Directing succession by altering nutrient availability
by Gretchen J Herron
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Land
Rehabilitation
Montana State University
© Copyright by Gretchen J Herron (1999)
Abstract:
Spotted knapweed reduces wildlife and livestock habitat, biodiversity, and increases erosion.
Manipulating nutrient availability may be used to accelerate succession away from spotted knapweed.
Early successional plant communities often thrive in sites with high nutrient availability, whereas
late-successional communities are often found on lower nutrient availability. This study tested the
ability to direct succession by altering nutrient availability in a growth chamber experiment and in the
field. My hypotheses were: removal of nutrients changes the competitive advantage from spotted
knapweed to bluebunch wheatgrass (late-seral), and nutrient addition will favor spotted knapweed in a
growth chamber study. The field study tested the hypothesis that using either mid-seral species or
annual rye would direct succession away from a weedy plant community.
In addition series matrices (densities ranging from 100 seeds m-2 to 5,000 seeds m-2), background
densities of rye and bottlebrush squirreltail (3,000 seeds m-2) were used to remove nutrients from the
soil in the growth chamber. Nitrogen or phosphorus (33 kg ha'1) were added to the soil. The field study
applied annual rye, bottlebrush squirreltail, nitrogen (31 kg ha'1), and bluebunch wheatgrass as a
completely randomized incomplete factorial design.
In the growth chamber, nutrient analysis of soil and vegetation indicated that rye and bottlebrush
squirreltail reduced nutrient availability in soils. Data were fit to Watkinson’s curvilinear model to
determine the competitive relationship between bluebunch wheatgrass and spotted knapweed.
Competition coefficients indicated that without nutrient manipulation, spotted knapweed was more
competitive than bluebunch wheatgrass. Annual rye changed the competitive balance in favor of
bluebunch wheatgrass (equivalence ratio = 9.9). Addition of nitrogen, phosphorus, or the mid-seral
species did not change the competitive relationship between the two species. The field study,
demonstrated that the application of nitrogen and rye increased spotted knapweed density over rye
alone. Bluebunch wheatgrass tended to enhance spotted knapweed establishment and growth.
However, bottlebrush squirreltail and rye had negative effects on spotted knapweed without affecting
bluebunch wheatgrass. The growth chamber study and the field study suggest that succession from
spotted knapweed to late-seral bluebunch wheatgrass community may be accelerated by altering
resource availability. DIRECTING SUCCESSION BY ALTERING NUTRIENT AVAILABILITY
by
Gretchen J. Herron
A thesis submitted in partial fulfillment
o f the requirements for the degree
of
M aster o f Science
in
Land Rehabilitation
MONTANA STATE UNIVERSITY-BOZEMAN
Bozeman, Montana
January 1999
N3^
\A4SU4'
APPROVAL
o f a thesis submitted by
Gretchen J. Herron
This thesis has been read by each member o f the thesis committee and has been
found to be satisfactory regarding content, English usage, format, citations, bibhograp c
style, and consistency, and is ready for submission to the College o f Graduate Studies.
Roger L Sheley
Approved for the Department o f Land Resources and Environmental Sciences
Jeffrey S. Jacobsen
Date
Approved for the College o f Graduate Studies
Bruce R McLeod
iii
STA TEM EN T O F PE R M ISSIO N TO USE
In presenting this thesis in partial fulfillment o f the requirements for a master’s
degree at M ontana State University-B ozeman, I agree that the Library shall make it
available to borrowers under rules o f the Library.
I f I have indicated my intention to copyright this thesis by including a copyright
notice page, copying is allowable only for scholarly purposes, consistent with “fair use” as
prescribed in the U S . Copyright Law. Requests for permission for extended quotation
from or reproduction o f this thesis in whole or in parts may be granted only by the
copyright holder.
Signature
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iv
TABLE OF CONTENTS
1. INTRODUCTION.................................................................................................................... I
Successional introduction and history............................................................................2
O bjectives...........................................................................................................................9
2. INFLUENCE OF NUTRIENT AVAILABILITY ON THE INTERACTION
BETW EEN SPOTTED KNAPWEED AND BLUEBUNCH
WHEAT GRAS S...............................................................................................................11
Introduction.......................................................................................................................11
Materials and M e th o d s................. ,.............................................................................. 13
Interference ........................................................................................................13
Sampling ................................................................................................15
A nalysis..................................................................................................15
Growth o f Isolated Individuals........................................................................16
Sampling ................................................................................................17
Analysis .................................................................................................17
Results .......
17
N u trie n ts............................................................................................................. 17
Soil Nitrogen and P hosphorus............................................................ 17
Nitrogen and Phosphorus Uptake by Bluebunch W heatgrass.......18
Nitrogen and Phosphorus Uptake by Spotted K napw eed............. .19
Interference........................................................................................................ 20
Bluebunch Wheatgrass B iom ass........................................................ 20
Spotted Knapweed B io m ass.............................................................. 20
Growth Rate o f Isolated Individuals.............................................................. 22
Discussion ....................................................................................................................... 22
3. DIRECTING SUCCESSION BY ALTERING NUTRIENT A V A ILA BILITY ........ 27
Introduction...................................................................................................................... 27
Materials and M e th o d s................................................................................................. 30
Study Sites ........................................................................................................ 30
P ro ced u res..........................................................................................................30
Sam pling................................................ ,............,............................................. 31
Analysis .... :........................................................................................................31
TABLE OF CONTENTS - Continued
Results..............................................................................................................................32
Spotted K napw eed............................................................................................32
D ensity...................................................................................................32
C o v e r.................................................................................................... 32
B iom ass................................................................................................. 36
Bluebunch W heatgrass..................................................................................... 39
D ensity...................................................................................................39
C o v e r.....................................................................................................39
B iom ass................................................................................................. 39
Bottlebrush Squirreltail....................................................................................39
D ensity...................................................................................................39
Cover and Biomass............................................................................... 42
Annual R y e ........................................................................................................ 42
D ensity......................,........................................................................... 42
Cover and B iom ass...........................................
42
Discussion ....................................................................................................................... 43
LITERATURE CITED............................................................................................................... 46
A PPEN D ICES..............................................................................................................................54
A- SPOTTED KNAPWEED AND BLUEBUNCH WHEAT GRAS S DENSITY,
COVER, AND BIOMASS MEANS GENERATED FOR TREATMENT M AIN
EFFECTS..........................................................................................................................55
Spotted knapweed..................................................................................56
Bluebunch wheatgrass..........................................................................57
B- RESPONSE SURFACES FOR THE PREDICTION OF BLUEBUNCH
WHEATGRASS BIOMASS..........................................................................................58
Response surface for bluebunch wheatgrass biomass control
treatment................................................................................................ 59
Response surface for bluebunch wheatgrass biomass rye
treatment................................................................................................ 60
C- RESPONSE SURFACES FOR THE PREDICTION OF SPOTTED
KNAPWEED BIOM ASS.............................................................................................. 61
Response surface for spotted knapweed control treatment............62
Response surface for spotted knapweed bottlebrush squirreltail
treatment................................................................................................ 63
Response surface for spotted knapweed annual rye treatment....... 64
Response surface for spotted knapweed nitrogen treatm en t..........65
Response surface for spotted knapweed phosphorus treatment.....66
LIST OF TABLES
Table I .
Residual soil N and P content following the plant grow th study................. 18
Table 2.
Nitrogen, and phosphorus uptake by bluebunch wheatgrass and
spotted knapweed.............................................................................................. 19
Table 3.
Table 4.
Equivalence ratios generated from the prediction o f bluebunch
wheatgrass biomass.......... .....................
21
Equivalence ratios generated from the prediction o f spotted
knapweed............................................
21
Table 5.
P -values for spotted knapweed density, cover, and biomass (1998) as
the dependent variables generated from ANOVA.........................................33
Table 6.
P-values for bluebunch wheatgrass density, cover, and biomass (1998)
as the dependent variables generated from ANOVA....................................40
Table 7.
