the potential to use growth analysis o f isolated individuals to predict
interference and resource use among one weed and three desirable.species.
Hypothesis: Relative growth rate o f isolated individuals does not predict resource
use and interference in mixtures.
Alternative hypothesis: Relative growth rate predicts resource use and
interference in mixtures.
Rationale: Species shown to differ in relative growth rate when grown in
isolation will differ when grown in mixtures.
22
(4) Determine possible long-term community dynamics by incorporating data
from a multiple replacement series experiment into a predictive model. This
hypothesis-generating tool will elucidate key processes and mechanisms involved
in weed invasion and revegetation o f weed-infested rangeland.
23
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C
31
CHAPTER 2
COMPARATIVE GROWTH AMONG INTERMEDIATE WHEATGRASS, CRESTED
WHE AT GRAS S3 ALFALFA, AND SPOTTED KNAPWEED
Introduction
Loss o f native perennial vegetation on North American rangelands has been
accompanied by invasions o f aggressive non-indigenous weeds. Grassland ecosystems o f
the West, once dominated by native perennial bunchgrasses, now contain extensive areas
dominated by exotic undesirable species (Roche and Talbott 1986). Invasive weedy
species have been associated with reduced biodiversity (Tyser and Key 1988, Belcher and .
Wilson 1989, Kedzie-Webb 2000), reduced wildlife and livestock forage production
(Bucher 1984, Spoon et al. 1983), and altered functioning o f the ecosystem (Hooper and
Vitousek 1997, Vinton and Burke 1995).
Revegetation o f weed-dominated rangeland that has focused on establishment of
grass monocultures and the use o f broadleaf herbicides has produced variable and
unpredictable results (Hubbard 1975, Berube and Myers 1982, H uston et al. 1984, Larson
and McTnnis 1989). The goal o f establishing persistent, biologically diverse plant
communities has often been sacrificed to accommodate the short-term goals of plant
establishment and soil stabilization (Call and Roundy 1991). Existing revegetation
technology often fails to address spatial and temporal diversity within the community.
32
It has been suggested that long-term, sustainable weed management must focus on
establishing desirable species with plant traits and population strategies that promote
continuous resource capture and enhance niche occupation (Larson et al. 1994, Chapin
1992, Sheley et al. 1996). Plant communities that lack the niche occupation necessary to
capture available resources may be functioning below their productive capacity and have
an increased risk o f weed invasion (Larson et al. 1994, Tilman 1997). Establishment o f
desirable species having contrasting above- and below-ground growth patterns may
minimize interspecific competition, maximize community structure, and enhance resource
capture (Pyke and Archer 1991, Tilman 1986). Sheley and Larson (1995) proposed that
maximizing community structure and resource capture limits weed invasion through
resource preemption. However, the theory is largely untested, especially with
nonindigenous desirable species.
During the establishment phase o f revegetation, the relative timing o f emergence and
growth rates o f species may be an important determinant o f community dynamics (Ross
and Harper 1972, Harper 1977, Radosevich and Holt 1984). Rehabilitation o f yellow
starthistle (Centaurea solstitialis L.) dominated rangeland has been attempted by
revegetating with desirable grasses. These attempts typically fail because o f resource
preemption by annual weeds which germinate sooner and have a higher initial growth rate
than the desirable grasses (Borman, et al. 1991). In ah experiment by Harris (1967), the
winter annual, cheatgrass (Bromus tectorum L.), preempted resources from fall-sown
bluebunch wheatgrass \Agropyron spicatum (Pursh) Scribn. & Smith], Both species
germinated in the fall, but cheatgrass had faster winter root growth than did bluebunch
33
wheatgrass, thus allowing it to gain control o f the site before bluebunch wheatgrass
established. Borman et al. (1991) found that established perennial grasses that initiate
growth early and maintain some growth through the winter can limit reinvasion by alien
weeds.
During the early stages o f secondary Succession (e.g. revegetation), availability o f
light, water, and nutrients may be particularly high relative to the demand by a sparse
community o f establishing plants. In this case, resource preemption may be more
important than competition in determining community dynamics (Goldberg 1990). Those
plants with the highest growth rates have the highest likelihood o f establishing to the .
exclusion o f plants with slower growth rates (Harper 1977). Since establishment is the
most critical phase o f revegetation, knowledge o f the initial growth rate o f a species may
be the best predictor o f the short-term outcome o f a revegetation effort (James 1992).
Growth analysis o f plants grown in isolation has been used to describe their growth'
rate, phenology, and resource allocation (Radford 1967, Evans 1972, Grime and Hunt
1975, Erickson 1976, Hunt 1978, Hunt 1982, Hunt 1988). Growth analysis o f isolated
individuals may have the potential to predict the outcome o f a revegetation effort by
providing information about relative growth rates o f species involved. Where weed seed
is present in the seed bank o f a site recently sown with desirable species, the relative
growth rates o f the weed and the desirable species may determine community
I ■
_
composition, at least for that period o f time when resource preemption is more important
than competition.
34
The objective o f this experiment was to quantify the growth rate o f three desirable
species commonly used in revegetation o f weed-infested rangeland and one common
rangeland weed species.
Null hypothesis: The relative growth rates o f the four species are similar.
Alternative hypothesis: Alfalfa and spotted knapweed (small-seeded dicots) have
higher growth rates than intermediate wheatgrass and crested wheatgrass (largeseeded monocots).
■ Rationale: In studies by Grime and Hunt (1975) and Hunt and Cornelissen (1997),
relative growth rate was negatively related to seed size and unrelated to whether
species were monocots or dicots.
Materials and Methods
Plant materials
/
The three desirable species used in this study were crested wheatgrass [Agropyron
cristatum (L.) Gaertn., var. Hycrest], intermediate wheatgrass \Elytrigia intermedia
(Host) Nevski, var. Rush], and alfalfa (Medicago sativa L., var. Arrow). Crested
wheatgrass is a shallowly rooted, early emerging, cool-season bunchgrass. Intermediate
wheatgrass, a moderately deeply rooted, cool season, sod-forming grass, emerges later
than crested wheatgrass. Alfalfa is a deeply taprooted broadleaf with an extended growing
period. These three perennials are commonly used in revegetation in the West. They
were chosen because o f their potential to maximize niche occupation when used together
in a revegetation seed mix (Holzworth and Lacey 1991). The invasive weed species
35
chosen for this experiment was spotted knapweed (Centaurea maculosa Lam.), a deeply
taprooted Eurasian perennial forb. This species was chosen because o f its ecological
significance and wide distribution.
Experimental design
Isolated plants o f spotted knapweed, crested wheatgrass, intermediate wheatgrass,
and alfalfa were arranged in a randomized-complete-block design. Each o f four blocks
contained 20 plants (4 species X 5 harvest dates). Plants were grown in pots in an
environmental chamber (12°C, 12-h daylength, 500 pE m"2 s"1 visible radiation measured
at pot height). The pots (100 mm diameter X I m length polyvinyl chloride tubes) were
split vertically and taped back together to facilitate post-harvest root removal. The pots
t,
were filled with sterilized Farland silt loam (fine-silty, mixed Typic Argiboroll; A horizon),
water-saturated, and allowed to drain to pot capacity. Ten seeds o f a given species were
evenly spaced on the soil surface o f each pot and covered lightly with soil (<2 mm). The
surface was lightly misted with water once daily and covered with clear plastic until
emergence (7 days), after which no further watering occurred. Pots were thinned to a
single individual per pot (10 days).
Sampling
Seed weight o f each species was determined by taking the mean o f four subsamples o f
100 seeds. First plant harvest occurred 14 days after emergence (DAE). Harvests
continued on 14-day intervals until final harvest (70 DAE). Soil was manually rinsed from
roots and maximum rooting depth was measured. Root systems were extracted from each •
36
pot, separated from shoots, divided into upper (0 cm - 33 cm), middle (33 cm - 67 cm),
and lower (67 cm -100 cm) depth increments, washed, and measured for root length using
a root length scanner (Comair Corp., Melbourne, Australia). L eaf area was measured
using a LICO R-3100 with conveyor belt (LI-COR, Inc., Lincoln, NE). All plant material
(root and shoots) was dried (48 h, 60 °C) and weighed.
D ata analysis
A regression analysis was used to estimate the instantaneous relative growth rate
(RGR) and instantaneous unit leaf rate (ULR) calculated over the 70-day period (Hunt
1982). Slopes were compared using the extra sums o f squares procedure (P<0.05,
Ratkowski 1983). Analysis o f variance (PROC GLM, SAS Institute Inc. 1991) was used
to test for absolute differences among species in total weight, shoot weight, root weight,
leaf area, leaf area ratio, and maximum rooting depth. Mean separations were made using
LSD comparisons (P<0.05). ■
Results
Growth rates
Relative growth rates o f intermediate and crested wheatgrasses were similar to each
■ •
I
other, and approximately twice that o f alfalfa and spotted knapweed, which were similar
to each other (Figure I). Unit leaf rates o f the four species were similar (Figure 2).
ELIN
A
MESA
R2 = 0.93
y = 1.53 + 0.044x
Days after emergence
Days after emergence
AGCR
R2 = 0.95
CEMA
R2 = 0.79
y = 1.32 + 0.085x
y = 1.40 + 0.047x
Days after emergence
Days after emergence
Figure I. Relative growth rates o f intermediate wheatgrass (A), crested wheatgrass (B), alfalfa (C), and spotted
knapweed (D). Acronyms ELIN, AGCR, MESA, and CEMA represent intermediate wheatgrass, crested wheatgrass,
alfalfa, and spotted knapweed, respectively.
ELIN
E
A
0.8
R2 = 0.47
2
MESA
R2 = 0.65
0.2
y = 0.090 + 0.0082%
y = 0.088 + O.OOSOx
Days after emergence
Days after emergence
AGCR
E
CEMA
0.8
R1 = 0.61
2
u>
oo
0.2
R2 = 0.42
y = 0.090 + 0.0070x
Days after emergence
y = 0.066 + 0.0091x
Days after emergence
Figure 2. Unit leaf rates o f intermediate wheatgrass (A), crested wheatgrass (B), alfalfa (C), and spotted knapweed
(D).
39
Total weight
Seed weights (0 DAE) were different for the four species. Intermediate wheatgrass
had the highest seed weight followed in descending order by crested wheatgrass, alfalfa,
and spotted knapweed (Table I). Intermediate wheatgrass and crested wheatgrass grew
much faster than alfalfa and spotted knapweed. At 70 DAE, intermediate wheatgrass and
crested wheatgrass weights were similar and more than 10 times greater than alfalfa and
spotted knapweed weights, which were similar (Figure 3 A, Table I).
Table I. Mean total dry weight per plant (mg).
Days after
emergence
ELIN
AGCR
M ESA
0
6.72
3.49
2.24
CEMA
,
2.09
LSDnns
0.11
14
10.88
7.50
8.80
5.45
2.29
28
66 78
19.55
20.70
42
218.38
40.73
188.50
33.83
36.00
29.21
74.40
56
771.95
459.35
55.10
62.20
319.20
70
1031.35
1243.40
92.43
92.45
437.03
Leaf-related measures
At 14 D AE intermediate wheatgrass had the greatest leaf area, followed by alfalfa,
spotted knapweed, and crested wheatgrass; however, no single species was significantly
different from all others (Figure 3B, Table 2). By 28 DAE, intermediate wheatgrass and
crested wheatgrass were similar and had at least three times more leaf area than did alfalfa
and spotted knapweed, which were similar. By 70 DAE, intermediate wheatgrass and
crested wheatgrass were similar and had at least 12 times more leaf area than alfalfa and
spotted knapweed, which were similar.
40
L eaf area ratios o f the four species showed only slight differences over the 70-day
period (Figure SC, Table 3). There was no obvious trend in rank among the four species.
By 70 DAE, the only difference was between the intermediate wheatgrass (highest) and
alfalfa (lowest).
Table 2. Mean leaf area per plant (mm2).
Days after
emergence
ELIN
AGCR
M ESA
14
135
60
122
28
1020
710
178
42
3580
300
3239
CEMA
99
LSDn ns
41
227
555
1053
412
338
415
4729
732
1023
6592
Table 3. Mean leaf area ratio (mm2 mg'1).
Days after
emergence
ELIN
AGCR
M ESA
14
11.68
7.55
14.00
28
14.83
16.81
9.27
CEMA
LSDn n<
4.33
4.98
56 ■
70
9260
13439
6859
16398
42
17.53
17.11
56
70
11.51
13.54
18.58
11.39
9.19
4.98
.16.71
8 82
7.56
6.74
5.17
12.82
8.19
9.79
3.72
Root weight
Root weights were similar at 14 DAE (Figure 4A, Table 4). Thereafter, intermediate
wheatgrass and crested wheatgrass roots grew faster than alfalfa and spotted knapweed.
By 70 DAE, intermediate wheatgrass and crested wheatgrass were similar and had more
41
Errorbars = LSD 005
V //A ELIN
EM3 A G C R
IfZxSxXl MESA
Error bars = LSD,
E S
5
CEM A
600
Days after emergence
Days after emergence
25000
20000
15000
S 10000
E rro r bars = LSD,
ELIN
?
I AGCR
E S 9 M E SA
^
CEM A
Days after emergence
Figure 3. Total dry weight (A), leaf area (B), and leaf area ratio (C) means by harvest
date.
42
Table 4. Mean dry root weight per plant (g).
Days after
emergence
ELIN
AGCR
M ESA
14
0.002
0.002
0.003
28
0.018
0.009
0.008
42
0.034
0.024
0.016
56
0.094
0.053
0.022
70
0.092
0.079
0.040
Table 5. Mean dry shoot weight per plant (g).
Days after
emergence
ELIN
AGCR
M ESA
14
0.009
0.005
0.006 '
CEMA
0.002
LSDnn<
0.009
0.008
0.018
0.029
0.036
0.016
0.051
0.043
CEMA
LSDnns
0.004
0.002
0.001
28
0.049
0.032
0.012
0.011
0.022
42
0.184
0.164
0.018
0.018
0.065
56
0,678
0.407
0.034
0.033
0.270
70
0.939
1.165
0.052
0.057
0.404
Table 6. M ean root-to-shoot ratio per plant.
Days after
emergence
ELIN
AGCR
M ESA
14
0.325
0.378
0.483
28
• 0.357
0.289
0.658
CEMA
LSDnns
0.481
0.209
0.856
0.130
42 .
0.185
0.148
0.866
0,988
0.158
56
0.133
0.120
0.649
0.886
0.190
0.096
0.066
0.786
0.852
0.461
70
’
Table 7. Mean maximum rooting depth per plant (m).
Days after
emergence
ELIN
AGCR
M ESA
CEMA
LSDnns
14
0.114
0.179
0.196
0.153
0.053
28
0.361
0.452
0.238
0.318
0.083
42
0.591
0.543
0.286
0.442
0.157
56
0.921
0.816
0.347
0.542
0.122
70
0.930
0.905
0.328
0.605
0.253
43
Error bars = LSD 0 05
Error bars = LSD 0
E Z 3 ELIN
i-----1 AGCR
KKKiI MESA
CEMA
Days after emergence
Days after emergence
Error bars = LSD005
V m ELIN
1.00
EHHD AGCR
BKAKd MESA
CEMA
0.75
0.50
0.25
0.00
28
42
56
Days after emergence
Figure 4. Mean root weight (A), shoot weight (B), root-to-shoot ratio (C), and
maximum rooting depth (D) by harvest date.
44
14 DAE
Depth increment
Ocm - 33 cm \ / / / A
33 cm - 67 cm lyxvxi
67 cm - IOOcm
Root weight
Root length
ELIN
AGCR MESA CEMA
0.0000
W 0.0025
J
S
%0.0050
J
^ 0.0075
0 .0 1 0 0 L
28 DAE
0.000
ELIN
AGCR MESA CEMA
42 DAE
ELIN
AGCR MESA CEMA
53 0.010
S 0.015
70 DAE
56 DAE
ELIN
AGCR MESA CEMA
ELIN
AGCR MESA CEMA
o 0.08
Figure 5. Mean root dry weight and mean root length by depth increment and harvest
date: 14 (A), 28 (B)1 42 (C), 56 (D), and 70 (E) days after emergence (DAE),
respectively. Scales o f y-axes change by harvest interval, but remain proportionally
similar, thus allowing for visual comparison o f weight-to-length relationships among
45
Table 8. Root weight (g) by depth increment and harvest date.