Spotted knapweed means for density, cover, and biomass generated by
main effects o f treatments..................................................................................56
Table 8.
Bluebunch wheatgrass means for density, cover, and biomass generated
by main effects o f treatments.............................................
57
Vll
LIST OF FIGURES
Page
Figure I .
Growth rate o f isolated individuals................. .................................................23
Figure 2.
Interaction o f annual rye and N on spotted knapweed density at
site 2 in 1998......................................................................................................34
Figure 3.
Interaction o f annual rye, N, and bluebunch wheatgrass on spotted
knapweed cover at site I in 1997................................................................... 35
Figure 4.
Interaction o f bluebunch wheatgrass and bottlebrush squirreltail on
spotted knapweed cover at site I in 1997......................................................37
Figure 5.
Interaction o f annual rye, N, and bluebunch wheatgrass on spotted
knapweed biomass at site 2 in 1988................................................................ 38
Figure 6.
Interaction o f bluebunch wheatgrass and N on bluebunch
wheatgrass density at site 2 in 1997................................................................ 41
Figure 7.
Interaction o f bluebunch wheatgrass and N on bluebunch
wheatgrass at site 2 in 1998..............................................................................41
Figure 8.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict bluebunch wheatgrass control treatm ent.........................59
Figure 9.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict bluebunch wheatgrass annual rye treatm ent................... 60
Figure 10.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed control treatm ent...............................62
Figure 11.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed bottlebrush squirreltail treatment.....63
viii
L IS T O F FIG U R ES - Continued
Figure 12.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed annual rye treatm ent..........................64
Figure 13.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed nitrogen treatm ent..............................65
Figure 14.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed phosphorus treatm ent........................66
i
IX
ABSTRACT
Spotted knapweed reduces wildlife and livestock habitat, biodiversity, and
increases erosion. Manipulating nutrient availability may be used to accelerate succession
away from spotted knapweed. Early successional plant communities often thrive in sites
with high nutrient availability, whereas late-successional communities are ,often found on
lower nutrient availability. This study tested the ability to direct succession by altering
nutrient availability in a growth chamber experiment and in the field. M y hypotheses
were: removal o f nutrients changes the competitive advantage from spotted knapweed to
bluebunch wheatgrass (late-seral), and nutrient addition will favor spotted knapweed in a
growth chamber study. The field study tested the hypothesis that using either mid-seral
species or annual rye would direct succession away from a weedy plant community.
In addition series matrices (densities ranging from 100 seeds m"2 to 5,000
seeds m"2), background densities o f rye and bottlebrush squirreltail (3,000 seeds m"2 )
were used to remove nutrients from the soil in the growth chamber. Nitrogen or
phosphorus (33 kg ha'1) were added to the soil. The field study applied annual rye,
bottlebrush squirreltail, nitrogen (31 kg ha'1), and bluebunch wheatgrass as a completely
randomized incomplete factorial design.
In the growth chamber, nutrient analysis o f soil and vegetation indicated that rye
and bottlebrush squirreltail reduced nutrient availability in soils. D ata were fit to
W atkinson’s curvilinear model to determine the competitive relationship between
bluebunch wheatgrass and spotted knapweed. Competition coefficients indicated that
without nutrient manipulation, spotted knapweed was more competitive than bluebunch
wheatgrass. Annual rye changed the competitive balance in favor o f bluebunch
wheatgrass (equivalence ratio = 9.9). Addition o f nitrogen, phosphorus, or the mid-seral
species did not change the competitive relationship between the two species. The field
study, demonstrated that the application o f nitrogen and rye increased spotted knapweed
density over rye alone. Bluebunch wheatgrass tended to enhance spotted knapweed
establishment and growth. However, bottlebrush squirreltail and rye had negative effects
on spotted knapweed without affecting bluebunch wheatgrass. The grow th chamber study
and the field study suggest that succession from spotted knapweed to late-seral bluebunch
wheatgrass community may be accelerated by altering resource availability.
I
CHAPTER I
INTRODUCTION
Spotted knapweed (Centaurea maculosa L.) is a perennial forb native to central
Europe and Russia (Rees et at, 1996). Spotted knapweed currently infests every county in
Washington, Idaho, Montana, and Wyoming (Sheley et ah, 1998). In Montana, spotted
knapweed infests over 2.2 million hectares and is spreading at a rate o f 27% per year
(Lacey et a l, 1989; Chicoine et a l, 1985). The environmental impacts o f spotted
knapweed include reduced forage production (Watson and Renney 1974; Harris and
Cranston 1979), reduced plant species diversity (Tyser and Key 1988), reduced wildlife
habitat (Bedunah and Carpenter 1989) as well as increased erosion and stream
sedimentation (Lacey et al, 1989). The economic impact o f spotted knapweed to the
M ontana livestock industry has been estimated at about $11 million each year (Hirsch and
Leitch 1996).
Spotted knapweed is well-adapted to a wide range o f environmental and climatic
conditions (Watson and Renney 1974; Chicoine et a l, 1985). Spotted knapweed is highly
competitive and relatively drought tolerant (Berube and Myers 1982). The weed displays
germination and emergence polymorphism, allowing it to avoid intraspecific competition
and occupy all available safe-sites by developing a hierarchy o f age classes within a
population (Sheley and Larson 1996). In addition, early establishment and rapid growth
2
rates enable knapweed to capture resources prior to neighboring species (Sheley et al,
1993). These characteristics allow spotted knapweed to establish and out compete
established native species (Harris and Cranston 1979; Watson and Renney 1974).
Large scale spotted knapweed infestations become intractable due to their size.
Cultural, mechanical, and chemical means o f management have not proved sustainable and
are not cost effective on most lands (Griffith and Lacey 1991). Development of
sustainable cost-effective weed management is imperative to rehabilitate and preserve
native plant communities. Sustainable methods for weed management focus on natural
processes that govern plant community dynamics (Sheley et a l, 1996). It is a reasonable
assumption that mechanisms which control community dynamics could be used to manage
weed invasion and spread (Sheley et al, 1996). Mechanisms that influence plant
establishment and competitive ability such as disturbance, nutrient availability, growth rate
and resource use efficiency are potential factors to sustainably manage invasive plants.
Successional Introduction and History
The process o f succession occurs as plant populations within a community change
over time. I f the environment remains relatively constant, the change in composition
becomes very slow or even ceases. Environmental stasis may occur when disturbance is
removed (e.g. a stream meanders away from original site) or an exotic species creates a
monoculture. Succession is divided into two parts, pertaining to the conditions of
disturbance, primary and secondary. Primary succession occurs when plants establish on
substrate that was not previously vegetated. An example o f primary succession could
3
include a lava flow or a desiccated lake bed. Secondary succession is the pattern o f
establishment that follows a disturbance on vegetated land. An example o f secondary
succession is when a plant community establishes after a fire or clear-cut logging event. I f
undisturbed, secondary succession plant community composition will transition toward its
predisturbance community structure and composition. Three successional stages (early,
mid, and late) are recognized as containing unique species with distinct evolutionary
characteristics. Late-succession communities are often idealized as a community having
constant species composition. This plant community is associated with the efficient
capture o f resources by late-successional species creating a resilient community.
The theory o f plant community change over time was originally discussed by
Clements (1916) in the early 20 century. Clementian ecology focuses on a singular
“climax” community. EQs successional theory is unidirectional and determined by
individual performance. Clements brought an element o f individual species influence to
the pervasive landscape theory o f the era. He emphasized vegetational homogeneity,
dynamic nature o f plant communities, and recognized that each species requires particular
conditions to become established in the community. Clements theorized that over large
tracts o f land, vegetation can be characterized by a single climate-determined mature
phase. AU other populations reflect immature stages o f the self-sustaining climatic climax
vegetation (Allen and Hoekstra 1992)
Gleason expanded (1917; 1926) upon previous theories by introducing seed
source, habitat condition, and dispersal’s effect on species composition. Gleason (1926)
also theorized that multiple steady states or climax communities exist and abiotic/biotic
4
processes select for organisms on a site. His approach expanded on the organismal
concept Clements had developed. However, Gleason did not refute Clements theory; he
focused on the underlying processes that select individual organisms for a community
rather than emphasizing Clement’s community constraints on a large scale (Allen and
Hoekstra 1992).