0 cm - 33 cm
Species
Days after
emergence
14
ELIN
AGCR
M ESA
CEMA
'
Mean
Depth Increment
33 cm - 67 cm
67 "cm - 100 cm
0.0024
SE
0.0012
Mean
0
SE
0
Mean
0
SE
6
28
0.0151
0.0077
0.0024
0.0044
0
0
.42
56
70
0.0311
0.0155
0.0032
0
0
0.0696
0.0418
0.0212
0.0017
0.0146
0.0664
0.0191
0.0219
0.0113
0.0033
0.0037
0.0021
0.002
14
0.0022
0.0014
0
0
0
0
28
0.0085
0.0021
0.0003
0.0004
0
0
42
0,0233
0.0109
0.0009
0.0008
0
0
. 56
0.0339
0.0164
0.0173
0.0111
0.0015
0.0017
70
0.0486
0.0157
0.0228
0.0222
0.0071
0.0085
14
0.0027
0.0004
0
0
0
0
28
0.0078
0.0018
0
.0
0
0
42
0.01.56
O.0O22
0
0
0
0
56
0.0216
0.0042
0
0.0001
0
0
70
0.0404
0.015
0.0001
o .o o o i
0
0
14
0.0017
0.0005
0
0
0
0
28
0.0095
0.0018
0
0.0001
0
■ 0
42
0.017
0.0006
0.0007
0.0008
0
0
56
0.0284
0.0083
0.0007
0.0005
0
0
70
0.0335
0.0165
0.0023
0.0019
0.0001
0.0001
46
Table 9. Root length (m) by depth increment and harvest date.
_____________________ Depth Increment
0 cm - 33 cm
33 cm - 67 cm
Species
Days after
emergence
14 .
ELIN
.
AGCR
M ESA
CEMA
Mean
SE
Mean
3.10
2.77
28
2.93
42
56
70
67 cm - 100 cm
Mean
SE
0.00
SE
' 0.00
0.00
0.00
1.61
0.04
0.05
0.00
0.00
10.05
4.89
0.47
Q.36
0.00
0.00
22.53
7.48
6.40
5.71
1.61
1.34
22.53
6 63
8.35
2.82
2.88
2.95
14
1.28
0.45
0.00
0.00
0.00
0.00
28
2.20
0.45
0.09
0.07
. 0.00
0.00
42
5.75
1.32
0.30
0.16
0.00
0.00
' 56
11.78
5.12
3.79
3.85
0.46
0.36
70
19.03
5.35 '
8 05
6.46
3.24
3.87
14
1.00
0.90
0.00
0.00
0,00
0.00
28
0.90
0.28
0.00
0.00
0.00
0.00
42
1.60
0.37
0.00
0.00
0.00
0.00
56
2.48
0.49
0.03
0.06
0.00
0.00
70
6.05
2.11
o io i
0.01
0.00
0.00
14
3.15
2.43
0.00
0.00
0.00
0.00
28
1.58
0.34
0.01
0.03
0.00
0.00
42
2.50
0.91
0.12
0.09
0.00
0.00
56
4.33
1.49
0.21
0.01
0.00
0.00
70
.
8.23
4.72
1.31
2.20
0.09
0.11
'■
.
47,than twice the root weight o f alfalfa and spotted knapweed; however, only intermediate
wheatgrass was significantly different from alfalfa and spotted knapweed at 70DAE.
Shoot weight
Over the first 28 days, shoot weights o f intermediate wheatgrass and crested
wheatgrass were only slightly greater, if at all, than that o f alfalfa and spotted knapweed
(Figure 4B). By 42 DAE, shoot weights o f intermediate wheatgrass and crested
wheatgrass were similar and approximately 10 times greater than that o f alfalfa and
spotted knapweed. By 70 DAE, intermediate wheatgrass and crested wheatgrass had
approximately 20 times the shoot weight o f alfalfa and spotted knapweed, which were
similar.
Root-related measures
Root-to-shoot ratios o f the four species were similar at 14 DAE (Figure 4C, Table 6).
By 28 DAE, alfalfa and spotted knapweed were similar and approximately twice that of
intermediate wheatgrass and crested wheatgrass, which were similar. At 70 DAE, rootto-shoot ratios o f alfalfa and spotted knapweed were similar and about 10 times greater
than those o f intermediate wheatgrass and crested wheatgrass, which were similar.
At 14 DAE, differences in maximum rooting depth among the four species were slight
(Figure 4 D, Table 7). At 28 DAE, crested wheatgrass had greater maximum rooting
depth than the other three species. At 56 DAE and 70 DAE, intermediate wheatgrass and
crested wheatgrass were similar and 1.5 times greater than spotted knapweed, and
between 2 and 3 times greater than alfalfa.
J
48
R oot weight and length by depth and harvest interval is presented in Figure 5, A-E
(Tables 8 and 9). This series o f graphs depicts the greater proportion o f root weight and
length at depths greater than 33 cm o f intermediate wheatgrass and crested wheatgrass as
compared to alfalfa and spotted knapweed.
Discussion
‘Hycrest’ crested wheatgrass growth in this experiment was similar to that from
greenhouse studies by Asay et al. (1985) and Aguirre and Johnson (1991). Svejcar (1990)
reported similar total dry weight for the closely related ‘Nordan’ crested wheatgrass
[Agropyron desertorum (Fisch. ex Link) Schult. cv. Nordan] grown with warmer days and
■cooler nights. However, the root-to-shoot ratio for ‘Nordan’ was seven times greater
than that for ‘Hycrest.’ Growth measurements in this experiment and the other
experiments may have differed because o f differences in environmental conditions. Asay
et al. (1985) compared growth o f ‘Hycrest’ with two other crested wheatgrass cultivars at
both seedling (greenhouse) and adult (field conditions) stages. ‘H ycrest’ grew more than
the other cultivars at both-stages, suggesting that growth analysis o f seedlings may provide
clues to adult growth.
Alfalfa growth in this experiment was 20 to 40 times less than that in other
experiments by Ueno and Smith (1970), Pearson and Hunt (1972), Daday and Williams
(1976), and MacDowall (1983); however, those studies used higher temperatures, more
light, or more frequent watering. In those studies, temperature was a treatment, and
growth increased with temperature up to 25 °C, then decreased with increasing
temperature.
49
Jacobs and Sheley (1999) studied spotted knapweed in its first 84 days o f growth
under conditions nearly identical to those in this experiment. In that experiment, spotted
knapweed growth rate over the first 70 DAE was about half o f that from this experiment, •
while root-to-shoot ratio was. similar. Velagala et al. (1997) analyzed growth o f spotted
knapweed grown in pots under field conditions for 60 days. Under those conditions,
ambient light was five times greater than in this experiment (Jacobs 1989). Their linear
regression equations predicted spotted knapweed root weight and shoot weight at 70
DAE to be 8 and 24 times higher, respectively, than the means from this experiment.
Perhaps the greater light, warmer days, and precipitation in the field contributed to the
increased growth. Linear regressions from that experiment predicted ‘Oahe’ intermediate
wheatgrass root weight and shoot weight at 70 DAE to be 21 and 7 times greater,,
respectively (Velagala et al. 1997), than ‘Rush’ intermediate wheatgrass in this
experiment. Annual production o f ‘Rush’ is typically higher than that o f ‘Oahe’ (USDANatural Resources Conservation Service 1994). Perhaps the greater growth o f ‘Oahe’ in
the field experiment was caused by more light, warmer days, and precipitation. It is also
possible that growth rates o f these two cultivars in their first 60 to 70 days is not
indicative o f their annual production.
Sheley and Larson (1994) performed growth analysis on yellow staithistle (Centaurea
solstitialis L.), which is closely related to spotted knapweed, under similar conditions to
those o f this experiment. Yellow staithistle growth was similar to that for spotted
knapweed in this experiment. The success o f these related weeds may be due less to their
initial growth rate and more to their deep taproots and extended growing period as mature
50
plants, allowing for water uptake and growth after shallower-rooted species senesce
(Jacobs and Sheley 1998).
The four species in this experiment had similar unit leaf ratios; therefore, they appear
to be equally “efficient” carbon fixers. However, intermediate wheatgrass and crested
wheatgrass had greater absolute and relative growth than alfalfa and spotted knapweed.
In addition, their greater rooting depth than spotted knapweed suggests that intermediate
wheatgrass and crested wheatgrass may be excellent candidates for revegetation o f sites
previously dominated by spotted knapweed because o f their potential to preempt
resources.
si
LITERATURE CITED
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growth in seedlings o f four range grasses. I. Range Manage. 44(4):341-346.
Asay, K.H., D R Dewey, F E. Gomm, D.A. Johnson, and J.R. Carlson. 1985.
Registration o f 'Hycrest' crested wheatgrass. Crop Sci. 25:368-369.
Belcher, J. W., and S.D. Wilson. 1989. Leafy spurge and the species composition o f a
mixed grass prarie. J. Range Manage. 42:172-175.
Berube, D.E. and J.H. Myers. 1982. Suppression o f knapweed invasion by crested
wheatgrass in the dry interior o f British Columbia. J. Range Manage. 35:459-461.
Borman, M.M., W.C. Krueger, and D.E. Johnson. 1991. Effects o f established perennial
grasses on yields o f associated annual weeds. J. Range Manage. 44:318-326.
Bucher, R F. 1984. Potential spread and cost o f spotted knapweed on rangelands.
MontGuide 8423, Montana State University, Bozeman, MT.
Call, C A . and B A . Roundy. 1991. Perspectives and processes in revegetation o f arid and
semiarid rangelands. J. Range Manage. 44:543-549.
Callihan, R H., F E. Northam, J.B. Johnson, E.L. Michalson, and T.S. Prather. 1989.
Yellow starthistle management in pasture and rangeland. Univ. o f Idaho. Curr. Inf.
Ser. No. 634. Moscow, ID. 4 p.
Chapin, F. S. III., E D. Schulze, and H.A. Mooney. 1992. Biodiversity and ecosystem
process. Trends Ecol. Evol. 7:8-9.
Chicoine, T.K., P.K. Fay, and G A. Nielsen. 1985. Predicting weed migration from soil
and climate maps. Weed Sci. 34:57-61.
Daday, H. and J.D. Williams. 1976. Changes in lateral root growth o f lucerne cultivars
with temperature. J. Austral. Inst. Agile. Sci. 42:119-121.'
Erickson, R.O. 1976. Modeling o f plant growth. Ann. Rev. Plant Physiol. 27:407-434.
Evans, G.C. 1972. The Quantitative Analysis o f Plant Growth. University o f California
Press, Berkeley.
Goldberg, D.E. 1990. Components o f resource competition in plant communities. In. J.B.
Grace and D. Tilman (eds.), Perspectives on Plant Competition. Academic Press,
San Diego, pp. 27-49.
\
52
Grime, J.P. and R. Hunt. 1975. Relative growth-rate: its range and adaptive significance in
a local flora. J. Ecol. 63:393-422.
Harper, J.L. 1977. The Population Biology o f Plants. Academic Press, London.
Harris, G A. 1967. Some competitive relationships between Agropyron spicatum and
Bromus tectorum. Ecol. Monogr. 37:89-111.
Holzworth, L. and I. Lacey. 1991. Species selection criteria for seeding dryland pastures
in Montana. Montana State University Extension Bull. 19. Bozeman, MT.
Hooper, D.U., and P M . Vitousek. 1997. Plant composition and diversity effects on
ecosystem processes. Sci. 277:1302-1305.
Hubbard, W. A. 1975. Increased range forage production by reseeding and the chemical
control o f knapweed. J. RangeM anage. 28:406-407.
Hunt, R. 1978. Plant Growth Analysis. Studies in Biology No. 96. Edward Arnold,
London.
Hunt, R. 1982. Plant growth curves: The functional approach to plant growth analysis.
University Park Press. Baltimore, MD. 555 p.
Hunt, R. 1988. Analysis o f growth and resource allocation. Weed Res. 28:459-463.
Hunt, R. and L H C . Comelissen. 1997. Components o f relative growth rate and their
interrelations in 59 temperate plant species. New Phytol. 135:395-417.
Huston, C.H., R H. Callihan, and R U . Sheley. 1984. Reseeding intermediate wheatgrass
in yellow starthistle-infested rangeland. In: Proc. Knapweed Symp., Montana State
University Cooperative Extension Bull. 1315. Bozeman, MT. pp. 42-44.
Jacobs, I S . 1989. Temperature and light effects on seedling performance o f Pinus
albicaulis. M.S. Thesis. Montana State University. Bozeman, MT.
Jacobs, J.S. arid R U . Sheley 1999. Competition and niche partitioning among
Pseudoroegmria spicata, Hedysarum boreale, and Centaurea maculosa. Great
Basin Nat. 59:175-181.
Jacobs, J.S. and R U . Sheley. 1998. Observation: life history o f spotted knapweed. J.
Range Manage. 5 1:665-673.
James, D. 1992. Some principles and practices o f desert reyegetation seeding. Arid Lands
Newsletter 32:22-27.
53
Kedzie-Webb, S.A., R.L. Sheley, J.J. Borkowski, and J.S. Jacobs. 2000. Relationships
between Centaurea maculosa and indigenous plant assemblages. Great Basin Nat.
(In press)
Larson, L.L. and M.L. Mclnnis. 1989. Impact o f grass seedlings on establishment and
density o f diffuse knapweed and yellow starthistle. Northwest Sci. 63:162-166.
Larson, L.L., R.L. Sheley, and M.L. Mclnnis. 1994. Vegetation management and weed
invasion. In. Sustaining rangeland ecosystems, USDA-SARE Symposium,
LaGrande, OR.
MacDowall, F.D.H. 1983. Effects o f light intensity and CO2 concentration on the kinetics
o f 1st month growth and nitrogen fixation o f alfalfa. Can. J. Bot. 61:731-40.
Nolan, D.G. and M.K. Upadhyaya. 1988. Primary seed dormancy in diffuse and spotted
■knapweed. Can. I. Plant Sci. 68:775-783.
Pearson, C. J. and L A . Hunt. 1972. Effects o f temperature on primary growth o f alfalfa.
Can. J. Plant Sci. 52:1007-1015.
Peterson, R.G. 1985. Design and analysis o f experiments. Marcel Dekker, Inc. New York.
Pyke, D A. and S. Archer. 1991. Plant-plant interactions affecting plant establishment and
persistence on revegetated rangeland. I. Range Manage. 44:550-557.
Radford, P J . 1967. Growth analysis formulae—their use and abuse. Crop Sci. 7:171-5.
Radosevich, S R. and J.S. Holt. 1984. Weed Ecology. John Wiley & Sons, Inc. New
York. 265 p.
Ratkowski, D A. 1983. Nonlinear regression modeling: a unified practical approach.
Marcel Dekker, Inc., New York, N.Y. 276 p.
Roche, B.F., Jr., and C.J. Talbott. 1986. The collection history o f Centaurea found in
Washington State. Agri. Res. Center. Res. Bull. XB0978. Washington State
University Cooperative Extension, Pullman, WA. 36 p.
Ross, M.A. and TL. Harper. 1972. Occupation o f biological space during seedling
establishment. J. Ecol. 60:77-88.
SAS Institute Inc. 1991. SAS/STAT U ser’s Guide, Release 6.03 edition. SAS Institute
Inc., Cary, NC. 1028 p.
Sheley, R U ., and LU . Larson. 1994. Competitive growth and interference between
cheatgrass and yellow starthistle seedlings. J. Range Manage. 47:470-474.
54
Sheley, R.L. and L.L. Larson. 1995. Emergence date effects on resource partitioning ■
between diffuse knapweed (Centaurea diffusa) seedlings. J. Range
M anage.49:241-44.
Sheley, R L., T. J. Svejcar, and B. D. Maxwell. 1996.,A theoretical framework for
developing successional weed management strategies on rangeland. Weed
Technol. 10:712-720.
Spoon, C.W., H R. Bowles, and A. Kulla. 1983. Noxious weeds in the Lolo National
Forest. A situation analysis staff paper. USDA Forest Service. 33 p.
Svejcar, T. 1990. Root length, leaf area, and biomass o f crested wheatgrass and cheatgrass
seedlings. J. Range Manage. 43(5):446-448.
Tilman, D. 1986. Resources, competition and the dynamics o f plant communities. In:
Michael Crawley (ed.), Plant Ecology. Blackwell Scientific, Boston, pp. 51-75.
Tilman, D. 1997. Community invasibility, recruitment limitation, and grassland
biodiversity. Ecol. 78(l):81-92. '
Tyser, R.W. and C.H. Key. 1988. Spotted knapweed in natural area fescue grasslands: an
ecological assessment. Northwest Sci. 62:151-160.
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carbohydrate composition o f three alfalfa cultivars. Agron. I. 62:764-767.