According to Cause’s (1934) competitive exclusion principle, species that are in
direct competition cannot coexist permanently on the same site. Therefore, the Iatesuccessional stage must be comprised o f species that complement, rather than directly
compete, with each other. Initial stages o f succession include early-successional (pioneer)
and mid-successional (intermediate) phases. However, currently, this principle is not
accepted as the best explanation of species diversity and coexistence (Silvertown 1987).
Recent theories claim that plant species have various life cycle characteristics that
allow them to succeed other species in a niche. Resource allocation is the term used to
denote plant traits that allow species to survive (Radosevich et a l, 1997). The theory
addressing patterns o f evolutionary development is that o f r- and.K-selection. This theory
is based on carrying capacity limits. A resource limitless environment would increase
exponentially whereas in a real environment growth levels off as the population
approaches the carrying capacity. MacArthur (1962) and Pianka (1970; 1994) developed
the theory o f r- and K-selection. The theory hypothesizes that within the range o f
resource availability, species develop two contrasting strategies for survival. Extreme Kstrategists tend to be long lived, have prolonged vegetative stages, allocate a small portion
o f biomass to reproduction, are nutrient competitive, and occupy late stages o f succession
5
(MacArthur 1962; Pianka 1970; Radosevich et a l, 1997). Extreme r-selection species are
often short-lived, colonize open spaces, allocate resources to reproduction, possess high
growth rates, and occupy early successional stages.
Connell and Slatyer (1977) described several alternative mechanisms through
which species replace each other via successional changes in the community. After a
disturbance in the environment, opportunistic species classified as early-seral species
colonize. Seeds o f early-seral species are abundant in the seed bank or are adapted to
migrate to disturbances. These species have the ability to first colonize a disturbed site
because they allocate resources to rapid growth rates, dispersal mechanisms, and seed
production (Tilman 1991). Early-seral species are most often annuals that occupy safesites becoming less competitive in sites where mature species are already established.
Allocation o f resources to above ground biomass decreases competitive ability for soil
nutrients (Radosevich et a l, 1997). According to Connell and Slatyer, (1977) the species
that replace these early communities may be influenced by one o f three mechanisms.
Early-seral species may be replaced through facilitation, tolerance, or inhibition.
Facilitation, requires early-seral species to change the soil and above ground environment
prior to the establishment o f mid- or late-seral species. The second mechanism
emphasizes species tolerance. A predictable successional sequence occurs because o f the
existence o f species that have evolved different strategies for exploiting environmental
resources. Late-seral species are able to tolerate lower levels o f resources than early-seral
species. The third mechanism thought to be associated with successional changes is
inhibition, whereby, all species resist invasion by competitors. The first occupants pre­
6
empt the space and continue to exclude or inhibit later colonists until the inhibitor species
die or are damaged; releasing environmental resources. When resources are released later
colonizers establish and reach maturity. This model has not been thoroughly investigated.
Grime (1979) expanded on the r and K theory o f resource allocation developed by
Pianka (1970) and MacArthur (1962) to include stress and disturbance. H e defined stress
as an external factor that limits production when resources (light, nutrients, water) are
limited. Disturbance is perturbation o f plant biomass. Ifth e extremes o f high and low
stress and disturbance are considered, four possible combinations occur. These
combinations allow four evolutionary strategies:!) ruderals: low stress, high disturbance
2) stress tolerators: high stress, low disturbance and 3) competitors: low stress, low
disturbance. The fourth combination o f high stress and high disturbance are intolerable for
plant establishment (Radosevich et al., 1997). Grime arranges the three evolutionary
strategies depicting various degrees o f stress, disturbance, and competition. Model
arrangement of various strategies provides categorical information about plant life cycles.
M ore recently, Tilman theorized that regardless o f which species colonizes first the
species which is most competitive for the limiting resource(s) will eventually dominate the
site (Wedin and Tilman 1993). This theory o f succession is termed the “resource-ratio”
hypothesis (Tilman 1985). This hypothesis suggests that succession results from a
gradient over time in the relative availabilities o f limiting resources on a site. Resources
emphasized are soil nutrient availability and light at the soil surface. It is assumed that
these resources are inversely related. In addition, each plant species is assumed to be a
superior competitor for a particular proportion o f the limiting resources. Therefore,
7
community composition should change when the ratio o f the two limiting resources
changes. The resource-ratio theory suggests that competition drives succession.
Tilman (1990) introduced a supporting theory to the resource-ratio hypothesis
termed the R* theory. The R* theory delineates a basic mechanism o f competition for a
soil resource. The level that a soil limiting resource is reduced by a monoculture o f a
species is called R*. R* is the resource level a species requires for it to be able to persist
in that environment. The species with the lowest R* for a limiting soil resource is
predicted to be the superior competitor for the resource (Tilman 1980; 1990) and will
displace less competitive species. R* is predicted to depend on plant traits (root mass,
nutrient conservation abilities, photosynthetic rates, and maximal grow th rate).
Study o f successional catalysts has indicated that there may be a relationship
between nutrient availability and successional stages (Wedin and Tilman 1993; Tilman
1990). Reflecting on what is known regarding plant serai stages and their characteristics,
it is logical to assume that early-successional species that colonize open disturbed sites are
poor competitors for nutrients. In these systems nutrient availability is not limiting. The
limiting resources are often propagules, light, or space. This theory is reinforced by
examining life cycle traits o f early-seral species. Early-seral species have evolved to
deseminate seeds, establish, and grow quickly to capture light resources and exclude
slower growing species through development o f above-ground biomass. Resource
allocation for early-seral species includes prolific seed production increasing the
probability that the species will be represented on site in the event o f a disturbance. In
addition, the evolution o f high above ground biomass enables early-seral species to
8
exclude neighboring species. Resources are not limited in disturbed sites because o f the
rerelease o f nutrients previously constrained in plant biomass. Alternatively, late-seral
species have highly developed root systems that, although decreases their ability to grow
quickly, allows them to be highly competitive for nutrients and deplete the soil o f .
available N (Wedin and Tilman 1993). Late-seral species have the ability to capture and
hold nutrients present at low levels. High competitivness allows late-seral species to
displace earlier-serai stages over the course o f time as nutrient levels decline. Wedin and
Tilman (1993) identified lower levels o f soil NO3" and NH4"1". in stands o f late-seral
Agropyron. Late-seral species are highly competitive in low nutrient soils but are inferior
competitors under highly productive light limited habitats. (Wedin and Tihnan 1993;
Tilman 1988; 1990).
W eed scientists have begun to develop management methods based upon
ecological principles. Maxwell et al. (1988) have developed a weed population model that
serves as a framework to organize weed population biology information and develop weed
control strategies. The model distinguishes stages in a weed’s life cycle when it is most
susceptible to management efforts. These models are used to identify information gaps,
set research priorities, develop hypotheses pertinent to weed population regulation, and
suggest management strategies. Modeling w ork depicting weed/crop interactions have
also been developed by Wilkerson et al. (1991), Shribbs et al. (1990), and Kropff and van
Laar (1993). These models focus on weed threshold levels and give insight into when to
employ control measures. Models were developed under various crop environments and
are individually based upon factors such as abiotic influence, timing o f herbicide
application, seed bank content, biomass, and plant interaction (Radosevich e ta l, 1997).
Resource management models developed by Rosenberg and Freedman (1984) and
Pickett et al. (1987) have been utilized as the basis for a conceptual successional
management model for weed infested rangelands (Sheley et al., 1996). The approach used
by Sheley et al. (1996) incorporates successional components in managing exotic weed
infestations. Successional weed management recognizes that plant communities are
dynamic and employs technology to enhance natural processes that regulate vegetation
change (Sheley et a l, 1996). The conceptual model is based upon three components o f
succession (i.e. disturbance, colonization, and species performance).
Objectives
Our objective was to test early- and late-seral species response to nutrient addition
and reduction. It may be possible to use species with demonstrated abilities to sequester
nitrogen, such as annual rye (Secale cerale L.) or mid-seral species, to reduce resource
availability during seedling establishment in order to favor late-seral species. The objective
o f the growth chamber study was to determine the effect o f additions and reductions o f
nitrogen (N) and phosphorus (P) on the relationship between spotted knapweed and
bluebunch wheatgrass (Pseudoroegneria spicata [Pursh.] Scribn and Smith). We
hypothesized that nutrient additions would favor spotted knapweed, while nutrient
reductions would favor bluebunch wheatgrass. In addition, resource additions or
extractions may also be used to direct plant community succession. The objective o f the
field study was to determine the potential for directing weed dominated plant communities
10
along a trajectory toward more desirable communities by altering resource availability.
The specific objective was to determine the effects o f applied N, annual rye, and
bottlebrush squirreltail (Sitanion hystrix (Nutt.) J. G. Smith) on the establishment and
growth o f spotted knapweed and the late-seral species, bluebunch wheatgrass. Annual rye
and bottlebrush squirreltail, considered a mid-seral species, were seeded in an attempt to
reduce the availability o f nutrients to other species. We hypothesized that annual rye and
bottlebrush squirreltail would decrease establishment and growth o f spotted knapweed and
favor the establishment and growth o f bluebunch wheatgrass. The value o f this w ork is to
create an example o f using successional principles to disturbed weed infested sites and
direct a plant community change from weed dominated to desirable species dominated.
11
CHAPTER 2
INFLUENCE OF NUTRIENT AVAILABILITY ON THE INTERACTION
BETW EEN SPOTTED KNAPWEED AND BLUEBUNCH W HEAT GRASS
Introduction
Invasion o f rangelands by spotted knapweed (Centaurea maculosa Lam.) in the
northwestern United States and Canada has altered plant community composition and
reduced forage value (Watson and Renney 1974), wildlife habitat (Bedunah and Carpenter
1989), and species diversity (Tyser and Key 1988). Spotted knapweed displaces perennial
grasses and increases bare-ground (Tyser and Key 1988), surface w ater runoff, and stream
sedimentation (Lacey et al, 1989). Spotted knapweed infests over 7 million hectares in
N orth America (Sheley et al, 1998). Early germination, rapid growth, and allocation o f
resources to above ground biomass enable knapweed to preempt resource use by its
competitors (Sheley et al, 1993). Typically, knapweeds are more competitive than
perennial grasses in high nutrient environments (Prather and Callihan 1991; Velagala et
al,. 1997).
Over the past several decades, spotted knapweed management has focused mainly
on chemical control. Because o f environmental, ecological, and economic concerns, the
appropriateness and effectiveness o f chemical weed control strategies are being questioned
(Sheley et al, 1996). There has been an increasing call for weed management strategies
that are based on an understanding o f ecology (Radosevich et al, 1997). Understanding
12
the mechanisms and processes associated with plant community dynamics is central to
developing sustainable weed management strategies on rangeland (Maxwell etal., 1988;
Sheley et al, 1996).
Nutrient availability may be a driving force in grassland community composition
dynamics (Seastadt et ah, 1991; Wedin and Tilman 1996; Vitousek et al, 1997). In
disturbed, semiarid steppe, the rate o f species replacement was more rapid on plots with
less available soil nitrogen (N) and slowest on plots with more available N (McLendon and
Redente 1992). Similarly, Tilman and Wedin (1991) studied a range o f species with
different successiona!niches and found that late-seral species were very competitive for N.
Late-seral species had high below-ground biomass and created soils with high C:N, and
consequently low N mineralization. Early-seral species were poor competitors for
nitrogen, but persisted by maintaining rapid growth rates and high seed production. When
growm in pairwise competition experiments, the late-seral species displaced early and midseral species (Tilman and Wedin 1991). The ability o f a late-seral species to lower
available ammonium and nitrate accounted for the change to a late-seral plant community.
Although Story et al. (1989) found that increasing available N increased the
biomass o f spotted knapweed, few studies address the influence o f nutrient availability on
the interactions o f invasive non-indigenous plants and indigenous plants. It may be
possible to use species with demonstrated abilities to sequester N, such as annual rye
(Secale cerale L.) or mid-seral species, to reduce resource availability during seedling
establishment in order to favor late-seral species. The objective o f this study was to
determine the effect o f additions and reductions o f N and phosphorus (P) on the
13
relationship between spotted knapweed and bluebunch wheatgrass (Pseudoroegmria
spicata [Pursh.] Scdbn and Smith). We hypothesized that nutrient additions would favor
spotted knapweed, while nutrient reductions would favor bluebunch wheatgrass.
Materials and Methods
Interference
Spotted knapweed, bottlebrush squirreltail (Sitanion hystrix [Nutt.] I. G. Smith),
and bluebunch wheatgrass were used as a model plant community to test the hypothesis
that succession from a weedy plant community toward a desired late-seral plant
community could be accelerated by altering nutrient availability. This study examined the
effect of additions o f N and P; annual rye; and bottlebrush squirreltail (mid-seral) on the
interaction between spotted knapweed and bluebunch wheatgrass. Annual rye is reported
to reduce nutrient availability in .the soil (Mugwira et al, 1980). Densities and
proportions o f spotted knapweed (early-seral) and bluebunch wheatgrass (late-seral) were
arranged to provide five addition series matrices (Radosevich 1987). Seeding densities o f
spotted knapweedibluebunch wheatgrass in each matrix were 100:100, 100:1000,
100:3000, 100:5000, 1000:100, 1000:1000, 1000:3000, 1000:5000, 3000:100,
3000:1000, 3000:3000, 3000:5000, 5000:100; 5000:1000, 5000:3000, and 5000:5000m"2.
Five treatments that alter nutrient availability were applied to each addition series matrix.
Treatments consisted o f (I) 0.65g N/pot, in the form o f urea (2) 0.2004g P2O5Zpot (3) an
annual rye covercrop (3,000 seeds m"2), (4) bottlebrush squirreltail (mid-seral species)
(3,000 seeds m"2), and (5) no nutrient manipulation (control). Each treatment matrix was
14
replicated three times. Nutrients were applied in liquid form. Experiments were
conducted in pots (15 cm x 15 cm x 38 cm) which were filled with a low nutrient soil
mixture collected from a grassland dominated by late successional species, including
bluebunch wheatgrass and Idaho fescue (Festuca idahoensis Elmer) (Mueggler and
Stewart 1980). Soil was sieved, mixed at a rate o f 1:3 sand to soil; and pasteurized. Sand
was used to simulate a nutrient poor soil. The soil mixture was saturated with water and
allowed to equilibrate to field capacity. Seeds were broadcast on the soil surface and
manually arranged until a uniform distribution was achieved. A small amount (<2mm
depth) o f dry soil was used to cover the seeds. The soil surface was periodically
moistened with a fog mister and covered with plastic until seedlings emerged. Pots within
each matrix were randomly placed in an environmental chamber (12 C, 12-h day length;
200 uE/m2/s, spectral light). Conditions o f this study were within the range o f those found
during establishment o f bluebunch wheatgrass populations. Plants w ere allowed to grow
for 60 days.
In 1997, additional densities o f bluebunch wheatgrass and spotted knapweed were
seeded in combination with annual rye in the environmental chamber under identical
environmental conditions (12 C, 12-h day length; 200uE/m2/s, spectral light). Additional
densities o f bluebunch wheatgrass and spotted knapweed were created to quantify annual
rye’s effect on these species at low densities. Densities o f I, 2, 3, and 4 seeds o f bluebunch
wheatgrass were seeded with annual rye (3,000 seeds m"2). Spotted knapweed densities
o f I, 2, 3, and 4 seeds were sown in the presence o f annual rye at the same density.