USDA-Natural Resources Conservation Service. 1994. Notice o f Release o f "Rush"
Intermediate Wheatgrass. Aberdeen Plant Materials Center, Aberdeen, ID. 7 p.
Velagala, R.P., R.L. Sheley, and J.S. Jacobs. 1997. Influence o f density on intermediate
wheatgrass and spotted knapweed interference. J. Range Manage. 50:523-29.
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nutrient status in short-grass steppe. Ecol. 76:1116-1133.
55
CHAPTER 3
DESIGNING W EED-RESISTANT PLANT COMMUNITIES B Y MAXIMIZING
NICHE OCCUPATION BY DESIRABLE SPECIES
Introduction
It has been suggested that long-term, sustainable weed management must focus on
establishing desirable species with plant traits and population strategies that promote
continuous resource capture and enhance niche occupation (Larson et al.1994, Chapin et
al. 1992, Chapin et al. 1998, Sheley et al. 1996). Plant communities that lack the structure
(i.e. niche occupation) necessary to capture available resources may be functioning below
their productive capacity and may have an increased risk o f weed invasion (Tilman 1997,
Larson et al. 1994). Furthermore, establishment o f desirable species having contrasting
above- and below-ground growth patterns may minimize interspecific competition,
maximize community structure, and enhance resource capture (Pyke and Archer 1991,
Tilman 1986).
The goal o f this study was to determine the potential to design weed-resistant plant
communities by maximizing niche occupation and resource capture. The overall objective
was to determine the relationship between species diversity/richness and spotted
knapweed establishment in a well-established desirable plant community. I hypothesized
that as species diversity or richness increases, spotted knapweed establishment decreases.
I believed that as species diversity and richness increase, niche occupation increases and
56
resources will be preempted from spotted knapweed. Specific objectives were (I) to
quantify the relationship between species diversity/richness and niche occupation, and (2)
to determine the relationship between species diversity/richness and spotted knapweed
establishment. In order to test the hypothesis that soil water is an important resource
determining spotted knapweed establishment in semiarid environments, the relationship
between species diversity/richness and soil water content was quantified. I also grew
isolated individuals o f three desirable species and spotted knapweed to determine the
potential to use growth analysis information to describe each species’ niche.
Materials and Methods
Study sites
This study was conducted at two field sites in southwest Montana: Red Bluff
Research Ranch and the Arthur Post Research Farm. Red BluffResearch Ranch (45° 34'
N, 111° 40' W; hereafter referred to as Red Bluff) is located 40 km west o f Bozeman, MT.
Elevation at Red Bluff is 1505 m, and mean annual precipitation is 305 mm. The Red
Bluff soil is a Varney clay loam (fine-loamy, mixed, frigid Calcidic Argiustoll). The
Arthur H. Post Research Farm (45° 40' N, 111° 09' W; hereafter referred to as the Post
Farm) is located 6 km w est o f Bozeman, MT. Elevation at the Post Farm is 1463 m, and
mean annual precipitation is 457 mm. The Post Farm soil is an Amsterdam silty loam
(fine-silty, mixed, frigid Typic Haplustoll).
57
Plant materials
The three desirable species used in this study were crested wheatgrass [Agropyron
cristatum (L.) Gaertn., var. Hycrest], intermediate wheatgrass [Elytrigia intermedia
(Host) Nevski, var. Rush], and alfalfa (Medicago sativa L., var. Arrow). Crested
wheatgrass is a shallowly rooted, early emerging, cool season bunchgrass. Intermediate
wheatgrass, a moderately deeply rooted, cool-season, sod-forming grass, emerges later
than crested wheatgrass. Alfalfa is a deeply taprooted broadleaf with an extended growing
period. These three perennials are commonly used in revegetation in the West. They
were chosen because o f their potential to maximize niche occupation when used together
in a revegetation seed mix (Holzworth and Lacey 1991). The weed species chosen for this
experiment was spotted knapweed (Centaurea maculosa Lam.), a deeply taprooted
Eurasian perennial forb. This species was chosen because o f its ecological significance
and wide distribution. Seeds o f the desirable species were obtained from the Bridger Plant
Materials Center, Bridger, MT. Spotted knapweed seeds were collected from Deer Lodge
County, MT, in August 1995.
Competition Experiment
Experimental design. Four background densities (blocks) o f spotted knapweed (500,
1000, 1500, and 3000 seeds/m2) were sown in a multiple replacement series (de Wit 1960;
Figure 6). Each block contained seven 4-m2 plots: three plots with one desirable species
per plot (60 plants/m2), three plots with each combination o f two desirable species (30
plants/m2 per species), and one plot containing all three desirable species (20 plants/m2 per
58
species). Nine plots with monocultures o f the three desirable species at three densities
(20, 30, and 60 plants/m2) accompanied the multiple replacement series. Location o f all
37 plots was completely randomized. Species not planted or sown were hand-weeded
from the plots for the duration o f the experiment. This experiment was repeated at the
Post Farm and Red Bluff.
Planting schedule. Individual plants o f the three desirable species were grown in
plastic “conetainers” (3 cm diam. X 15 cm) in a greenhouse for three months prior to
transplanting in the field. Plants were established between 01JUN95 and 23JUN95 at Red
Bluff and between 19JUN95 and 21JUN95 at the Post Farm. Plants were spaced
uniformly so that each plant was equidistant from nearest neighbors; and in polycultures,
nearest neighbors were nonconspecific. Plots were watered daily for one week following
planting to facilitate establishment. Spotted knapweed was sown into the plots at the end
o f the following growing season. Spotted knapweed seeds were hand-sown to minimize .
aggregated distributions. Sowing occurred on 17NOV96 at Red B luff and on 18NOV96
at the Post Farm.
1997 Sampling. Harvest occurred on 20SEP97 at Red Bluff and on 24SEP97 at the
Post Farm. A 510-cm2 circular frame was placed in the center o f a randomly selected
quadrat in the southern half o f each plot. Stem density o f each species was recorded, and
all aboveground biomass was clipped, separated by species, dried (168 hours, 60° C), and
weighed.
59
' A.
AGCRv3 ELINiz3
I M ESAiz3 CEM A1
AGCR1 ELIN0
MESA0 CEMA1
AGCR0 ELIN1
MESA0 CEMA1
I AGCRiz2 ELIN iz2
AGCRiz2 ELIN0
M ESAiz2 CEMA1
AGCR0 ELINiz2
M ESAiz2 CEMA1
I M ESAiz3 CEMA2
AGCR1 ELIN0
MESA0 CEMA2
AGCR0 ELIN1
MESA0 CEM A2
I AGCRiz2 ELINiz2
I MESA0 CEMA2
AGCR172 ELIN0
M ESAiz2 CEMA2
AGCR0 ELINiz2
M ESAiz2 CEMA2
AGCRiz3 ELINiz3
M ESAiz3 CEMA3
AGCR1 ELIN0
MESA0 CEMA3
AGCR0 ELIN1
MESA0 CEMA3
AGCRiz2 ELIN iz2
MESA0 CEMA3
AGCRiz2 ELIN0
M ESAiz2 CEMA3
AGCR0 ELINiz2
M ESAiz2 CEMA3
I MESA0
CEMA1
AGCRiz3 ELINiz3
AGCRiz3 ELIN iz3
I M ESAiz3 CEMA4
AGCR1 ELIN0
MESA0 CEMA4
AGCRiz2 ELIN iz2
I MESA0 CEMA4
AGCRiz2 ELIN0 ■
M ESAiz2 CEMA4
AGCR0 ELINiz2 .
M ESAiz2 CEMA4
AGCR0 ELINiz3
MESA0 CEMA0
AGCR0 ELIN0
M ESAiz3 CEMA0
AGCRiz2 ELIN0
AGCR0 ELINiz2
MESA0 CEMA0 ■ MESA0 CEMA0
AGCR0 ELIN0
M ESAiz2 CEMA0
AGCR1 ELIN0
MESA0 CEMA0
AGCR0 ELIN0
M ESA1 CEMA0
AGCR0 ELIN1
• MESA0 CEMA4
AGCR0 ELIN0
M ESA1 CEMA1
AGCR0 ELIN0
M ESA1 CEMA2
AGCR0 ELIN0
M ESA1 CEMA3
. AGCR0 ELIN0
-MESA1 CEMA4
B.
AGCRiz3 ELIN0
MESA0 CEMA0
AGCR0 ELIN1
MESA0 CEMA0
Figure 6. Plot plan for multiple replacement series (A) and monocultures (B ). Each cell
represents one mixture; field location o f all plots was completely randomized. AGCR =
crested wheatgrass; ELIN = intermediate wheatgrass; CEMA = spotted knapweed; M ESA
= alfalfa. CEMA0, CEMA1, CEMA2,, CEMA3, and CEMA4 represent background spotted
knapweed sowing densities o f 0, 1250, 2500, 3750, and 7500 seeds/m2, respectively.
Subscripted proportions o f 0, 1/3, 1/2, and I represent planted densities o f desirable
species: 0, 20, 30, and 60 plants/m2, respectively.
I
j
I
60
1998
Sampling. Aboveground volume was used to estimate aboveground biomass
(Bussler et al. 1995). Aboveground volume o f the desirable species was visually
estimated. Volume was calculated as cover X average height. Aboveground biomass o f
the desirable species was sampled from one randomly selected quadrat from plots
containing three desirable species (four plots per site). Aboveground biomass was
clipped, separated by species, dried (168 hours, 60° C), and weighed. D ata from these
volume and biomass sub samples were used in linear regressions (PROC REG, SAS
Institute .Inc. 1991) to estimate the biomass o f each desirable species in each plot from
their respective volume estimates (Table 10). At Red Bluff, all alfalfa aboveground
biomass was clipped, dried, and weighed because o f poor biomass-volume regression, and
its actual weight was used in analysis. All spotted knapweed plants were counted in each
plot. Sampling occurred between 28SEP97 and 140CT97 at Red Bluff and between
19SEP97 and 23SEP97 at the Post Farm.
Table 10. Linear regression o f biomass (kg m'2) on volume (m3 m~2) for 1998 harvest.
Species
Alfalfa
Crested wheatgrass
Intermediate wheatgrass
Site
P
Post Farm
Intercept
R2
Coefficient
(volume)
0.13
0.34
0.098
R ed Bluff
0.29
0.22
■-0.057
204
Post Farm
0.36
0.14
0.50
Ill
Red Bluff
0.003
0.80
0.10
Post Farm
0.003
0.80
0.114
Red Bluff
. 0.066
0.46
0.063
.
82.5
.
5.08
-14.0
43.7 -
61
Soil water monitoring. Soil water content was measured biweekly during the 1997
)
growing season. A calibrated neutron probe (Appendix) was used to measure soil water
content in the center o f each plot from 10 cm to 150 cm below the soil surface in 20-cm
increments.
Data analysis
Niche differentiation. Niche differentiation between desirable species was quantified
using their relative competitive coefficients (Spitters 1983). For each combination o f site
and year, a model using three regressions was used: one for each desirable species
(response) being predicted by all three desirable species. Predictor variables were planted
density or measured density, and response variables were the average shoot weight per
plant or its inverse. The regressions were o f the general form:
Yz=Poi + PffN/ +■■•+ PyNy
where y is the response (average weight o f an individual or its inverse) o f species i, P0,- is
the y-intercept (interpreted as the weight o f an isolated individual), P„N, is the product o f
the coefficient o f intraspecific competition o f sp e c ie s(P „ ) and its density (Nz), and PzzNy-is
the product o f the coefficient o f interspecific competition o f species j on species i (Pzz) and
the density o f species j (Ny). The relative competitive ability o f each species is calculated
as:
RCz=
PzzZPzz and RCz= PzzZp7,
where RCzis the relative competitive ability o f species i a n d /o n species i. For example,
RCzis 1Z3 where I plant o f species i and 3 plants o f species j have equal influence on the
62
average weight per plant o f species i (Spitters .1983). Niche differentiation (ND) is
calculated from the relative competitive abilities o f each species:
ND ^ (PVP6) / (PVPj) - RC, • RCy= (P 8ZPs) ■(p/p,)
Niche.differentiation increases as ND departs from unity; that is, species i and j are
decreasingly limited by the same resources (Spitters 1983).
,
Response and predictor variable combinations within models were uniform, and
models o f different variable combinations were compared. Model preference was given to
that which provided the highest total number o f significant predictors (P<0.05) from the
three regressions.. Using a coefficient o f 0 for nonsignificant predictors yields values o f
infinity or undefined value for competition coefficients and niche differentiation values.
To avoid this, nonsignificant predictors were assigned coefficients o f one order of
magnitude less than the smallest significant coefficient (smallest absolute value) in their
respective models. Nonsignificant predictors retained the sign (+ or -) o f their respective
regression coefficients. Regardless o f sign, a nonsignificant coefficient indicates the
predictor species has no effect on the response species. Therefore, where species
combinations have an ND o f exactly I or -I because nonsignificant predictors have
cancelled, each other out, they should be interpreted as being highly niche-differentiated.
That is, where species have no effect on each other, they occupy different niches.
The .response variable and the sign (+ or -) o f the coefficients are important in
interpreting niche differentiation ratios. Where the response is the inverse o f the average
weight per plant, a positive sign denotes negative interference (e.g. competition.
63
amensalism), and a negative sign denotes positive interference (e.g. mutualism,
commensalism). The opposite is true where the response is average weight per plant.
To estimate average shoot weight per plant for 1998 models, plant densities were
estimated using regressions o f density on shoot weight for each species and site from 1997
(Table 11). This limited 1998 Spitters models’ predictor variables to planted density, as
derived density and average shoot weight per plant (and its inverse) were algebraically
related. An advantage o f using planted density as a predictor is that it includes the effect
o f individuals no longer alive at the time o f sampling. An indirect measure o f niche
occupation was made by regressing total shoot biomass o f desirable species on desirable
species richness and diversity.
Spotted knapweed recruitment. Spotted knapweed recruitment [(spotted knapweed
density/spotted knapweed sowing density) X 100%] was regressed on desirable species
richness and diversity. Desirable species diversity was based on shoot biomass and
f
calculated using the Shannon function (Shannon and Weaver 1949). Differences between
communities based on species richness were compared using one-tailed f-tests (P<0.05).
An indirect measure o f this effect was made by regressing total shoot biomass o f desirable
species on spotted knapweed recruitment (PROC REG, SAS Institute Inc. 1991).
Analysis o f variance was used to test the effect o f desirable species mixture on spotted
knapweed recruitment (PROC GLM, SAS Institute Inc. 1991). Means were compared
(P<0.05) using Fisher’s protected LSD test (Peterson 1985).
64
Table 11. Linear regression o f density (stems m'2) on biomass (g m'2) for 1997 harvest.
Species
Alfalfa
Crested wheatgrass
Site
P
R2
Post Farm
<0.001
0.50
0.586
Red Bluff
<0.001 . 0.72
0.248
171
Post Farm
<0.001
0.64
2.45
250
Red Bluff
<0.05
0.28 .
1.48
621
Post Farm
<0,001
0 66
0.845
Red Bluff
<0.01
0.43
0.861
Coefficient
(biomass)
Intercept
75.4
.
178
Intermediate wheatgrass
331
Soil w ater. Spotted knapweed recruitment was regressed on season-average soil
water content for the depth increments 10 cm - 30 cm and 30 cm - 50 cm. The 10 cm - 30
cm depth increment represents the moisture control sections for both soils (Soil Survey
Staff 1999). Soil water depletion ([beginning soil water content] - [ending soil water
content]) was regressed on desirable species richness and mixture. Analysis o f variance
was used to test for absolute differences among desirable species mixtures on soil water
depletion. M ean separations were made using LSD comparisons (P<0.05).
•
Growth Analysis Experiment
{
Experimental design and procedure. This experiment was conducted at the Arthur
Post Research Farm. Plots were arranged in a randomized-complete-block design. Each
o f four blocks contained 20 (4 species, 5 harvest dates) 6.25-m2 plots. Individual plants o f
the three desirable species were grown in plastic “conetainers” (3 cm diam. X 15 cm) in a
greenhouse for three months prior to transplanting in the field on 29JUN96. Spotted
65
knapweed was sown on 21MAY97. Twenty seeds were placed in the center o f each plot
and lightly covered (< 2 mm) with soil. In the month following sowing, plots with spotted
knapweed were lightly watered daily and progressively thinned to one individual. All plots
were hand-weeded for the duration o f the experiment.