15
Sampling. The soil was sampled and analyzed prior to seeding to determine
available levels o f N and P (Page's* al, 1982; Olsen et al, 1954). Soil analysis prior to
mixing indicated that the soil contained about 3 1 mg kg"1 NH4, 138 mg kg'1N 0 3> and 13
mg kg'1 o f phosphorus. Organic matter was approximately 8%. Initial seedling densities
were recorded two weeks after planting. All vegetation was harvested 60 days following
seeding. Final harvest densities by species and above and below ground biomass were
determined. At the end o f the study, soil samples were collected and analyzed for
available N (NO"3 and NH+4) and P (PO4"3) levels. Above ground biomass was collected
and analyzed for N and P concentration. Total N and P uptake (concentration*plant
biomass) was used to quantify nutrient dynamics. Analysis o f variance was used to
quantify nutrient addition and depletion effects on spotted knapweed and bluebunch
wheatgrass N and P uptake and soil nutrient levels.
Analysis. Nonlinear regression analysis was used to determine the effect o f the
nutrient addition, annual rye, and a mid-seral species on equivalence ratios o f spotted
knapweed and bluebunch wheatgrass. D ata were iteratively fit to models o f the following
forms (Firbank and Watkinson 1985):
bluebunch wheatgrass Ww= Wmw [I + AwQSlw+ CwkN k) ] 'Bw
spotted knapweed Wk = Wmk [I + AkQSlk + CkwNw)] "Bk
where Ww and Wk are the weight per plant o f bluebunch wheatgrass and spotted
knapweed, respectively. Wmw and Wmk are the maximum weight o f a plant grown in
isolation, and Aw and Ak represents the area required by a plant to achieve Wm. N w and
N k are the harvest densities o f bluebunch wheatgrass and spotted knapweed, respectively.
The parameters, Cwk and Ckw, are the per-plant equivalence o f w and k and can be
interpreted as the ratio o f intradnterspecific competition. The C parameters are also the
equivalence ratio o f the number o f plants (Nk and N w) it takes to have equivalent effect on
bluebunch wheatgrass or spotted knapweed biomass. For example, in determining
bluebunch wheatgrass biomass, if the C parameter is 0.5, then 0.5 unit o f spotted
knapweed density has the same ability to reduce biomass o f bluebunch wheatgrass as one
unit density o f bluebunch wheatgrass. Bw and Bk may be used to estimate the efficiency o f
resource utilization by each population. The asymptotic 95% confidence interval around
fit values for Cwk and Ckwwere used to determine the relative intensity o f competition
under the different nutrient and plant community treatments.
Growth o f Isolated Individuals
Growth analysis was conducted at the same time and under the same
environmental conditions as the addition series experiments. Absolute growth rate was
used to compare inherent growth differences o f each species. Individual species were
seeded in pots 15 cm x 15 cm x 38 cm. The soil used in the growth analysis experiment is
same soil described in the interference study. Pots were filled with soil, saturated with
water and allowed to equilibrate to field capacity. Each o f the four species were seeded m
individual pots. Five seeds o f each individual specie; were broadcast on the soil surface
and arranged in a uniform distribution. A small amount (<2 mm depth) o f dry soil was
used to cover the seeds. The soil surface was covered with plastic and moistened with a
17
fog mist until seedlings emerged. Upon emergence, plants were thinned to a single
individual plant. Pots were randomly placed in a growth chamber. The experiment was
replicated three times.
Sampling. Each pot was harvested for plant biomass every 12 days over 60 days.
Above-ground biomass was harvested by clipping plants at the soil surface. Plants were
dried (60 C, 48 hr) to a constant weight and weighed.
Analysis. Linear regression was used to quantify the absolute growth rate o f each
species. Regression lines were compared using the extra sums o f squares procedure
(Snedecor and Cochran 1980).
Results
Nutrients
Soil Nitrogen and Phosphorus. Analysis o f variance indicated a difference in
residual soil NO 3 and P (p<0.05) among treatments imposed on the addition series o f
spotted knapweed and bluebunch wheatgrass (Table I). Available soil N was 0.40 mg N
kg'1 soil in the control addition series. Annual rye, P, and bottlebrush squirreltail
decreased available N to 0.11, 0.18, 0.31 mg N kg'1 o f soil, respectively. N addition
increased final N levels in the soil (0.64 mg N kg'1). Soil P was 21.67 mg P kg'1 in the
control. P addition increased soil P to about 143 mg P kg"1- N o other treatments affected
soil P levels.
18
Nitrogen and Phosphorus Uptake by Bluebunch Wheatgrass. N and P uptake by
bluebunch wheatgrass differed among treatments (p<0.05). Without nutrient manipulation,
N uptake by bluebunch wheatgrass was 10.5 mg N o f plant material per pot (Table 2).
Table I. Residual soil N and P content following the plant growth study.
Treatment
Control
Nitrogen
Phosphorus
Squirreltail
Rye
L S D (0.05)
Soil P
SoilN O 3-N
(mg/kg)(standard deviation)
0.40
(± 0.064)
0.64
(+ 0 .0 6 2 )
0.18
(+ 0.061)
0.31
(± 0.061)
0.11
(± 0.060)
0.09
(mg/kg)(standard deviation)
2 1 .7
(±13.43)
9.0 (±13.10)
143.0
(±12.80)
12.7
(± 12.80)
9.8
(±12.57)
18.3
N uptake was unaffected by addition o f N or P. Bottlebrush squirreltail and annual rye
decreased N uptake by bluebunch wheatgrass when grown in the addition series mixture.
Annual rye reduced N uptake 5 times more than did bottlebrush squirreltail. P uptake by
bluebunch wheatgrass in the control was 0.71 mg P o f plant material per pot, which was
similar to the N and bottlebrush squirreltail nutrient manipulations. P application
dramatically increased uptake to 1.63 mg P o f plant material per pot. Annual rye
decreased P uptake to the lowest level by bluebunch wheatgrass at 0.12 mg o f plant
material per pot.
19
Nitrogen and Phosphorus Uptake bv Spotted Knapweed.
N uptake by spotted
knapweed ranged from 1.43 to 6.42 mg N o f plant material per pot (Table 2). N or P
addition did not affect N uptake by knapweed as compared to the control. Bottlebrush
squirreltail increased N uptake by spotted knapweed, while annual rye decreased uptake o f
N in spotted knapweed more than all other treatments. Annual rye and bottlebrush
squirreltail dramatically decreased P uptake by knapweed. N addition did not alter P
uptake by spotted knapweed, while P addition increased P uptake nearly 3-fold that o f the
control.
Table 2. Nitrogen and phosphorus uptake by bluebunch wheatgrass and spotted
knapweed.
■
-------------------------------------------------------—---------!----- :------------------------ v
_________ Nitrogen_________
________ Phosphorus________
Treatment -
Bluebunch Wheatgrass Spotted Knapweed Bluebunch Wheatgrass Spotted knapweed
Control
(mg/kg)
10.49 (±1.16) 4.49 (±0.90)
0.71 (±0.12)
(mg/kg)
0.19 (±0.04)
Nitrogen
' 10.22 (±1.20)'5.49 (±0.94)
0.65 (±0.12)
0.14 (±0.04)
11.49 (±1.10) 3.70 (±0.87)
1.63 (±0.11)
0.54 (±0.04)
6.34 (±1.16)
6.42 (±0.74)
0.63 (±0.12)
0.01 (±0.04)
1.26 (±2.14)
1.43 (±0.98)
0.12 (±0.22) <0.01 (±0.05)
Phosphorus
Squirreltail
Rye
LSD (0.05)
.
1.76
1.24
0.18
0.06
20
Interference
Bluebunch Wheatgrass Biomass. Nonlinear regression coefficient predicting
bluebunch wheatgrass mean plant biomass from spotted knapweed and bluebunch
wheatgrass densities indicated that 0.17 spotted knapweed plants was competitively
equivalent to 1.0 bluebunch wheatgrass when no addition o f nutrients or other species
were included in the experiment (Table 3). N addition to the system had no effect on the
equivalence o f spotted knapweed and bluebunch wheatgrass in determining individual
bluebunch wheatgrass biomass. P addition or a background o f bottlebrush squirreltail
reduced the equivalence influence o f spotted knapweed with bluebunch wheatgrass when
predicting individual bluebunch wheatgrass biomass. Annual rye shifted the competitive
advantage from spotted knapweed to bluebunch wheatgrass where it to o k 9.9 units o f
spotted knapweed to have an equivalent impact as 1.0 unit o f bluebunch wheatgrass.
Based on R2, competition became more important in the annual rye (R2= 0.40) and
bottlebrush squirreltail (R2=0.33) treatments than the other treatments (Weldon and
Slauson 1986).
Spotted Knanweed Biomass. It took 0.38 bluebunch wheatgrass plants to have an
equivalent impact on spotted knapweed mean individual plant biomass in the control
addition series where no N or P were added and no other species were added as
background (Table 4). Addition o f N, P, bottlebrush squirreltail, or annual rye did not
alter the competitive balance from that o f the control. R2 for the nonlinear regression
were relatively low in all treatments. However, the system with the P addition showed a
21
trend o f increasing the equivalence impact o f bluebunch wheatgrass biomass on spotted
knapweed and increased the importance (R2) o f competition.
Table 3. Equivalence ratios generated from the prediction o f bluebunch wheatgrass
biomass.
T reatm ent
Equivalence ratio
Confidence In terv al
R2
Control
0.17
0 .0 3 -0 .3 0
0.13
Nitrogen
0.14
0.03 - 0.24
0.12
Phosphorus
0.64 '
0.21 - 1.06
0.07
Squirreltail
0.82
0 .2 7 -1 .3 6
0.33
Rye
9.90
2.57 - 17.4
0.40
Table 4. Equivalence ratios generated from the prediction o f spotted knapweed.
T reatm en t
Equivalence ratio
Confidence In te rv a l
R2
Control
0.38
0 .1 7 -0 .5 7
0.10
Nitrogen
0.23
0.07 - 0.39
0.05
Phosphorus
0.54
0 .1 0 -0 .9 7
0.27
Squirreltail
0.59
0 .1 0 -1 .0
0.13
Rye
0.85
-1.05-2.75
0.07
22
Growth Rate o f Isolated Individuals
Linear regression equations determining change in plant weight over time indicate
that all species grew at different rates when compared using the extra sums o f squares
procedure (Figure I). Annual rye had the fastest growth rate, followed by bluebunch
wheatgrass, then spotted knapweed, and lastly, bottlebrush squirreltail.
Discussion
Many members o f the knapweed genus are reported to grow rapidly (Sheley and
Larson 1994, 1995; Velagala e ta l, 1997), whereas late-seral species, such as bluebunch wheatgrass, typically have slower growth rates, but are more competitive (Gnme 1979).
Species with high intrinsic growth rates are generally favored in high N environments (Heil
and Bruggink 1987; Aerts and Benende 1988). In this study, bluebunch wheatgrass
seedlings grew faster than spotted knapweed seedlings. Furthermore, N uptake was
unaffected by N addition. We believe this was because o f the low N level in the soil, which
was collected from a bluebunch wheatgrass/Idaho fescue community type dominated by
late-seral species. Soil low in N may not support the high production requirements of
early-seral species, thus the species with the lower nutrient requirement should have a
more rapid growth rate (Grime 1979; M cGraw and Chapin 1989). It is also possible that
the growth rate o f seedlings is different than that o f mature plants.
N is a major determinant o f successional patterns in many ecosystems (Parrish and
Bazzaz 1982; Tilman 1984, 1986; Wedin and Tilman 1996). Plant community shifts are
attributed to changes in the competitive relationships among species as the level o f N
--- # ----
bolllebrush squlrrellall
knapweed
whealgrass
rye
T
>3 \
Y
1.4 -
Y
»
E
CO
1.0
-
CO
CO
♦
rd
E
o
a
0.G "
0.2
-
0.2
W
U)
CD
-
-
_ _ _ _ _ _ _ _ _ I_ _ _ _ _ _ _ _ _ _ _ _ ,_ _ _ _ _ _ _ _ _ _ _ _ I_ _ _ _ _ _ _ _ _ _ _ _ ,_ _ _ _ _ _ _ _ _ _ _ _ I_ _ _ _ _ _ _ _ _ _ _ _ i- - - - - - - - - - - - - - - - - - - 1- - - - - - - - - - - - - - - - - - - '- - - - - - - - - - - - - - - - - - - - L
1
2
3
Harvest Increments
Figure I.
Growth rate o f isolated individuals
4
5
availability changes (Parrish and Bazzaz 1982; Tilraan 1982, 1986, McLendon and
Redente 1991; Miller and W erner 1987; Carson and Barrett 1988). It has been shown that
increasing N availability facilitates establishment and persistence o f species with high
growth rates such as early-seral weed species (Bernese and Aerts 1984; Heil and Bruggink
1987). N amendments o f 56 kg N ha'1, 112 kg N ha \ and 224 kg N ha favored spotted
knapweed in field trials in Montana (Story et a l, 1989). In our study, increasing N did not
alter the competitive relationship between spotted knapweed and bluebunch wheatgrass.
Either the soil was so low in N that adding low amounts o f N may not have been sufficient
r
to alter the competitive relationship between the species, or N was not limiting. Leaching
was not a factor in soil available N because pots were not watered after emergence.
Addition o f P decreased soil N below that o f the control. Reduction o f available N
by P may be explained using the resource-ratio hypothesis. N and P are essential
resources for these plants, with their growth rates determined solely by the one resource in
lower supply relative to their requirements (Tilman 1980, 1982). In this case, if P is the
most limited resource it would limit growth in unamended soil. W e believe addition o f P
allowed increased plant growth which, in turn, increased N uptake and decreased N
availability at the end o f the study. Reduction o f available N by P shifted the competitive
balance in favor o f bluebunch wheatgrass.
Species composition can directly influence nutrient dynamics. Wedin and Tilman
(1993) studied a range o f species and found that the later the successional niche, the more
competitive the species was for N. Late-seral species had high below-ground biomass.
This creates soils with high C:N and low N mineralization. In this study, addition o f the
25
mid-seral bottlebrush squirreltail reduced the N availability and correspondingly reduced
the effect o f spotted knapweed on bluebunch wheatgrass (Table 2). It is possible that midseral species persist by maintaining rapid growth rates and high seed production and begin
to lower the availability o f N in the root zone o f surrounding species (Tilman 1991). This
would create an environment that facilitates the establishment o f late-seral species like
bluebunch wheatgrass (Tilman 1991).
In restoration o f spotted knapweed infested rangeland, establishment o f late-seral
species is often difficult because o f weed competition during the establishment phase
(Velagala et a l, 1996). Sheley et al (1996) proposed that it may be possible to use
species with demonstrated abilities to sequester N, such as annual rye, to reduce resource
availability. In this study, annual rye dramatically lowered N and shifted the competitive
relationship from spotted knapweed to bluebunch wheatgrass. We believe that bluebunch
wheatgrass has a lower N requirement than spotted knapweed and, therefore, has the
ability to out-compete spotted knapweed at lower N levels (Tilman 1996). The shift in
competitive balance from spotted knapweed to bluebunch wheatgrass when N levels were
reduced is consistent with McLendon and Redente (1992) who demonstrated that
reductions in N availability by adding sucrose accelerated succession in a sagebrush steppe
site in northwestern Colorado.