Sampling. All shoot material from individual plants was harvested on 14-day intervals
beginning on 08JUL97. At first harvest, most spotted knapweed plants were early in the
first true-leaf stage o f growth, and plants o f the desirable species all possessed currentyear shoot growth. Final harvest was on 04SEP97. Shoot material was dried (168 hours,
60° C) and weighed. At time o f harvest, soil cores were collected directly below shoot
crowns with a Giddings® (Ft. Collins, CO) soil corer (10 cm diam. x I m). Soil cores
were separated into upper (0 cm - 33 cm), middle (33 cm - 67 cm) and lower (67 cm - 100
cm) depth increments. R oot material was washed and sieved from soil cores, dried (168
hours, 60° C), and weighed.
Data analysis
Linear regression analysis was used to describe increase o f shoot biomass and sample
root mass density over time. Slopes were compared using the extra sums o f squares
procedure (Ratkowski 1983). Analysis o f variance was used to test for absolute
differences among species for shoot biomass and root mass density. M ean separations
were made using LSD comparisons (P<0.05).
66
Results
Competition Experiment
Niche differentiation. Intermediate wheatgrass, crested wheatgrass, and alfalfa were
highly niche-differentiated at the Post Farm in 1997 (Table 12) and 1998 (Table 13).
Niche differentiation ratios among the three desirable species were all at least an order o f
magnitude from unity. At Red Bluff in 1997, intermediate wheatgrass was highly niche
differentiated from crested wheatgrass and alfalfa, while crested wheatgrass and alfalfa
showed little niche differentiation (Table 14). At Red Bluff in 1998, intermediate
wheatgrass and alfalfa were highly niche-differentiated, while crested wheatgrass showed
little niche differentiation from intermediate wheatgrass and alfalfa (Table 15). Total
shoot biomass o f desirable species was positively related to desirable species richness and
diversity at the Post Farm in 1998 (Figures 7 A and 7B).
Spotted knapweed recruitment. Spotted knapweed recruitment was negatively
related to desirable species richness and diversity at the Post Farm in 1997 (Figures 8A
and SB) and 1998 (Figures SC and 8D). Spotted knapweed recruitment was not related to
species richness at Red Bluff in either year.
Analysis o f variance revealed an overall trend o f declining spotted knapweed
recruitment with increasing desirable species richness at the Post Farm. At the Post Farm
in 1997, recruitment was higher in one-species plots than in two- and three-species plots,
but two- and three-species plots were similar (Figure 9A). In 1998, recruitment was
higher in one- and two-species plots than in three-species plots, but one- and two-species
Table 12. Relative competitive abilities and niche differentiation among desirable species for the Post Farm in 1997. The
y-intercept (yint) is interpreted as the inverse of the weight of an isolated individual. Numbers in parentheses = I SE.
Response:
inverse of the average shoot
weight per plant (mg'1)
Predictor:
measured density
(plants m'2)
Coefficient
Intermediate wheatgrass (I)
R2= 0 . 4 9
Yint = 8 . 6 5 ( 1 . 9 1 )
Intermediate wheatgrass
Pu = -0 .0 0 8 8
Relative competitive
ability.
^response predictor
Pintraspecific
pE / p IC =-22.o
P lc = 0 .0 0 0 4
(N S )
Alfalfa
P ia = 0 . 0 0 0 4
(N S )
Intermediate wheatgrass
P ci = 0 .0 0 0 4
(NS)
Alfalfa (A)
no model
Y ia = IU (N S )
Pcc =
Alfalfa
P ca = 0 .0 0 0 4
(N S )
Intermediate wheatgrass
P ai = - 0 .0 0 0 4
(N S )
-0 .0 0 4 7
- 1 1 7
(0 .0 0 2 )
Pcc I
P ca
=
Alfalfa
P ac = - 0 .0 0 0 4
Paa
=
(N S )
Paa Z Pac = I
-0 .0 0 0 4
(N S )
z
P V P cc
Intermediate-Alfalfa
PhZPia / PaiZPaa
-22
- 1 1 7
PaaZPa i = I
Crested wheatgrass
P iiZ P ic
P ia = - 2 2 . 0
Pcc / Pci =
Crested wheatgrass
. Intermediate-Crested
257
P V
Crested wheatgrass (C)
R2 = 0.52
= 8 .7 4 (1.70)
/ Pinterspecifie
(0 .0 0 3 )
Crested wheatgrass
Niche differentiation
Crested-Alfalfa
Pcc/P ca Z PacZPaa
-11.7
Table 13. Relative competitive abilities and niche differentiation among desirable species for the Post Farm in 1998. The
y-intercept (yint) is interpreted as the average weight o f an isolated individual. Numbers in parentheses = I SE.
Response:
average shoot weight per ■
plant (g)
Predictor:
planted density
(plants m"2)
Coefficient
Intermediate wheatgrass (I)
R 2 = 0.80
Intermediate wheatgrass
Pn = 0.0001
Xa=TOlS (0.094)
Crested wheatgrass
Pic = -0.004 (0.002)
Alfalfa
Pia = -0.010 (0.002) .
Crested wheatgrass (C)
R 2 = O Jl
Intermediate wheatgrass
Pc1= -0.0001
Xa = 0.29 (0.024)
Crested wheatgrass
pcc = 0.0001
Alfalfa
Pca = -0.002(0.0005)
Alfalfa (A)
no model
Intermediate wheatgrass
Pai = -0.0001
X* =1.47 (NS)
Crested wheatgrass
pAC= 0.0001 (NS) '
Alfalfa
p ^ = 0.0001 (NS)
Relative competitive
ability
Presponsepredictor
Niche differentiation
Pintraspecific/Pinterspecific
(NS)
P u / Pic = -0.0245
Pn/ Pia = -O-OI
(NS)
Pcc / Pci = "I
(NS)
Pcc / Pca = -0.0658
(NS)
Paa/ Pai = -I^
Paa / Pac = I
Intermediate-Crested
Pn/Pic / Pci/Pcc
0.0245
Intermediate-Alfalfa
Pn/PiA / Pai/P aa
0.01
Crested-Alfalfa
Pcc/P ca / Pac/P aa
-0.0658
Table 14. Relative competitive abilities and niche differentiation among desirable species for Red Bluffin 1997. The
y-intercept (yint) is interpreted as the weight of an isolated individual. Numbers in parentheses = I SE.
Response:
average shoot weight per
plant (mg)
Predictor:
planted density
(plants m"2)
Coefficient
Intermediate wheatgrass (I).
R2 = 0.69
Y^ = S l G ( N S )
Intermediate wheatgrass
Pe = I O
Presponsepredictor
Relative competitive
ability
Niche differentiation
Pintraspecific/ Pinterspecific
(NS)
Pn / Pic = -I
Crested wheatgrass
Pic= -I-O
Alfalfa
P u = 14.3 (3.59)
Intermediate wheatgrass
Pd=TO
(NS)
Intermediate-Crested
Pn^Pic / Pci/Pcc
-I
Pn / PiA = 0.0698
Crested wheatgrass (C)
no model
Ya = 96.5 (NS)
Alfalfa (A)
R2 = 0.40
Ya = 6346 (1668)
(NS)
Pcc / P a = I
Crested wheatgrass
Pcc= TO
(NS)
Alfalfa
Pca= T O
(NS)
Intermediate wheatgrass
PAi = -99.2 (33.4)
Pcc / Pca = I
PAA/ PAI =0.889
Crested wheatgrass
PAc = -75.6 (33,4)
Alfalfa
P ^ = -88.1 (31.6)
Intermediate-Alfalfa
Pn/PiA / Pai/P aa
0.062
Paa/ Pac= 1-17
Crested-Alfalfa
Pcc/P ca / Pac/P aa
1.17
Table 15. Relative competitive abilities and niche differentiation among desirable species for Red Bluff in 1998. The
y-intercept (yint) is interpreted as the inverse of the weight of an isolated individual. Numbers in parentheses = I SE.
Response:
inverse o f the average shoot
weight per plant (g'1)
Predictor:
planted density
(plants m"2)
Intermediate wheatgrass ( I ) . Intermediate wheatgrass
R2 = 0.55
Crested wheatgrass
y^= 1,50 (0.366)
Coefficient
Relative competitive
' ability
Presponsepredictor
Pintraspecific/Pinterspecific
Pn = - 1.91 (0.500)
Pu/Pie =1.093
Pic = -1.74 (0.731)
Alfalfa
Pia = -0.652 (0.240)
Crested wheatgrass (C)
no model
Intermediate wheatgrass
P a = -0.06
y„, = 0.057 (NS)
Crested wheatgrass
Alfalfa (A)
R2 = 0.98
Xa = 0.059 (0:017)
Niche differentiation
Pu/PiA= 2.925
(NS)
Pcc = -0.06 (NS)
Alfalfa
Pca= 0.06
(NS)
Intermediate wheatgrass
Pai = 0.06
(NS)
Pcc / Pci = I
Pcc / PCA= I
Paa/P ai= I
Crested wheatgrass
Pac = -0.06 (NS).
Alfalfa
Paa = 0.06 (NS)
Paa/P ac= I
Intermediate-Crested
Pn/Pic / Pci/Pcc
1.093
Intermediate-Alfalfa
Pn/PiA / Pai/Paa
2.925
Crested-Alfalfa
Pcc/PcA / Pac^Paa
I
71
P <0.01
y = 0.668 + 0.242x
Desirable species richness
y = 0 .9 0 8 + 0.501x
R1 = 0.20
Shannon index
Figure 7. Regressions of total shoot biomass o f desirable species on desirable species
richness (A) and desirable species diversity (B) for the Post Farm in 1998.
A
Post Farm 1997
i?
100
O
Post Farm 1997
E
S
B
o
100
C
S
75
«
3
I
J2
50
I
y = 37.9 - 13.5x
R2 = 0.18
P = 0.03
g
S
75
I
50
3
it
25 -
°
25 -
o
§ ---
I
3
~
&
CO
0
~—
8
?
I
---- “
i
i
I
~~
y = 24.9 - 24. Ix
g
8 ______
—■
6
O
0
2
3
o
0.0
0.4
C
Post Farm 1998
12
I
3
it
0
S
3
O
I
O
6
I
0
i
I
P = 0.06
t—
•2
3
g
°
@
~~-©
8
4
8
I
3
P =0.07
R 2 = 0.12
o
©
CP
O
O
I
i
2
Desirable species richness
D
-
y = 3.38 - 3.26x
3
I
1.2
I
8 -
4
0.8
Shannon index
8
#
I
©o
r
8
R 2 = 0.13
<9>ce>
Post Farm 1998
12
y = 4.98 - 1.60x
O
------------
Desirable species richness
I
I
P = 0.03
R 2 = 0.17
°o
0
0.0
0.2
0.4
O ooo
0.6
°o
0<J
0.8
1.0
Shannon index
Figure 8. Regressions of spotted knapweed recruitment on desirable species richness and desirable species diversity for the Post
Farm in 1997 (A and B) and 1998 (C and D). Note overall reduction in recruitment from 1997 to 1998.
A
P ost Farm 1997
B
Post Farm 1998
I
S
I
I
i
E rror bars = I S E
I
i
Error bars = I SE
*73
I
I
D esirable species richness
R e d B lu ff 1997
Desirable species richness
C
D
Red B lu ff 1998
&
=
I
E rror bars = I SE
2
"S
Error bars = I S E
I
I
"S
"S
I
D esirable species richness
Desirable species richness
Figure 9. Effect of desirable species richness on spotted knapweed recruitment at the Post Farm in 1997 (A) and 1998 (B) and at
Red BlufFin 1997 (C) and 1998 (D).
74
plots were similar (Figure 9B). There were no differences based on species richness at
Red Bluff in 1997 (Figure 9C). At Red Bluff in 1998, recruitment was higher in one- and
two-species plots than in three-species plots, while one- and two-species plots were
similar (Figure 9D).
Analysis o f variance comparisons o f species mixture on spotted knapweed
recruitment detected some differences, but no overall trends. At the Post Farm, crested
wheatgrass monocultures had higher spotted knapweed recruitment than any o f the threespecies mixtures in 1997 (Figure 10A) and higher recruitment than all other mixtures in
1998 (Figure I OB). At Red Bluff, alfalfa monocultures had lower recruitment than
crested wheatgrass monocultures in 1997 (Figure 10C) and lower recruitment than crested
wheatgrass and intermediate wheatgrass monocultures in 1998 (Figure 10D). It appears
that the presence o f alfalfa in any mixture may have had a negative effect on recruitment at
the Post Farm and to a lesser extent at Red Bluff. However, no linear regression with
alfalfa as the sole predictor o f recruitment was significant for either site in either year.
Regressions o f spotted knapweed recruitment on total shoot biomass o f desirable
species were significant (P<0.05) at both sites. However, the relationship was negative at
the Post Farm (Figure 11 A) and positive at Red Bluff (Figure I IB).
Soil w ater. Spotted knapweed recruitment was positively related to season-average
soil water content for the 10 cm - 30 cm depth increment for the Post Farm in 1997,
( P O .03, Figure 12). Spotted knapweed recruitment was not related to season-average
P ost Farm 1997
B
P ost F arm 1998
11
2 10
Error bars = L S D 0 05
E rro r bars = L SD 0 05
9
I
E
8
7
I
•s
AGCR
MESA
I
I
ELIN
MESA
6
AOCR
ELIN
MESA
AGCR
ELIN
MESA
AGCR
5
MESA
4
3
a
AGCR
ELIN
ELIN
CZD
ELIN
2
I
0
AGCR
ELIN
D esirable species m ixture
MESA
MESA
AGCR
ELIN
D esirable species m ixture
R ed B lu ff 1997
D
Red B lu ff 1998
LA
30
£
E rror bars = LSD 0 G
C
40
I
I
E rror bars = L SD 0 05
25
20
II
15
10
73
AGCR
MESA
AGCR
ELIN
AGCR
MESA
D esirable species m ixture
ELIN
MESA
ELIN
MESA
I
5
0
AGCR
ELIN
MESA
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MSSA
D esirable species m ixture
Figure 10. Effect of desirable species mixture on spotted knapweed recruitment at the Post Farm in 1997 (A) and 1998 (B) and
at Red Bluffin 1997 (C) and 1998 (D)
76
Post Farm 1998
y = 6.83 - 4.22x
R2 = 0.23
P = 0.01
Total shoot biomass o f desirable species (kg m"2)
Red B luff 1998
R2 = 0.20
P <0.02
Total shoot biomass o f desirable species (kg m"2)
Figure 11. Regression of spotted knapweed recruitment on total shoot biomass of
desirable species for the Post Farm (A) and Red Bluff (B) in 1998. N ote scales o f xand y-axes.
77
y = -111 + 884x
JO----£.
0.13
0.14
0.15
Soil water content (m 3 m"3)
Figure 12. Regression o f spotted knapweed recruitment on soil water content (season
average for 10 cm - 30 cm soil depth) for the Post Farm in 1997.
soil water content for the 30 cm - 50 cm depth increment at the Post Farm in 1997 or
either depth increment at Red B lulfin 1997.
Soil water depletion showed no consistent trends or differences by species mixture at
the Post Farm (Figures 13 and 14) or Red Bluff (Figures 15 and 16). However, depletion
in crested wheatgrass monocultures at the Post Farm in the 10 cm - 30 cm depth
increment was six to eight times less than that o f the other mixtures, which were similar
(Figure 13). At Red Bluff, total depletion (all depths) in the alfalfa monocultures was
higher than all other mixtures except for the three-species mixtures.
10 c m - 3 0 c m
30 cm - 50 cm
0.09
0.10
0.09
0.08
U
E 0.08
T
E
%
0.07
pIs
J
0.06
J5
'
•S
f
0.04
I
0.03
7B
0.02
(Z)
o.oi
0.06
0.05
D.
"H. 0.05
I
0.07
-O 0.04
6
I
AGCR
ELIN
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
0.03
i CM
CA
0.01
AGCR
ELIN
AGCR
0.00
AGCR
MESA
ELIN
MESA
0.00
Desirable species mixture
AGCR
ELIN
MESA
Desirable species mixture
50 cm - 70 cm
70 cm - 90 cm
OO
0.06
0.05
'5
O. 0.04
I
q . 0.04
I
k
§
0.03
0.03
I
iCA 002
0.01
AGCR
ELIN
AGCR
MESA
Desirable species mixture
ELIN
MESA
AGCR
ELIN
MESA
0.01
ELIN
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
Desirable species mixture
Figure 13. Effect of desirable species mixture on soil water depletion by depth increment at the Post Farm from 02JUN97 to
04SEP97. Error bars = LSD005.
90 cm - 110 cm
HO cm - 130 cm
0.06
£
0.05
^
0.04
§
'€
0.03
p.