Results for the prediction o f spotted knapweed biomass indicate that no treatments
effected the competitive ability o f spotted knapweed. Low R2 for the nonlinear regression
indicate that competition under these treatments was not an important factor.
26
Our study provides initial evidence supporting the theory that nutrient levels can be
altered to accelerate successional change from a weedy plant community toward a desired
plant community. Ephemeral cover crops or mid-seral species could be used in restoration
projects to lower N availability. Lower N availability could accelerate the establishment
and domination o f late-seral species over earlier successional weedy species. Because the
growth chamber experiment studied seedlings that were not grown to reproduction, these
results need to be tested in a long-term field study.
27
CHAPTER 3
DIRECTING SUCCESSION BY ALTERING NUTRIENT AVAILABILITY
Introduction
Over the past several decades, rangeland managers have focused weed
management on controlling weeds, with limited regard for existing or resulting plant
communities. While the appropriateness and effectiveness o f weed management practices
are being questioned, it is becoming increasingly clear that weed management decisions
must consider ecological, environmental, and economic principles. The development o f
future rangeland weed management practices must be based on our understanding o f the
biology and ecology o f rangeland ecosystems (MacMahon 1987; Allen and Allen 1988;
Call and Roundy 1991; Pyke and Archer 1991).
Ecologically-based weed management will require scientists and managers to
develop strategies based on an understanding o f succession. Successional weed
management depends on the principle that plant communities are dynamic and technology
can be used to enhance natural processes and mechanisms that regulate vegetation change
(Sheley et a l, 1996). Ultimately, the goal is to direct weed-infested communities along a
trajectory toward more desirable plant communities.
Resource availability may be one factor that can be regulated in successional
management. In some cases, changes in plant communities are related to resource
availability, and the relative ability o f the species to use those resources at particular levels
28
(Sheley and Larson 1996). To quote Tilman (1988), "Because each plant species is
. constrained to being a superior competitor for particular resource levels, the forces that
determine resources are critically important in determining vegetation pattern".
Researchers generally agree that the availability o f soil nutrients change during
succession, but there is a lack o f agreement on the direction and significance o f such
changes. One line o f argument suggests that the availability o f all major resources (e.g.
light, water, nutrients) is elevated at the soil surface shortly after disturbance, and that
succession thereafter involves a diminution in resource availability (Grime 1979). For
example, McLendon and Redente (1991) demonstrated that additions o f N inhibited
succession from annual to perennial species in a sagebrush steppe site in northwestern
Colorado. They concluded that dominance by annuals during the early stages o f secondary
succession was related to high nutrient availability. Similarly, Story et al. (1989) and
Carter et al. (1999) found that increased nitrogen favored spotted knapweed {Centaurea
maculosa Lam.). However, Myers and Berube (1983) found no effect o f nitrogen on
diffuse knapweed {Centaurea diffusa L.) on a dry site in Canada. Because little is known
about the ecological role and successional status o f perennial non-indigenous weeds in
assemblages o f native plants, directing and predicting successional trajectories based on
resource availability will not be possible until more information is available.
The potential for altering nutrient availability to direct succession has not been
adequately explored. M ost o f the emphasis in nutrient management has been on increasing
availability. However, various crops and cropping systems have been used to reduce
nitrate leaching on agricultural fields (Chaubey et al., 1994; Coyne et al., 1995). Tilman
29
and Wedin (1991) studied a range o f species with different successional niches and found
late-seral species were very competitive for nitrogen. Late-seral species had high below­
ground biomass and created soils with high C:N, resulting in low nitrogen mineralization.
Early-seral species were poor competitors for nitrogen, but persisted by maintaining rapid
growth rates and high seed production. When grown in pairwise competition experiments,
late-seral species displaced early to mid-seral species (Tilman and Wedin 1991). The
ability o f a species to lower plant available ammonium and nitrate accounted for the results
o f the competition trials. It may be possible to use species with demonstrated abilities to
sequester nitrogen, such as annual rye (Secale cereal L.) or mid-seral species to reduce
resource availability.
Resource additions or extractions may be used to direct succession. The overall
objective o f this study was to determine the potential for directing weed dominated plant
communities along a successional trajectory toward more desirable communities by
altering resource availability. The specific objective was to determine the effects of
nitrogen (N), annual rye, and bottlebrush squirreltail (Sitanion hystrix (Nutt.) I. G. Smith)
on the establishment and growth o f spotted knapweed and the late-seral species,
bluebunch wheatgrass (Pseudoroegneria spicatum [Pursh.] Scribn and Smith). Annual rye
and bottlebrush squirreltail, considered a mid-seral species, were seeded in an attempt to
reduce the availability o f nutrients. We hypothesized that annual rye and bottlebrush
V
squirreltail would favor the establishment and growth o f bluebunch wheatgrass and have a
negative impact on spotted knapweed.
30
Materials and Methods
Study Sites
The study was conducted during 1996, 1997 at site I and 1996, 1997, and 1998 at
site 2. Site I was located 25 km southwest o f Bozeman, Mont. (45°36"N, 1110T l 1W) at
an elevation o f 1524 m. Site 2 was located 11 km east-northeast o f Hamilton, Mont. (46°
17' N, 114° I' W) at an elevation o f 1341 m. Site I lies on a Festuca
idahoensis/Agropyron spicatum habitat type and site 2 lies on a Festuca
scabrella/Agropyron spicatum habitat type (Mueggler and Stewart 1980). Soil at site I is
a loamy-skeletal over sandy or sandy skeletal mixed typic Argiboroll. Site 2 soil is a
Stecum stony-loamy coarse sand, moderately steep mixed typic Cryothents. Both sites
were dominated by spotted knapweed with an understory o f downy brome (Bromus
tectorum L.) with few forbs present. Annual precipitation at both sites ranges from 406 to
457 mm. M ean annual temperature at site I is 6 .1 0C and 6.6 0C at site 2.
Procedures
Treatments were N, annual rye, bottlebrush squirreltail, and bluebunch wheatgrass
arranged in an incomplete factorial arrangement (2 X 2 X 2 X 2). Each treatment was
applied as either with or without that treatment including a combination without any
treatment (control). All bottlebrush squirreltail by annual rye combinations were omitted.
Treatments were arranged in a completely-randomized-design and replicated 4 times at
each site.
In the fall o f 1996, both sites were disced to a depth o f 25 cm, which did not kill
existing plants. Each site was divided into 48, 2 by 3 m plots. Seeds were broadcast, ■
31
raked, compacted with a roller, and lightly watered once in the fall o f 1996 to enhance
seed germination. AU species were seeded at rate o f 32 kg ha"1 to ensure establishment. N
(31 kg ha'1) was broadcast via a hand-spreader at a rate o f 67 kg ha'1 (46-0-0; N-P-K).
Sampling
Spotted knapweed, annual rye, bottlebrush squirreltaU, and bluebunch wheatgrass
density and cover were measured at peak standing crop (July) o f 1997 using a 0.445 m'2
circular hoop on both sites. Because site I was inadvertently destroyed by the landowner,
the same data, plus biomass, were only coUected on site 2 in 1998 at peak standing crop.
Harvesting included clipping all plants within a single randomly placed hoop by species to
ground level in each plot. Biomass samples were dried to a constant weight (60° C) and
weighed.
Analysis
Each site was analyzed separately using analysis o f variance. N, annual rye,
bottlebrush squirreltaU, bluebunch wheatgrass, and aU interactions (except interactions
with annual rye by bottlebrush squirreltaU) were included in the model, and tested using
the residual error term. When a significant F-test (P<0.05) was calculated, differences
among means were tested using Fishers protected least significant difference LSD (0.05)
comparisons (Peterson 1985). In cases where low numbers caused lack o f variance
homogeneity (i.e. annual rye and bottlebrush squirreltaU), main effects (N, annual rye,
bottlebrush squirreltaU, and bluebunch wheatgrass) were tested using standard t-tests.