•O 0.02
S
I
0.01
•g o.oo
MESA
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
Desirable species mixture
10 cm - 150 cm
<i
VO
Error bars = LSD,0.05
'5
i
0.03
i.,,
rg
AGCR
ELIN
MESA
AGCR
ELIN
AGCR
ELIN
ELIN
MESA
MESA
MESA
Desirable species mixture
Figure 14. Effect of desirable species mixture on soil water depletion by depth increment at the Post Farm from 02JUN97 to
04SEP97. Error bars = LSD005.
10 c m - 3 0 c m
30 cm - 50 cm
0.07
<7
z-s 0.06
0.05
E
E
T=OW
§
% OW
"E 0.04
O
%
o . 0.03
-8
Q.
•§ 0.03
=S 0.02
■5
M 0.01
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
i
"
0.01
AGCR
AGCR
ELIN
ELIN
AGCR
MESA
ELIN
AGCR
ELIN
MESA
MESA
0.00
Desirable species mixture
Desirable species mixture
50 cm - 70 cm
70 cm - 90 cm
00
o
0.05
'E 0 04
"te
g 0.01
I
O. 0.00
*8
fe - 0.01
0.01
AGCR
ELIN
MESA
AGCR
ELIN
AGCR
MESA
Desirable species mixture
ELIN
MESA
AGCR
ELIN
MESA
MESA
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
Desirable species mixture
Figure 15. Effect of desirable species mixture on soil water depletion by depth increment at Red Bluff from 27MAY97 to
07SEP97. Error bars = LSDn
'0 .0 5 '
9 0 c m - 1 10 c m
1 10 c m - 1 3 0 c m
0.03
'T
E
§
'5
§ 0.01
‘5
001
C.
O. -0.00
I
5
0.02
-0.00
•3
jg -0.01
- 0.01
I
=
- 0.02
■"3 "°'02
AGCR
AGCR
ELIN
AGCR
MESA
ELIN
MESA
ELIN
MESA
AGCR
ELIN
ELIN
ELIN
MESA
AGCR
ELIN
MESA
-0.04
-0.04
Desirable species mixture
Desirable species mixture
130 cm - 150 cm
10 cm - 150 cm
Error bars = LSDn
0.03
E
AGCR
MESA
7E 0.03
"E
0.02
%
§ 0.01
1C- -0.00
'■5
F ”
I
2 -o.oi
I
"
- 0.02
rz
-0.03
ELIN
AGCR
ELIN
AGCR
MESA
Desirable species mixture
ELIN
MESA
AGCR
ELIN
MESA
0.01
(g
MESA
AGCR
ELIN
AGCR
MESA
ELIN
MESA
AGCR
ELIN
MESA
Desirable species mixture
Figure 16. Effect of desirable species mixture on soil water depletion by depth increment at Red Bluff from 27MAY97 to
07SEP97. Errorbars = LSD005.
82
Growth Analysis Experiment
Shoot growrth. Only alfalfa and spotted knapweed had a significant regression o f
shoot biomass over time (P < 0.05).. Growth was linear for alfalfa and exponential for
spotted knapweed (Figure 17). At the fifth harvest, shoot biomass o f intermediate
wheatgrass and alfalfa were similar, about three times greater than that o f crested
wheatgrass, and orders o f magnitude greater than spotted knapweed which was in its first
growing season (Figure 18):
R oot mass density. None o f the four species had a significant regression o f root mass
density over time (Figures 19A-D). At the fourth harvest, root mass density o f
intermediate wheatgrass was about six times greater than that o f crested wheatgrass and
alfalfa, and orders o f magnitude greater than spotted knapweed (Figure 20 A). At the fifth
harvest, crested wheatgrass and alfalfa were similar. N o data were available for
intermediate wheatgrass and spotted knapweed at the fifth harvest because o f lack o f
establishment, death, or absence o f the soil corer due to breakage (Figure 20B). Root
mass density by depth increment for each harvest is depicted in Figure 21..
Discussion
Niche differentiation at the Post Farm was greater than at Red B luff in both years.
Perhaps the higher average annual precipitation at the Post Farm allowed the desirable
species to establish more quickly, thus “finding their niches” more quickly than at Red
MESA
y = 0.269 + 0.056x
0.00 08JUL97
22JUL97
05AUG97
19AUG97
04SEP97
08JUL97
22JUL97
Harv est date
1.00
19AUG97
04SEP97
Harvest date
B
AGCR
1.25
05AUG97
A2 = 0.19
CEMA
-
"3
a
0.75
R2 = 0.01
No Model
-0.014 + 0.00223x3
5
I
0.50 -
e2
0.25
°
a
e
.
°
0.00 08JUL97
8
8
19AUG97
04SEP97
R2 = 0.42
I
22JUL97
05AUG97
Harvest date
08JUL97
22JUL97
05AUG97
19AUG97
04SEP97
Harvest date
Figure 17. Regressions of shoot weight over time for intermediate wheatgrass (A), crested wheatgrass (B), alfalfa (C),
and spotted knapweed (D) The variable “x” represents harvest intervals I through 5. Note CEMA units are grams.
84
S p e c ie s n a m e s w ith in
Error bars = I S E
0.800
b a rs = s ig n ific a n tly d iffe re n t
(P < 0.05)
0.600
^
0.400
0.002
0.000
AGCR
MESA
CEMA
Species
Figure 18. ANOVA for growth analysis mean shoot biomass at final harvest. Note break
in y-axis.
Bluff In the drier climate o f Red Bluff establishment may have been slower, thus
delaying below-ground partitioning among species.
Below-ground partitioning at Red Bluff may also have been limited by soil physical
properties. Singh and Sainju (1998) have shown that the presence o f a hardpan impedes
root growth. Soil coring at Red Bluff (for neutron probe access tubes) revealed an
abundance o f rock or hardpan as shallow as I m. Thus, root distributions at Red Bluff
may have been confined to a shallower depth than at the Post Farm whose soil developed
from deep loess and has little coarse fraction and no hardpan.
The positive relationship between desirable species richness/diversity and total shoot
biomass at the Post Farm in 1998 indicates niche differentiation. Increasing species
richness or diversity with species that are not niche-differentiated would not be expected
ELIN
MESA
R1 = 0.04
08JUL97
22JUL97
05AUG97
19AUG97
No model
04SEP97
08JUL97
R1 = 0.08
22JUL97
Harvest date
05AUG97
19AUG97
04SEP97
Harvest date
AGCR
CEMA
OO
Ui
No model
No model
08JUL97
22JUL97
05AUG97
Harvest date
19AUG97
04SEP97
08JUL97
22JUL97
05AUG97
19AUG97
04SEP97
Harvest date
Figure 19. Root mass density (0 cm - 100 cm depth increment) plotted over time for intermediate wheatgrass (A),
crested wheatgrass (B), alfalfa (C), and spotted knapweed (D) None had a significant regression. Note scales of y-
86
A
Fourth Harvest
Error bars = I S E
E
2
S p e c ie s n a m e s w ith in b a rs =
s i g n i f i c a n t l y d i f f e r e n t (P < 0 . 0 5 )
•t
I
I
I
ELIN
AGCR
MESA
CEMA
Species
Fifth Harvest
E
Error bars = I S E
.S1
I
N o t s ig n ific a n tly d iffe re n t
( P < 0 .0 5 )
AGCR
MESA
Species
Figure 20. ANOVA for growth analysis root mass density (0 cm - 100 cm depth
increment) at fourth harvest (A) and fifth harvest (B). Sample size = 0 for ELIN and
CEMA at fifth harvest.
87
Harvest I
Harvest 2
6
.
B
I
-
I
I
D e p th in c r e m e n t
D e p th in c re m e n t
Harvest 3
Harvest 4
S
6
S
I
I
I
I
D e p th in c re m e n t
D e p th in c re m e n t
Harvest 5
04SEP97
E rro r b a rs = I S E
////1
0)
2)
2)
0)
I
ELIN (n =
FttttttI AGCR (n =
MESA (n =
KVWI CEMA (n =
6
I
I
I
0 - 3 3 cm
33 - 67 cm
6 7 - 10 0 c m
D e p th in c re m e n t
Figure 21. Mean root mass density by depth increment and species for harvests I -5 , A E, respectively. Note sample sizes and scales o f y-axes.
88
to increase biomass production because previously unoccupied niche space would remain
unoccupied. Hooper and Vitousek (1998) found that increasing functional group diversity
increased resource use, and Tilman et al. (1997) and Brown (1998) found increasing
functional group diversity increased aboveground biomass. While intermediate wheatgrass
and crested wheatgrass may be considered by some to be in the same functional group (i.e.
C3 perennial bunchgrasses), their niche differentiation indicates a difference in function
(temporal, spatial, or rate o f resource use). The increase in biomass production with
increasing diversity in this study demonstrates how increasing niche occupation, even
within.a particular functional group, may increase resource use and productivity.
The negative effect o f increasing desirable species richness and diversity on spotted
knapweed recruitment at the Post Farm may be related to niche differentiation among the
desirable species, although the mechanism is unclear. Soil water availability may have
played a role in facilitating recruitment as suggested by the positive relationship between
recruitment and soil water content. However, there is no trend between soil water
depletion in the 10 cm - 30 cm portion o f the profile and species mixtures o f increasing
richness (Figure 15). This suggests that soil water in the uppermost 10 cm o f the soil
profile or resources other than water (e.g. light, nutrients) may also play important roles
during germination and establishment o f spotted knapweed! ■
The increase in total shoot biomass o f desirable species with increasing desirable
species richness and diversity at the Post Farm in 1998 (Figures 7A and 7B) was
accompanied by a corresponding decrease in spotted knapweed recruitment (Figures 8C,
8D, and 11 A). Kennett et al. (1992) found that spotted knapweed shoot growth increased
under full sunlight rather than half sunlight. Shoot biomass at the Post Farm in 1998 may
have decreased spotted knapweed recruitment by reducing light levels in the understory.
However, at Red Bluff in 1998, where desirable shoot biomass was half that o f the Post
Farm, spotted knapweed recruitment increased with increasing total shoot biomass (Figure
I IB). At Red Bluff, soil surface temperature may have influenced spotted knapweed
recruitment more than light or water, and spotted knapweed may have benefitted from
microsite^amelioration provided by increased overstory where overstory was sparse
overall.
In this study, spotted knapweed recruitment was lowest where niche occupation
among desirable species was highest. This suggests that diverse, well-established
rangeland plant communities may better resist weed invasion than those that are less
diverse. The results o f this study will help land managers maintain productive, weedresistant plant communities by maximizing niche occupation with desirable species.
90
LITERATURE CITED
Brown, C. S. 1998. Restoration o f California Central Valley grasslands: applied and
theoretical approaches to understanding interactions among prairie species. Ph D.
Dissertation. University o f California, Davis, CA.
Bussler, B H., B.D. Maxwell, and K.J. Puettmann. 1995. Using plant volume to quantify
interference in corn (Zea mays) neighborhoods. Weed Sci. 43:586-594.
Chapin, F.S. III., E D. Schulze, and H A . Mooney. 1992. Biodiversity and ecosystem
process. Trends Ecol. Evol. 7:8-9.
Chapin, F.S., O.E. Sala, TC. Burke, I P. Grime, D.U. Hooper, W.K. Lauenroth, A.
Lombard, H A . Mooney, A R . Mosier, S. Naeem, S.W. Pacala, I. Roy, W.L.
Steffen, and D. Tilman. 1998. Ecosystem consequences o f changing biodiversity.
B ioSci, 48:45-52.
de Wit, C L. 1960. On competition. Versl. Landbouwk. Onderz. 66, 1-82.
Holzworth, L. and I. Lacey. 1991. Species selection criteria for seeding dryland pastures
in Montana. Montana State University Extension Bull. 19. Bozeman, MT.
Hooper, D.U. and P M . Vitousek. 1998. Effects o f plant composition and diversity on
nutrient cycling in serpentine grassland. Ecol. Monogr. 68:121-149.
Kennett, G.A., LR. Lacey, C A. Butt, K.M. Olson-Rutz, and M R . Haferkamp. 1992.
Effect o f defoliation, shading and competition on spotted knapweed and bluebunch
. wheatgrass. I. RangeM anage. 45:363-369.
Larson, L.L., R.L. Sheley, and M L. Mclnnis. 1994. Vegetation management and weed
. invasion. In: Sustaining rangeland ecosystems, USDA-SARE Symposium,
LaGrande, OR.
Peterson, R.G. 1985. Design and analysis o f experiments. Marcel Dekker, Inc. New York.
Pyke, D.A. and S. Archer. 1991. Plant-plant interactions affecting plant establishment and
persistence on revegetated rangeland. I. Range Manage. 44:550-557.
Ratkowski, D.A. 1983. Nonlinear regression modeling: a unified practical approach!
Marcel Dekker, Inc., New York, N.Y. 276 p.
91
SAS Institute Inc. 1991. SAS/STAT U ser’s Guide, Release 6.03 edition. SAS Institute
Inc., Cary, NC. 1028 p.
Shannon, C E. and W. Weaver. 1949. The Mathematical Theory o f Communication.
University o f Illinois Press. Urbana, IL .
(
Sheley, RU.,- T I. Svejcar, and B.D. Maxwell. 1996. A theoretical framework for
developing successiorial weed management strategies on rangeland. Weed
Technol. 10:712-720.
Singh, B.P. and U.M. Sainju. 1998. Soil physical and morphological properties and root
growth. Proceedings o f the Colloquium on Soil Environment and Root Growth,
94th ASHS Annual Conference. Salt Lake City, UT.
Soil Survey Staff. 1999. Soil taxonomy: A basic system o f soil classification for making
and interpreting soil surveys. 2nd edition. U.S. Department o f Agriculture, Natural
Resources Conservation Service. U.S. Department o f Agriculture Handbook 436.
Spitters, CU T. 1983. An alternative approach to the analysis o f mixed cropping
experiments. I Estimation o f competition effects. Netherlands. I. Agric. Sci. 31:111.
Tilman, D. 1986. Resources, competition and the dynamics o f plant communities. In:
Michael Crawley (ed.), Plant Ecology. Blackwell Scientific, Boston, pp. 51-75.
Tilman, D. 1997. Community invasibility, recruitment limitation, and grassland
biodiversity. Ecol. 78(l):81-92.
?
.
Tilman, D., I. Knops, D. Wedin, P. Reich, M. Ritchie andE. Siemann. 1997. The
influence o f functional diversity and composition on ecosystem processes. Sci.
277:1300-1302.
92
CHAPTER 4
REVEGETATING WEED-INFESTED RANGELAND BY M AXIM IZING NICHE
OCCUPATION BY DESIRABLE SPECIES
Introduction
Revegetation o f weed-infested rangeland has often failed because o f poor
establishment o f desirable species, or because weeds soon reinvade. Cost-effective,
sustainable revegetation o f weed-infested rangeland must address both successful
establishment of desirable species and their ability to resist reestablishment o f weed
populations. Past revegetation o f weed-dominated rangeland that has focused on using
broadleaf herbicides to establish grass monocultures has produced variable and
unpredictable results (Hubbard 1975, Berube and Myers 1982, H uston et al. 1984, Larson
and McInnis 1989). The goal o f establishing persistent, biologically diverse plant
communities has often been sacrificed to accommodate the short-term goals of plant
establishment and soil stabilization (Call and Roundy 1991). Existing revegetation
technology often fails to address spatial and temporal diversity within the community.
During the establishment phase o f revegetation, the relative timing o f emergence and
growth rates o f species may be an important determinant o f community dynamics (Ross
and Harper 1972, Harper 1977, Radosevich and Holt 1984). Rehabilitation o f yellow
starthistle {Centaurea solstitialis L.)-dominated rangeland has been attempted by
revegetating with desirable grasses. These attempts typically fail because o f resource
93
preemption by annual weeds which germinate sooner and have a higher initial growth rate
than the desirable grasses (Borman et al. 1991). In an experiment by Harris (1967), the
winter annual, cheatgrass (Bromus tectorum L.), preempted resources from fall-sown
bluebunch wheatgrass [Agropyron spicatum (Pursh) Scribn. & Smith], Both species
germinated in the fall, but cheatgrass had faster winter root growth than did bluebunch
wheatgrass, thus allowing it to gain control o f the site. Borman et al. (1991) found that
established perennial grasses that initiate growth early and maintain some growth through
the winter can limit reinvasion by weeds.
During the early stages o f secondary succession (e.g. revegetation), availability o f
light, water, and nutrients may be particularly high relative to the demand by a sparse
community o f establishing plants. In this case, resource preemption may be more
important than competition in determining community dynamics (Goldberg 1990). Those
plants with the highest growth rates have the highest likelihood o f establishing to the
exclusion o f plants with slower growth rates (Harper 1977). Since establishment is the
most critical phase o f revegetation, knowledge o f the initial growth rate o f a species may
be the best predictor o f the short-term outcome o f a revegetation effort (James 1992).
The goal o f this study was to determine the potential to revegetate spotted knapweedinfested rangeland by maximizing niche occupation by desirable species that establish well
and persist. The overall objective was to determine the relationship among three desirable
species and spotted knapweed during the first two years o f establishment. I hypothesized
that as desirable species richness increases, spotted knapweed establishment and growth
decrease, as long as the desirable species differ in niche. I believed that as desirable
94
species richness increased, niche occupation would increase and resources would be
preempted from spotted knapweed. Specific objectives were to: (I) quantify niche
differentiation among three desirable species commonly used in revegetation, and (2)
determine the relationship between desirable species richness and spotted knapweed
establishment. To test the hypothesis that soil water influences spotted knapweed growth,
the relationship between soil water1content and spotted knapweed aboveground biomass
production was quantified. Observed demographic data from this experiment were
incorporated into a predictive model to identify key mechanisms and processes important
in establishment and sustainability o f revegetated plant communities. I also used growth
analysis o f three desirable species and spotted knapweed to determine the potential to
describe each species’ niche.
Materials and Methods
Study sites
This study was conducted at two field sites in southwest Montana: Red Bluff
Research Ranch and the Arthur Post Research Farm. Red BluffResearch Ranch (45° 34'
N, 111° 40' W; hereafter referred to as Red Bluff) is located 40 km w est o f Bozeman, MT.
Elevation at Red Bluff is 1505 m, and mean annual precipitation is 3 05 mm. The Red
Bluff soil is a Vamey clay loam (fine-loamy, mixed, frigid Calcidic Argiustoll). The
Arthur H. Post Research Farm (45° 40' N, 111° 09' W; hereafter referred to as the Post
Farm) is located 6 km west o f Bozeman, MT. Elevation at the Post Farm is 1463 m, and
95
mean annual precipitation is 457 mm. The Post Farm soil is an Amsterdam siltyjoam.
(fine-silty, mixed, frigid Typic Haplustoll).
Plant materials
The three desirable species used in this study were crested wheatgrass \Agropyron
cristatum (L.) Gaertn., var. Hycrest], intermediate wheatgrass [Elytrigia intermedia
(Host) Nevski, var. Rush], and alfalfa Qsdedicago sativa L., var. Arrow). Crested
wheatgrass is a shallowly rooted, early emerging, cool-season bunchgrass. Intermediate
wheatgrass, a moderately deeply rooted, cool season, sod-forming grass, emerges later
\
than crested wheatgrass. Alfalfa is a deeply taprooted broadleaf with an extended growing
period. These three perennials are commonly used in revegetation in the West because
they establish well, persist, and have the potential to maximize niche occupation when
used together in a revegetation seed mix (Holzworth and Lacey 1991). The weed species
chosen for this experiment was spotted knapweed (Centaurea maculosa Lam.), a deeply
taprooted Eurasian perennial forb. This species was chosen because o f its ecological
significance and wide distribution. Spotted knapweed seeds were collected from Deer
Lodge County, MT, in August 1995. Seeds o f the desirable species were obtained from
the Bridger Plant Materials Center, Bridger, MT.
Competition Experiment
Experimental design. Four background densities (blocks) o f spotted knapweed
(1250, 2500, 3750, and 7500 seeds/m2) were used in a multiple replacement series (de Wit
1960) (Figure 22). Each block contained seven 4-m2 plots: three plots with one desirable .
96
A.
I AGCRiz3 ELINiz3
I M ESAiz3 CEMA1
AGCR1 ELIN0
MESA0 CEMA1
AGCR0 ELIN1
MESA0 CEMA1
AGCRiz2 ELINiz2
I MESA0 CEMA1
AGCRiz2 ELIN0
M ESAiz2 CEMA1
AGCR0 ELINiz2
M ESAiz2 CEMA1
AGCRiz3 ELINiz3 .
I M ESAiz3 CEMA2
AGCR1 ELIN0
MESA0 CEMA2
AGCR0 ELIN1
AGCR0 ELIN0
MESA0 CEMA2
M ESA1 CEMA2
AGCRiz2 ELIN iz2
MESA0 CEMA2
' AGCRiz3 ELIN iz3
M ESAiz3 CEMA3
AGCR0 ELIN0
M ESA1 CEMA1
I
AGCRiz2 ELIN0
AGCR0 ELINiz2
M ESAiz2 CEMA2 • M ESAiz2 CEMA2
AGCR1 ELIN0
MESA0' CEMA3
AGCR0 ELIN1
MESA0 CEMA3
AGCR172 ELfNiz2
MESA0 CEMA3 '
AGCRiz2 ELIN0
M ESAiz2 CEMA3
AGCR0 ELIN172
M ESAiz2 CEMA3
AGCRiz3 ELINiz3
M ESAiz3 CEMA4
AGCR1 ELIN0
MESA0 CEMA4
AGCR0 ELIN1
MESA0 CEMA4 .
AGCR172 ELINiz2
MESA0 ' CEMA4
AGCRiz2 ELIN0
M ESAiz2 CEMA4
AGCR0 ELINiz2
M ESAiz2 CEMA4
AGCRiz3 ELIN0
MESA0 CEMA0
AGCR0 ELINiz3
MESA0 CEMA0
AGCR0 ELIN0
M ESA173 CEMA0
AGCRiz2 ELIN0
MESA0 CEMA0
AGCR0 ELINiz2
MESA0 CEMA0
AGCR0 ELIN0
M ESA172 CEMA0
AGCR1 ELIN0
AGCR0 ELIN1
MESA0 CEMA0
AGCR0 ELIN0
M ESA1 CEMA3
AGCR0 ELIN0
M ESA1 CEMA4
I
B.
MESA0 CEMA0
'
AGCR0 ELIN0
M ESA1 CEMA0
Figure 22. Plot plan for multiple replacement series (A) and monocultures (B). Each cell
represents one mixture; field location o f all plots was completely randomized. AGCR =
crested wheatgrass; ELESl = intermediate wheatgrass; CEMA = spotted knapweed; M ESA
- alfalfa. CEMA0, CEMA1, CEMA2, CEMA3, and CEMA4 represent background spotted
knapweed sowing densities o f 0, 1250, 2500, 3750, and 7500 seeds/m2, respectively.
Subscripted proportions o f 0, 1/3, 1/2, and I represent sowing densities o f desirable
species: 0, 1500, 2250, and 4500 seeds/m2, respectively.
97
species per plot (4500 seeds/m2), three plots with each combination o f two desirable
species (2250 seeds/m2 per species), and one plot containing all three desirable species
(1500 seeds/m2 per species). Nine plots with monocultures o f the three desirable species
at one o f three densities (1500, 2250, and 4500 seeds/m2) accompanied the multiple
replacement series. Location o f all.37 plots was randomized.
All plots were tilled to a depth o f 10 cm within two weeks prior to sowing to remove
existing vegetation and prepare the seedbed. Sowihg occurred on 24JUN96 and 25JUN96
at Red Bluff and on 28JUN96 and 29JUN96 at the Post Farm. Seeds were hand-sown to
minimize aggregated distributions. Plots were then covered with nylon-mesh backed
straw-mulch (North American Green®, Billings, MT). To facilitate germination and
establishment, plots were watered daily from 29JUN96 to 29JUL96. On 15JUL96, when
plants averaged approximately 5 cm tall, the mulch was removed from all plots.
Nonseeded species were hand-weeded for the duration o f the experiment.
1996 Sampling. Harvest at the Post Farm took place from 310C T 96 to 02NOV96.
A 5 10-cm2 circular frame was placed in the center o f a randomly selected quadrat in the
southern half o f each plot. Frame placement was limited to the southern half o f each plot
to avoid sampling the path to the neutron probe access tube o f each plot. Density o f each
species was counted,, and all aboveground biomass was clipped, separated by species,
dried (168 hours, 60° C) and weighed. Although Red Bluff was fenced, aboveground
biomass at Red Bluff was completely consumed by herbivores (grasshoppers, deer,
antelope, rabbits) and was not sampled.
98
1997 Sampling. Both sites were sampled in 1997. At Red Bluff, a 510-cm2 circular
frame was placed in the center o f a randomly selected quadrat in the southern half o f each
plot. At the Post Farm, the frame was placed in the southern quadrat not sampled in
1996. Sampling at both sites took place between 11SEP97 and 22SEP97. Density
(stems/m2) o f each species was recorded, and all aboveground biomass was clipped,
separated by species, dried (168 hours, 60° C) and weighed.
Soil water monitoring. Soil water content was measured weekly during the 1996
growing season and biweekly during the 1997 growing season. A calibrated neutron
probe (Appendix) was used to measure soil water content ii%the center o f each plot from
10 cm to 150 cm below the soil surface in 20-cm increments.
Data analysis ■
Niche differentiation. Niche differentiation between desirable species was quantified
using their relative competitive coefficients (Spitters 1983). For each combination o f site
and year, a model using three regressions was used: one for each desirable species
I
(response) being predicted by all three desirable species. Predictor variables were sown
density or measured density, and response variables were the average shoot weight per
plant or its inverse. The regressions were o f the general form:
Y/= Po, + P,M +...+ PyN7
where y,- is the response (average weight o f an individual or its inverse) o f species z, P0,. is
the y-intercept (interpreted as the weight o f an isolated individual), P„N, is the product o f
■
the coefficient o f intraspecific competition o f species i (p„) and its density (N,), and P^S(. is
99
the product o f the coefficient o f interspecific competition o f species j on species i (P6) and
the density o f species j (N7). The relative competitive ability o f each species is calculated
as:
RC,. - p,/p6 and RC7 =
where RC, is the relative competitive ability o f species / and j on species i. For example,
RC, is 1/3 where I plant o f species i and 3 plants o f species j have equal influence on the
average weight per plant o f species i (Spitters 1983). Niche differentiation (ND) is
calculated from the relative competitive abilities o f each species:
ND = (P„/P,) / (PyZPj) = RC, • RC, = (P,/P„) • (P/P„)
. Niche differentiation increases as ND departs from unity; that is, species i and j are
decreasingly limited by the same resources; (Spitters 1983).
. Response and predictor variable combinations within models were uniform, and
models o f different variable combinations were compared. Model preference was given to
that which provided the highest total number o f significant predictors (P < 0.05) from the
three regressions. Using a coefficient o f 0 for nonsignificant predictors yields values o f
infinity or undefined value for competition coefficients and niche differentiation values.
To avoid this, nonsignificant predictors were assigned coefficients o f one order of
magnitude less than the smallest significant coefficient (smallest absolute value) in their
respective models! Nonsignificant predictors retained the sign (+ or -) o f their respective
regression coefficients. Regardless o f sign, a nonsignificant coefficient indicates the
predictor species has no effect on the response species. Therefore, where species
combinations have an ND o f exactly I or -I because nonsignificant predictors cancel each
i
100
other out, they should be interpreted as being highly niche-differentiated. That is, where
species have no effect on each other, they occupy different niches.
The response variable and the sign (+ or -) o f the coefficients are important in
interpreting niche differentiation ratios. Where the response is the inverse o f the average
weight per plant, a positive sign denotes negative interference (e.g., competition,
amensalism), and a negative sign denotes positive interference (e.g., mutualism,
commensalism). The opposite is true where the response is average weight per plant.
Spotted knapweed recruitment. The effects o f desirable species richness on spotted
knapweed recruitment [(spotted knapweed density/spotted knapweed sowing density) x
100%] were compared using one-tailed f-tests (P < 0.05). Analysis o f variance was used
to test the effect o f desirable species mixture on spotted knapweed recruitment (PROC
GLM, SAS Institute Inc. 1991). Means were compared (P < 0.05) using Fisher’s
protected LSD test (Peterson 1985).
Spotted knapweed biomass and density. Linear regression was used to quantify the
relationship between desirable species richness and spotted knapweed biomass and
density. ANOVA was used to compare mean aboveground biomass o f spotted knapweed
to mean aboveground biomass o f the desirable species.
Soil w ater. Soil water content (average content o f all depths, averaged over the
entire season) was used to predict spotted knapweed growth using linear regression.
Total aboveground biomass was used to predict season-long soil w ater depletion
101
([beginning soil water content] - [ending soil water content]). ANOVA was used to
compare the effects o f desirable species mixture and richness on soil w ater depletion
(average o f all depths, averaged over the entire season).
Community dynamics model. The population dynamics model developed by Maxwell
and Sheley (1997) was adapted to accommodate the demographic data and competitive
coefficients from the observed field data. The model is based strictly on intra- and
interspecific interference, using competition coefficients derived from the Spitters equation
to predict the average weight per plant o f a given species. Total seed production per unit
area for a given species was defined as:
Average weight per plant X Seeds produced per plant X M ature plant density
Seed production was a linear function o f the average weight per plant (Table 16) and was
defined as:
Seeds produced / (gram o f average weigh, , plan,) End-of-season biomass per unit area for a given species was defined as:
Biomass = Density X Average weight
A 10% sensitivity analysis (Maxwell et al. 1988) is calculated on the model to identify
the relative importance o f the demographic and competitive influences. The effect o f a
10% reduction in a parameter value on the output is defined as:
Sensitivity value
A output
original output value
/
A parameter value
original parameter value
102
Table 16. D ata and sources for calculation o f seeds produced per plant.
Species
Maximum average
plant weight (g)
(observed,
Post Farm 1997)
Intermediate
wheatgrass
1.95
428
(Serin 1989)
219
Crested
wheatgrass
1.17
648
(Serin 1989)
556
Alfalfa
3.1
406 (Alfalfa Seed Survey 1992)
Spotted
knapweed
16.0
Maximum number o f seeds
produced per plant (source)
62
(Jacobs and Sheley 1998)
Seeds produced
per gram o f
average plant
weight (calculated)
131
4
The model was validated by comparing the predicted and observed (field data) values
using a two-tailed Mest for paired data (Box et al. 1978). The f -value represents the
probability that the predicted and observed populations are not significantly different.
Growth Analysis Experiment
Experimental design and procedure. This experiment was conducted at the Post
Farm. Plots were arranged in a randomized-complete-block design. Each o f four blocks
contained 20 (4 species, 5 harvest dates) 0.25-m2 plots. On 29JUN96, 20 seeds were
sown in the center o f each plot and lightly covered (< 2 mm) with soil. Plots were then
covered with nylon-mesh-backed straw-mulch (North American Green®). To ensure
germination and establishment, plots were watered daily from 29JUN96 to 29JUL96. On
15JUL96, when plants averaged approximately 5 cm tall, the mulch was removed from all
103
plots. Each plot was thinned to five individuals on 16JUL96. Plots were thinned to one
individual on 28JUL96. Plots were hand-weeded for the duration o f the experiment.
Sampling. All shoot material from individual plants was harvested on 14-day intervals
beginning on 16JUN97. Final harvest was on 18AUG97. Shoot material was dried (168
hours, 60° C) and weighed. At time o f harvest, soil cores were taken directly below shoot
crowns with a Giddings® (Ft. Collins, CO) soil corer (10 cm diam. x I m). Soil cores
were separated into upper (0 cm - 33 cm), middle (33 cm - 67 cm) and lower (67 cm - 100
cm) depth increments. R oot material was washed and sieved from soil cores, dried (168
hours, 60° C) and weighed.
D ata analysis. Linear regression analysis was used to describe increase o f shoot
biomass and sample root mass density over time (PROC REG, SAS Institute Inc. 1991).
Slopes were compared using the extra sums o f squares procedure (Ratkowski 1983).
Analysis o f variance (PROC GEM, SAS Institute Inc. 1991) was used to test for absolute
differences among species for shoot weight and root mass density. M ean separations were
made using LSD comparisons (P < 0.05).
Results
Competition Experiment
Niche differentiation. Niche differentiation values were near unity for all two-way
•
/
comparisons at the Post Farm in 1996 and Red Bluflfin 1997, indicating little niche
'
104
differentiation (Tables 17 and 18). At the Post Farm in 1997 the three desirable species
were highly niche-differentiated (Table 19).
Spotted knapweed recruitment. ANOVA was used to test for differences in spotted
knapweed recruitment by desirable species mixture and richness. Spotted knapweed .
recruitment was not affected by desirable species mixture or richness at the Post Farm in
1996 or at either site in 1997 (P < 0.05, Figures 23 and 24).
Spotted knapweed biomass and density. Total desirable plant sowing density was the
same in all plots containing spotted knapweed. Because spotted knapweed sowing density
varied, relationships among desirable species richness and spotted knapweed were
confounded by spotted knapweed sowing density. Regressions o f spotted knapweed
measured density on its sowing density were highly significant (P < 0.001) for the Post
Farm in 1996 and Red B luff in 1997; however, this relationship was not significant for the
Post Farm in 1997 (Table 20). As a result, relationships between desirable species
richness and spotted knapweed biomass and density were not confounded by spotted
knapweed sowing density. This allowed for the quantification o f the relationship between
desirable species richness and spotted knapweed biomass and density at the Post Farm in
1997. Neither relationship was significant (Table 21). Mean biomass o f spotted
knapweed was significantly greater than the mean biomass o f the desirable species, which
were similar (Figure 25). Biomass o f desirable species monocultures grown in the absence
o f spotted knapweed was similar to spotted knapweed biomass and total biomass (all
Table 17. Relative competitive abilities and niche differentiation among desirable species for the Post Farm in 1996. The
y-intercept (Yniay) is interpreted as the weight of an isolated individual. Numbers in parentheses = I SE.
.
Response:
average shoot weight per
plant (mg)
Predictor:
sown density
(seeds m'2)
Coefficient
Intermediate wheatgrass (I)
R2 = 0.83
Ym=- 4 0 7 (39.1)
Intermediate wheatgrass
Pn= -0.077
(0.01)
Crested wheatgrass
Plc= -0.070
(0.01)
Crested wheatgrass (C)
R2 = 0.68
Ymax= 342 (51.9)
Alfalfa (A)
R2 = 0.50
y_ = 280 (55.0)
Relative competitive
ability
Presponsepredictor
Niche differentiation
Pintraspecific/ Pinterspecific
Alfalfa
Pia = -0.078 (0.01)
Intermediate wheatgrass
Pci = -0.069 (0.014)
Pa/ Pic= I 09
Pa/Pia = 0.992
Pcc / Pci = 0.941
Crested wheatgrass
Pcc = -0.065
Alfalfa
Pca = -0.069 (0.014)
Intermediate wheatgrass
Pai = -O1OSI
Crested wheatgrass
Pac = -0.015 (0.015)
Alfalfa
Paa= -0.051
(0.013)
(0.015)
(0.014)
Pcc/PcA = 0.977
PAA/PAI=0.998
PAA/ PAC= 1.27
Intermediate-Crested
Pa/Pic / Pci/Pcc
1.03
Intermediate-Alfalfa
Pa/Pu / Pai/PAA
0.99
Crested-Alfalfa
Pcc/P ca / Pac/PAA
1.24
Table 18. Relative competitive abilities and niche differentiation among desirable species for the Post Farm in 1997. The
y-intercept (Yniay) is interpreted as the inverse of the weight of an isolated individual. Numbers in parentheses = I 'SE.
Response:
inverse o f the average shoot
weight per plant (mg'1)
Predictor:
measured density
(plants m"2)
Coefficient
Intermediate wheatgrass (I)
R2 = 0.58
Intermediate wheatgrass
Pu = -0.003 (NS)
y _ = 12.6 (2.97)
Crested wheatgrass
Pic = -0.003
-Alfalfa
Pia= 0.033 (0.011)
Crested wheatgrass (C)
R2 = 0.53
Intermediate wheatgrass
pCI = -0.003
y _ = 32.6 (NS)
Crested wheatgrass
Pcc = -0.003 (NS)
Alfalfa -
PcA = 0.62 (0.22)
Intermediate wheatgrass
Pa i= 0.003
Presponsepredictor
Niche differentiation
Pintraspecific/ Pinterspecific
Pn / Pic = I
(NS)
Pn / Pia = -0.091
(NS)
Pcc / Pci = I
Pcc/PcA = -0.005
(NS)
Crested wheatgrass.
PAc = 0.003 (NS)
Alfalfa
P^ = -0.003 (NS)
Intermediate-Crested
Pn/Pic / Pci/Pcc
I
106
Alfalfa (A)
no model
Ymax= 18.6 (NS)
.
Relative competitive
ability
Paa / Pai = -I
Paa / Pac = -I
Intermediate-Alfalfa
Pn/PiA / Pai/P aa
0.091
Crested-Alfalfa
Pcc/P ca / Pac/PAA
0.005
Table 19. Relative competitive abilities and niche differentiation among desirable species for Red Bluff in 1997. The
y-intercept Cytnay) is interpreted as the weight of an isolated individual. Numbers in parentheses = I SE.______
Response:
average shoot weight per
plant (mg)
Predictor:
sown density
(seeds m"2)
Coefficient
Intermediate wheatgrass (I)
R2 = 0.74
Ymm= 1729 (254)
Intermediate wheatgrass
Pu= -0.366
Relative competitive
ability
Presponsepredictor
Pintraspecific/ Pinteispecific
(0.064)
Plc= -0.391
0.897 .
Alfalfa
Pia = -0.383 (0.068)
Intermediate wheatgrass
Pcl = -0.310
(0.014)
Crested wheatgrass
Pcc = -0.297
.(0.013)
Alfalfa
PCA= -0.312 (0.014)
Intermediate wheatgrass
Pai = -0.03 (NS)
Pcc / Pci = 0.958
Pcc / Pca = 0.952
Paa/ P ai= I
Crested wheatgrass
PAC = -0-03 (NS)
Alfalfa
PAA = -0.03 (NS)
-
107
Alfalfa (A)
no model
ymax = 608 (NS)
PVPic / Pcj/Pcc
(0.068)
Pa / PiA = 0.956
Crested wheatgrass (C)
R2 = 0.98
.% _ = 1 4 1 7 (52.4)
Intermediate-Crested
P u / Pic = 0.936
Crested wheatgrass
Niche differentiation .
Paa / Pac = I
Intermediate-Alfalfa
PVPia / Pai/P aa
0.956
Crested-Alfalfa
Pcc/Pca /
Pac/P aa
0.952
108
Post Farm 1996
120
£ no
I
I
I
I
100
Error bars = LSD 0 05
90
80
70
60
50
40
30
20
10
0
AGCR
ELIN
AGCR
MESA
ELlN
MESA
AGCR
ELIN
MESA
Desirable species mixture
Post Farm 1996
£
I£
Error bars = I S E
I
"O
I
CO
Desirable species richness
Figure 23. Effect of desirable species mixture (A) and desirable species richness (B) on
spotted knapweed recruitment at the Post Farm in 1996.
Post Farm 1997
Post Farm 1997
Error bars = LSDn
&
Error bars = I SE
II
OQ
AGCR
I MESA I
AGCR
ELIN
AGCR
ELIN
MESA
MESA
AGCR
ELIN
MESA
Desirable species richness
Desirable species mixture
Red Bluflfl 997
Red Bluff 1997
&
Error bars = LSD0 05
I
90
I
g
I
I
5
S
S
•o
S
I
Error bars = I S E
I
80
70
I
60
50
40
30
20
10
109
100
AGCR
AGCR
MESA
AGCR
ELIN
AGCR
MESA
0
Desirable species mixture
ELlN
MESA
8.
ELIN
MESA
Desirable species richness
Figure 24. Effect of desirable species mixture and desirable species richness on spotted knapweed recruitment in 1997 at the Post
Farm (A and B), and Red Bluff (C and D).
no
Table 20. Linear regression o f spotted knapweed measured density on its sowing density.
R 2M o d e l
Year
Site
1996
Post Farm
0.0001
0.45
1997
Post Farm
0.0001
0.44
1997
Post Farm
0.63
0.01
^
M odel
Table 21. Linear regression o f spotted knapweed biomass and density on desirable species
richness for the Post Farm in 1997.
Independent variable
Dependent variable
Desirable species richness
^
M odel
R2M o d el
^
Spotted knapweed biomass
0.76
0.004
Spotted knapweed density
0.31
0.04
E rro r b ars = I S E
I
m m
O
I P o s t F a rm 1996
R e d B lu f f 1997
P o s t F a rm 1997
1.0
E L IN
AGCR
M ESA
CEM A
Figure 25. Mean shoot biomass by site and year. For each desirable species, data
represent the 12 plots (regardless of desirable species richness) where sown with spotted
knapweed (mean sowing density = 2750 seeds m"2). Data for spotted knapweed
represent all 28 plots where it was sown (mean sowing density = 3750 seeds m"2). At the
Post Farm in 1997, biomass of spotted knapweed was greater than each o f the desirable
species, which were similar.
Ill
species) o f mixtures at the Post Farm in 1997 (Figure 26) . There was no significant
relationship between spotted knapweed biomass and desirable species biomass in those
plots where spotted knapweed was sown (Figure 27).
Soil w ater. For reasons stated above, relationships between spotted knapweed
biomass and total plot biomass were not confounded by spotted knapweed sowing density
at the Post Farm in 1997. This allowed for the quantification o f the following two
relationships: (I) between spotted knapweed biomass production and soil water content,
and (2) between total biomass production by all species and soil w ater depletion. Neither
relationship was significant (Table 22). Neither desirable species mixture nor richness
affected soil water depletion at the Post Farm in 1997 (Figure 28).
Community Dynamics M odel. At the Post Farm in 1996 and at Red Bluff in 1997, no
plants reached the reproductive stage, and by the end o f the 1997 growing season at the
Post Farm, all plants were reproductive. As a result, seedling-to-adult transition data were
not available from Red Bluff, and the model was parameterized using Post Farm data only
(Figure 29). Years I and 2 represent Post Farm 1996 and 1997 data, respectively. Years
3 to. 10 are based on a combination o f Post Farm 1996 and 1997 data. The model’s
demographics represented the following field observations: (I) all Y ear-1 plants are
seedlings at the end o f the growing season, and (2) all Year-2 plants reach maturity. For
Years 3 to 10, all plants emerging in a given year reach maturity, whether they emerged
from the seed bank (from Post Farm 1996 data) or from regrowth from previous year’s
112
Error bars = I SE
ELIN
AGCR
MESA
CEMA
TOTAL
Species
Figure 26. Mean biomass o f monocultures o f intermediate wheatgrass (ELIN), crested
wheatgrass (AGCR), and alfalfa (MESA) in the absence o f spotted knapweed; mean
biomass o f spotted knapweed (CEMA) in mixtures; and mean total biomass o f all species
in mixtures (TOTAL) for the Post Farm in 1997. Mean sowing density o f desirable
species monocultures was 2750 seeds m"2. Mean sowing density o f spotted knapweed in
mixtures was 3750 seeds m"2. There were no significant differences among biomass
means. The difference between CEMA and TOTAL represents desirable species biomass.
4
r
Desirable species richness
o I
rC'E
S3
IE
J 2
I
2
■
3
O
o
D
D
D
I
'o
°D°o
5-o I
I
■0
° °D
CL
"
a
°.
O
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Desirable species biomass (kg m-2)
Figure 27. Regression o f spotted knapweed biomass over total desirable species biomass
for the Post Farm in 1997. The regression model was not significant.
113
Table 22. Linear regressions o f spotted knapweed biomass production on season-average
soil water content and season-long soil water depletion on total biomass production by all
species for the Post Farm in 1997.
R2M o d e l
Independent variable
Dependent variable
Soil water content
Spotted knapweed biomass
0.36
0.03
Total biomass production
Soil water depletion
0.51
0.01
M odel
plants (from Post Farm 1997 data). A ceiling on maximum seedling and mature plant
densities was imposed with values representing maximum observed seedling (Post Farm
1996) and mature plant (Post Farm 1997) densities from the field data.
Two-tailed Mests for paired data showed that for all species in Years I and 2,
biomass predictions were significantly different from the field data (Tables 23-25).
Because the predictions did not represent the field data, sensitivity analysis was not
conducted on the model. The lack o f agreement between the model and the field data may
have been due to the fact that o f the eight Spitters models only five were significant (P <
0.15), and those that were significant had an average R2 o f 0.27 (Table 26).
Growth Analysis Experiment
Shoot growth. Each species had a significant regression o f shoot biomass over time
(P < 0.05). All four species were significantly different from each other with spotted
knapweed having the fastest shoot growth and crested wheatgrass the slowest (Figure 30).
Shoot biomass at final harvest o f spotted knapweed and alfalfa were similar and
approximately four times greater than that o f crested wheatgrass and intermediate
wheatgrass which were similar (Figure 31).
114
B
Error bars = I SE
E
"E
I
S
I
I
2
3
Desirable species richness
Figure 28. Effects o f desirable species mixture (A) and desirable species richness (B) on
season-long soil water depletion (10 cm - 150 cm soil depth average) for the Post Farm
in 1997. Neither effect was significant.
115
Vcar 34-
Figure 29. Diagrammatic representation o f community dynamics model.
Root mass density. Only alfalfa had a significant regression o f root mass density over
time (Figure 32). Root mass densities o f alfalfa at the fourth and fifth harvests were about
four times greater than those o f crested wheatgrass, intermediate wheatgrass, and spotted
knapweed (fifth harvest only) which were similar (Figure 33). No data were available for
spotted knapweed at the fourth harvest because o f lack o f establishment, death, or
mechanical failure o f the soil corer. Root mass density by depth increment for each
harvest is depicted in Figure 34.
116
Table 23. Post Farm 1996 biomass (g m"2) by species and sowing density (seeds m"2).
Observed (obs) is from field data; predicted (pred) is from community dynamics model.
Intermediate wheatgrass
Crested wheatgrass
sown
obs
pred
sown
4500
97.5
78
0
4500
30.4
104
4500
128.3
4500
Alfalfa
Spotted knapweed
obs
pred
0
7500 142.1
155
0
0
3750
2.6
183
113
0
0
2500 155.5
146
187.9
122
0
0
1250
16.2
85
2250
395
60
2250
29.4
65
0
7500 164.8
155
2250
24 9
73
2250
35.1
65
0
3750
65 5
183
2250
76.4
77
2250
57.2
65
0
2500
3
146
2250
121.5
81
2250
75.6
65
0
1250 35.7
85
2250
42.2
39
0
2250
41.6
32
7500
94.9
155
2250
26 8
52
0
2250
39.5
62
3750
83.9
183
2250
79.3
56
0
2250
9.3
72
2500 34.1
146
2250
17.6
61
0
2250
73
82
1250 213.1
85
1500
54.5
35
1500
39.5
44
1500
23.5
21
7500 100.3
155
1500
29 8
44
1500
18.1
44
1500
22 5
41
3750 40.7
183
1500
22 3
47
1500
25.1
44
1500
66.7
48
2500
97.5
146
1500
11.8
50
1500
12.4
44
1500
37.9
55
1250
88.6
85
0
4500
26.1
131
0
7500 157.5
155
0
4500
169.9
131
0
3750
94 5
183
0
4500
80.7
131
0
2500
116
146
0
4500
109.1
131
0
1250
89.4
85
0
2250
49.1
65
2250
7.3
32
7500 41.8
155
0
2250
17.8
65
2250
97.9
62
3750 208.6
183
0
2250
18.9
65
2250
53.9
72
2500 100.8
146
0
2250
262
65
2250
73.4
82
1250 160.4
85
0
0
4500
92
64
7500 163.4
155
0
0
4500
73.4
124
3750
61.6
183
0
0
4500 127.9
144
2500 115.5
146
0
0
4500 126.9
164
1250 49.1
85
obs
pred sown
obs
pred
sown
117
Table 24. Post Farm 1997 biomass (g m"2) by species and sowing density (seeds m'2).
Observed (obs) is from field data; predicted (pred) is from community dynamics model.
Interm ediate w heatgrass
Crested w heaterass
Alfalfa
sown
obs
pred
sown
4500
32
10
0
0
4500
189
26
0
4500
55
57
4500
500
2250
obs
Pred sown
obs
Spotted knapweed
pred
sown
obs
pred
0
7500 1212
3750 465
2459
1884
0
0
2500 1277
1442
0
0
0
1250
801
913
463
4
2250
184
76
0
7500 1259
2442
2250
161
8
2250
74
71
0
3750 2949
1848
2250
90
12
2250
0
70
0
2500 2432
1437
2250
180
23
2250
224
74
0
1250
612
842
2250
135
11
0
2250
0
229
7500
385
2374
2250
81
0
0
2250
90
229
3750 1081
1784
2250
109
0
0
2250
22
229
2500
542
1378
2250
24
0
0
2250
392
229
1250 1961
806
1500
69
4
1500
68
11
1500
15
47
7500 1507
2396
1500
57
17
1500
59
12
1500
367
47
3750
927
1756
1500
0
0
1500
4
11
1500
201
47
2500
426
1381
1500
72
0
1500
57
11
1500
0
47
1250 2112
808
0
4500
75
10
0
7500 3101
2413
0
4500
301
10
0
3750
630
1794
0
4500
8
10
0
2500 2641
1387
0
4500
62
10
0
1250 1161
812
0
2250
488
6
2250
0
70
7500 1576
2363
0
2250
76
6
2250
122
70
3750 2621
1719
0
2250
0
6
2250
39
70
2500 1482
1317
0
2250
3
6
2250
16
70
1250
166
764
0
0
4500
296
459
7500
372
2312
0
0
4500
101
459
3750
527
1650
0
0
4500
193
459
2500
886
1254
0
0
4500
65
459
1250
0
721
118
Table 25 . Paired /-test comparisons o f observed and predicted biomass (g m'2) for Post
Farm. Numbers in parentheses represent I SE. The P -value represents the probability
that the observed and predicted populations are not significantly different. Observed
biomass for Red Bluff in 1997 is presented, but was not used in the model.
_________
Site
Post Farm
Year
1996
observed
predicted
r
Intermediate wheatgrass
62 #0)
68 (26)
0.71
0.497
Crested wheatgrass
49 (42)
76 (33)
0.68
0.004
72
(40)
0.77
0.094
Spotted knapweed .
60 #8)
96 (58)
142
(36)
-0.02
0.002
Intermediate wheatgrass
139 (145)
11
(15)
-0.14
0.003
Crested wheatgrass
105 (133)
25
(29)
0.07
0.031
Alfalfa
120(133)"
201 (170)
0.21
0.112
Spotted knapweed
1254 (879)
1588 (592)
0.16
0.082
. Alfalfa
Post Farm
RedBluff
1997
1997
. Mean
Species
Intermediate wheatgrass
90 (202)
Crested wheatgrass
112 (199)
Alfalfa
126 (169)
Spottedknapweed
179 (46)
P
Table 26. Spitters models predicting average weight per plant from density o f all species.
p2
Year
Predicted species
* * M o d el
P M o d el
1996
1997
.
0.36
Intermediate wheatgrass
0.1
Crested wheatgrass
No model
Alfalfa
0.08
0.20
Spotted knapweed
0.05
0.31
Intermediate wheatgrass
No model
Crested wheatgrass
0.08
Alfalfa
No model
Spotted knapweed
0.09
—
I
---
0.23
—
0.23
ELIN
y = 0.031 + 0.04Ox
16JUN97
08JUL97
21JLT97
MESA
R2 = 0.39
04AUG97
y = 0.022 + 0.086x
18AUG97
16JUN97
08JUL97
H arvest date
18AUG97
CEMA
R2 = 0.29
y = 0.150 + 0.170x
119
y = 0.034 + 0.016x
04AUG97
H arvest date
AGCR
0.00
21JUL97
R2 = 0.40
R2 = 0.51
-
16JUN97
08JUL97
21JUL97
H arvest date
04AUG97
18AUG97
000
16JUN97
08JUL97
21JUL97
04AUG97
18AUG97
H arvest date
Figure 30. Linear regression of shoot weight over time for intermediate wheatgrass(A), crested wheatgrass (B), alfalfa
(C), and spotted knapweed (D). The variable “x” represents harvest intervals I through 5. All four regressions are
significantly different from each other (?<0.05). Note scale of y-axes.
120
Error bars = I S E
S p e c i e s n a m e s w ith i n
b a rs = s ig n ific a n tly d iffe re n t
S
(P < 0.05)
I
ELIN
AGCR
AGCR
MESA
Species
Figure 3 1. Mean shoot biomass at final harvest.
Discussion
Increasing species richness with species that are niche-differentiated would be
expected to increase resource use because niche occupation increases. Hooper and
Vitousek (1998) found that increasing functional group diversity increased resource use,
and Tilman et al. (1997) and Brown (1998) found increasing functional group diversity
increased aboveground biomass. Carpinelli et al. (Submitted) have shown that in wellestablished communities, increasing species richness with niche-differentiated species
increased overall biomass production and reduced spotted knapweed invasion, suggesting
resources were preempted from establishing spotted knapweed. In this study, however,
where all species established simultaneously, increasing desirable species richness had no
effect on spotted knapweed even where the desirable species were niche-differentiated.
A
ELIN
2.5
0
MESA
•
O
<?
2.0 -
O
E
S
1.5
~V)
-S
^ °
R2 =0.13
No model
y = 1.37 + 1.06x
°
O
0
I
1.0
£§
0.5
°
o
°
■g
°
O
0.0
16JUN97
08JUL97
21JUL97
04AUG97
18AUG97
16JUN97
08JUL97
Harvest date
21JUL97
04AUG97
18AUG97
Harvest date
CEMA
E
I
16JUN97
08JUL97
21JUL97
Harvest date
04AUG97
18AUG97
R2 = OM
No model
16JUN97
08JUL97
21JUL97
04AUG97
18AUG97
Harvest date
Figure 32. Regressions of root mass density over time for intermediate wheatgrass (A), crested wheatgrass (B), alfalfa
(C), and spotted knapweed (D) for depth increment 0 cm - 100 cm. The variable “x” represents harvest intervals I
through 5. Note scales o f y-axes.
122
Fourth Harvest
Error bars = I S E
S p e c i e s n a m e s w ith i n
E
b a rs = s ig n ific a n tly d iffe re n t
S
(P 3 0.05)
I'
ELIN
AGCR
ELIN
AGCR
Species
Fifth Harvest
ii
10
9
6 8
B
Error bars = I S E
S p e c i e s n a m e s w ith i n
b a r s = s ig n i f i c a n t l y d i f f e r e n t
(/> < 0.05)
7
• i
6
I
5
4
3
2
ELIN
AGCR
I
ELIN
AGCR
MESA
Species
Figure 33. Mean root mass densities at fourth (A) and fifth (B) harvests. No samples
were available for spotted knapweed at the fourth harvest.
123
A
Harvest I
B
Harvest 2
16JUN97
0 - 33 cm
33 - 6 7 cm
E r r o r b a rs = I S E
E r r o r b a rs
EZZZ
FrrrrrI
ESXS)
KWvl
EZZZ
FrrrrrI
KVxKI
KWM
ELIN (n = 4)
AGCR (n =4)
MESA (n = 4)
CEMA (n = 4)
6 7 - 100 cm
0 - 3 3 cm
Depth increment
E
3
&
D
Harvest 4
04AUG97
E r r o r b a rs = I S E
E r r o r b a ts - I S E
v//A ELIN (n= 4)
ezza
ELIN (n =3)
AOCR (n =3)
DOOQ4 MESAfn =4)
NTTtI CEMA (n = 0)
prrrm AOCR (n = 4)
Q&XXI MESA (n =4)
KWH CEMA (n = 0)
8
*
C
21JUL97
10
67 - 100 cm
Depth increment
Harvest 3
12
33 - 67 cm
I SE
ELIN (n =3)
AGCR (n =3)
MESA (n = 4)
CEMA (n = 4)
6
4
2
0
_
0 - 33 cm
33 - 6 7 cm
_ C$3
6 7 - 100 cm
0 - 33 cm
Depth increment
33 - 67 cm
67 - 100 cm
Depth increment
E
Harvest 5
18AUG97
E r r o r b a rs = I S E
177771
FrrrrrI
E5S2
KVvM
0 - 3 3 cm
33 - 67 cm
ELIN (n = 3)
AGCR (n =3)
MESA (n = 3)
CEMA (n = 3)
67 - 100 cm
Depth increment
Figure 34. Mean root mass density by depth increment and species for harvests 1 - 5 , A E, respectively. Note variable sample sizes and scales o f y-axes.
124
This may be because this study was limited to the first two years o f establishment, when
occupation o f “biological space” may be more important than competition for resources
(Harper 1977). However, the fact that biomass o f desirable species monocultures was
similar to total biomass o f mixtures containing spotted knapweed (Figure 26) indicates,
that spotted knapweed was able to preempt resources from desirable species in the second
growing season, or that spotted knapweed was able to outcompete the desirable species
regardless o f the degree o f niche occupation by desirable species.
The lack o f effect o f mean soil water content on spotted knapweed growth, and the
lack o f effect o f spotted knapweed growth on soil water depletion, indicate water was .
neither limiting to, nor influenced by, spotted knapweed. In addition, the lack o f effect o f
desirable species richness on soil water depletion indicates that the desirable species were
not limited by soil water. This suggests niche differentiation among desirable species may
have resulted from competition for a resource other than water, and that same resource
.
•
J
may not have been limiting to spotted knapweed.
The predictive model o f Maxwell et al. (1988) was developed by using sensitivity
analysis on a simple “first generation” model to identify transition states that have large
effects on population dynamics o f leafy spurge {Euphorbia esuld). Their “second
generation” model included effects o f competition, physiological, and environmental
factors on these transition states. Their second-generation model proved very accurate in
predicting leafy spurge response to herbivory and chemical control (r = 97, r = 98,
respectively) (Maxwell et al. 1988). The model developed in this study did not accurately
represent the observed field data, possibly because the model was mainly based on intra-
125
and interspecific interference. The low i?2 values for the Spitters models suggest
interference played a minor role in explaining plant weight. Perhaps factors other than
competition played a major role in determining community dynamics. Inclusion of
physiological and environmental factors, as well as demographic data from future years,
may improve its accuracy. Once expanded, the model might help identify processes and
mechanisms that are key to successful revegetation o f spotted knapweed-infested
rangeland.
The ability o f spotted knapweed to dominate the desirable species in this study,
regardless o f species richness, demonstrates the necessity to consider site history in
revegetation. I f revegetation o f weed-infested rangeland is to be successful, it must
address potential weed reestablishment from seeds or propagules in the soil by depleting
the weed seeds in the seed bank or by using a persistent herbicide to provide a weed-free
window o f establishment for desirable species. For example, if weed seeds are allowed to
germinate, and the emerging weeds are then killed by mechanical or chemical means prior
to revegetation, the desirable species may dominate the site by preempting resources from
weeds that may later emerge. Only then can other strategies, such as maximizing resource
use by maximizing niche occupation by desirable species, be expected to provide long­
term success.
12.6
LITERATURE CITED
Alfalfa Seed Survey. 1992. M ontana Agricultural Statistics Service. Helena, MT.
Berube, D.E. and J.H. Myers. 1982. Suppression o f knapweed invasion by crested
wheatgrass in the dry interior o f British Columbia. J. Range Manage. 35:459-461.
Borman, M.M., W.C. Krueger, and D.E. Johnson. 1991. Effects o f established perennial
grasses on yields o f associated annual weeds. J. Range Manage. 44:318-326.
Box, G E P., W.G. Hunter, and ES. Hunter. 1978. Statistics for experimenters. John Wiley
and Sons, Inc., New York. 653 p.
Brown, C.S. 1998. Restoration o f California Central Valley grasslands: applied and
theoretical approaches to understanding interactions among prairie species. Ph D.
Dissertation. University o f California, Davis, C A.
Call, C A. and B A. Roundy. 1991. Perspectives and processes in revegetation o f arid and
semiarid rangelands. J. Range Manage. 44:543-549.
Carpinelli, M.F., R E. Sheley, and B.D. Maxwell. Submitted. Designing weed-resistant plant
communities by maximizing niche occupation. J. Range Manage, (under review)
de Wit, C.T. 1960. On competition. Versl. Landbouwk. Onderz. 66, 1-82.
Goldberg, D.E. 1990. Components o f resource competition in plant communities. Im TB.
Grace and D. Tilman (eds ), Perspectives on Plant Competition. Academic Press, San
Diego, pp. 27-49.
■'
Harper, J.L. 1977. The Population Biology o f Plants. Academic Press, London.
Harris, G A . 1967. Some competitive relationships bttWtenAgropyron spicatum m&Bromus
tectorum. Ecol. Monogr. 37:89-111.
Holzworth, L. and J. Lacey. 1991. Species selection criteria for seeding dryland .pastures in
Montana. M ontana State University Extension Bull. 19. Bozeman, MT.
Hooper, D.U. and P.M. Vitousek. 1998. Effects o f plant composition and diversity on
nutrient cycling in serpentine grassland. Ecol. Monogr. 68:121-149.
Hubbard, W: A. 1975. Increased range forage production by reseeding and the chemical
control o f knapweed. J. Range Manage. 28:406-407.
127
Huston, C.H., R H. Callihan, and R.L. Sheley. 1984. Reseeding intermediate wheatgrass in
yellow starthistle-infested rangeland. In. Proc. Knapweed Symp., Montana State
University Cooperative Extension Bull. 1315. Bozeman, MT. pp. 42-44.
Jacobs, J.S. and R L Sheley. 1998. Observation: life history o f spotted knapweed. J. Range
Manage. 51:665-673.
James, D. 1992. Some principles and practices o f desert revegetaion seeding. Arid Lands
Newsletter 32:22-27.
Larson, L.L. and M L . McInnis. 1989. Impact o f grass seedings on establishment and density
o f diffuse knapweed and yellow starthistle. Northwest Sci. 63:162-166.
Maxwell, B.D. and R.L. Sheley. 1997. Noxious weed population dynamics education model.
W eedTechnol. 11:182-188.
Maxwell, B.D., M.V. Wilson, and S R. Radosevich. 1988. Population modeling approach for
evaluating leafy spurge {Euphorbia esula) development and control. Weed Technol.
2:132-138.
Peterson, R.G. 1985. Design and analysis o f experiments. Marcel Dekker, Inc. New York.
Radosevich, S R. and J.S. Holt. 1984. Weed Ecology, John Wiley & Sons, Inc. New York.
265 p.
Ratkowski, D.A. 1983. Nonlinear regression modeling: a unified practical approach. Marcel
Dekker, Inc., New York, N.Y. 276 p.
Ross, M.A. and J.L. Harper. 1972. Occupation, o f biological space during seedling
establishment. J. Ecol. 60:77-88.
SAS Institute Inc. 1991. SAS/STAT User’s Guide, Release 6.03 edition. SAS Institute Inc.,
Cary, NC. 1028 p.
Serin, Y. 1989. Effect o f row spacing, nitrogen, and phosphorus rates on seed and herbage
yields and some yield components in crested wheatgrass {Agropyron cristatum (L.)
Gaertn.) grown under dry conditions at Erzurum. Turkish J. Agric. and Forest. 13:3a,
765-781.
Serin, Y. 1989. Effect o f row spacing, nitrogen, and phosphorus rates on seed yields and
some yield components in intermediate wheatgrass {Agropyron intermedium (Host.)
Beauv.). Turkish J. Agric. and Forest. 13:3a, 782-797.
128
Spitters, C J.T . 1983. An alternative approach to the analysis o f mixed cropping experiments.
I. Estimation o f competition effects. Netherlands. J. Agric. Sci. 31:1-11.
Tilman, D., I. Knops, D. Wedin, P. Reich, M. Ritchie and E. Siemann. 1997. The influence
o f functional diversity and composition on ecosystem processes, Sci. 277:1300-1302.
APPENDIX
130
NEUTRON PROBE CALIBRATION
The following procedure is used to covert neutron probe count ratio to soil water
content:
(1) Extract a soil core o f the same size necessary to accommodate the access tube
(e.g. PVC pipe) that will eventually be used for probe access.
(2) Divide the soil core into 20-cm increments. These 20-cm increments correspond
to the soil depth increments o f water measurement. For example, if the neutron probe
will be used to monitor soil depths from 10 cm to 70 cm in 20-cm increments, the soil
core should be divided into increments o f 10 cm - 30 cm, 30 cm - 50 cm, and 50 cm 70 cm. These soil cores are placed in airtight bags, weighed as soon as possible (wet
. weight), then oven dried to a constant weight and reweighed (dry weight).
(3) Insert access tube into cored location and take neutron probe readings at the
midpoint o f each depth increment. In the example in step 2 (above), neutron probe
readings would be taken at 20 cm, 40 cm, and 60 cm.
(4) Calculate volumetric soil water content from the soil samples for each increment.
Compute a. linear regression from water content (observed) over neutron probe count
ratio (observed). This regression is used to convert count ratio to volumetric soil
water content for future readings.
In order to represent a site accurately, multiple soil cores are taken to account for
variation in soil water content and soil type. To account for this variation, take a
minimum o f three cores in dry soil (e.g. end-of-growing-season) and three cores in moist
131
soil (e.g. fallow or spring) for each soil type and/or site. If regression residuals are
unevenly distributed by depth increment, separate regressions should be used for each
depth. If residuals are evenly distributed, a single regression using all depths may be used.
I f regressions are not satisfactory, increase sample size as needed by repeating steps I
through 4 (above).
MONTANA STATE UNIVERSITY - BOZEMAN