32
Results
Spotted Knapweed
Density. Spotted knapweed density was unaffected by any treatment at site I in
1997 (Table 5). At site 2, bluebunch wheatgrass decreased spotted knapweed by 22.2
plants m'2 from that o f the control (73.7 plants mf2; P=O.02) in 1997. By 1998, N and
annual rye interacted to affect spotted knapweed density at this site (Table 5). Treatments
without annual rye and/or N produced 173 plants m'2 (Figure 2). Adding N alone did not
affect spotted knapweed density. Adding annual rye reduced mean spotted knapweed
density to 46 plants m"2, which was below that o f all other treatments. Adding both N and
annual rye increased spotted knapweed density over the annual rye treatment.
Cover. Annual rye, bluebunch wheatgrass, and N also interacted to affect spotted
knapweed cover at site I in 1997 (Table 5). Within the three way interaction, (N by annual
rye by bluebunch wheatgrass), N plus annual rye produced the lowest spotted knapweed
cover (Figure 3). N, bluebunch wheatgrass, and annual rye combined had the highest
spotted knapweed cover but was similar to bluebunch wheatgrass and to N treatments, and
N plus bluebunch wheatgrass treatments. Bluebunch wheatgrass produced similar spotted
knapweed cover to that o f N. Without TM, bluebunch wheatgrass increased spotted
knapweed cover.
Table 5. P-valucs for spotted knapweed density, cover, and biomass (1998) as the dependant variables generated from A N O V A.
Source
Site I
1997
Density
df
Site 2
Cover
Density
Cover
Density
1998
Cover
(P-value)
(P-value)
(P-value)
(P-value)
(P-value)
(P-value)
(P-value)
1997
Biomass
N
I
0.67
0.41
0.58
0.07
0.15
0.85
0.25
Rye
I
0.68
0.17
0.65
0.34
0.37
0.85
0.85
Squirreltail
I
0.37
0.15
0.13
0.20
0.37
0.40
0.83
Wheatgrass
I
0.63
0.02
0.02
0.36
0.25
0.56
0.48
Rye+N
I
0.63
0.96
0.75
0.37
0.03
0.06
0.01
Squirreltail+N I
0.16
0.49
0.96
0.64
0.12
0.42
0.01
W beatgrass+N l
0.97
0.13
0.90
0.57
0.95
0.24
0.01
UJ
UJ
Squirreltail+
Wheatgrass
I
0.60
0.01
0.56
0.62
0.46
0.68
0.97
Rye+
Wheatgrass
I
0.60
0.15
0.16
0.48
0.60
0.94
0.18
Rye+
I
Wheatgrass+N
0.15
0.01
0.31
0.42
0.99
0.32
0.03
Squirreltail+ I
Wheatgrass+N
0.66
0.06
0.53
0.93
0.47
0.88
0.50
34
Spotted knapweed (plants m"2)
400
r
IA a a Al
W ithout ann u al rye
W ith annual rye
300 -
200
-
100
w itho u t nitro gen
w ith nitro gen
F ig u re 2.
Interaction o f annual rye and N on spotted knapweed density at site 2 in 1998
35
Spotted knapweed (percent cover)
a. W ith o u tN itro g en
fTTTH W ithout w heatgrass
I
I W ith w heatgntss
L5D=0.19
w ith o u t a n n u al rye
w ith a n n u a l rye
b W ith N itrogen
o
0 0.7
IX x">n W ith o u t w heatgrass
I
I W ith w heatgrass
1 0.6
LSD=O-IS
CJ-
E 0-5
'SO 0.4
^C3 0.3
5
^
0.2
O
O 0.1
EU
czo
0.0
w ithout a n n u a l rye
w ith a n n u al rye
Figure 3.
Interaction o f annual rye, N, and bluebunch wheatgrass on spotted
knapweed cover at site I in 1997
36
Bottlebrush squirreltail and bluebunch wheatgrass interacted to affect spotted
knapweed cover at site I in 1997 (Table 5). Treatments without bottlebrush squirreltail
produced similar spotted knapweed cover as those including bluebunch wheatgrass (Figure
4). Adding bottlebrush squirreltail alone reduced spotted knapweed cover by approximately
20% below all other treatments; however, the effect o f bottlebrush squirreltail was
removed in the presence o f bluebunch wheatgrass. In 1997 and 1998, spotted knapweed
cover was not affected by any treatment at site 2 (Table 5).
Biomass. Annual rye, bluebunch wheatgrass, and N interacted to determine spotted
knapweed biomass at site 2 in 1998 (Table 5). Without these treatments, mean spotted
knapweed biomass was about 144 g m"2 (Figure 5). Adding both bluebunch wheatgrass
and N reduced spotted knapweed biomass to about 86 g m"2. Adding annual rye and
bluebunch wheatgrass yielded the lowest spotted knapweed biomass (49 g m"2). Including
all three (annual rye, bluebunch wheatgrass, and N ) yielded the highest spotted knapweed
biomass. Within the three way interaction, (annual rye, bluebunch wheatgrass, and N), the
addition o f n ) bluebunch wheatgrass, or annual rye alone' had no effect bn spotted
knapweed biomass.
37
FV3 W ithout sq u irreltail
I I W ith squirreltail
0.5 r
ti
'
>
o
o
-M
e
a
u
<u
CL/
-—
rS
o
O
LSD=O-Il
0.4
<u
CL
O
0.3
0.2
"
0.1
CL
CZl
0.0
w ithout w heatgrass
w ith w heatgrass
Figure 4.
Interaction o f bluebunch wheatgrass and bottlebrush squirreltail on
spotted knapweed cover at site I in 1997
33
a. W ith o u t N itrogen
I W ith o u tw h e a te ra ss
250 r
W ith w heatgrass
w ith o u t a n n u al rye w ith a n n u a l rye
b. w ith N itrogen
250 r
w ith o u t a n n u a l rye
F igure 5.
w ith a n n u al rye
Interaction o f annual rye, N, and bluebunch wheatgrass on spotted
knapweed biomass at site 2 in 1988
39
Bluebunch Wheatgrass
Density. Bluebunch wheatgrass successfully established at site I and site 2 by 1997
(Table 6). Its density was 37 plants m"2 at site I. At site 2, adding N to those plots with
bluebunch wheatgrass increased density from 10 to about 25 plants m"2 (Table 6; Figure 6).
In 1998, bluebunch wheatgrass increased its density from zero (without bluebunch
wheatgrass) to 26 plants m"2 at site 2 (Table 6).
Cover. Plots receiving bluebunch wheatgrass had twice the bluebunch wheatgrass
cover (31%) than those without the seeding (15%) at site I in 1997 (Table 6). Bluebunch
wheatgrass cover interacted with N to affect bluebunch wheatgrass cover at site 2 in 1997
(P=0.04). Seeding bluebunch wheatgrass increased its cover from 0.0 to 0.6 percent
(LSD=O.004). At site 2 in 1998, adding N increased bluebunch wheatgrass cover to 1.2
percent. Bluebunch wheatgrass cover was unaffected by any treatment (Table 6).
Biomass. N and bluebunch wheatgrass interacted to affect bluebunch wheatgrass
biomass at site 2 in 1998 (Table 6). Including bluebunch wheatgrass increased its biomass
to 3.2 g m"2(Figure 7). AddingN with bluebunch wheatgrass more than doubled
bluebunch wheatgrass biomass.
Bottlebrush Squirreltail
Density. Including bottlebrush squirreltail among treatments resulted in 5.6
(P=0.0001) and 0.4 (P=0.0001) plants m'2 at site I and site 2, respectively, in 1997.
Including N with bottlebrush squirreltail increased its density to 9.9 plants m"2 (P=0.002) at
Table 6. P-vakies For bluebunch wheatgrass density, cover, and biomass (1998) as the dependant variables generated from ANOVA
Source
Site I
1997
df
Density
Site 2
1997
Cover
Density
(Pvalwe)
(Pvalue)
(Pvalue)
Density
1998
Cover
Biomass
(Pvalue)
(Pvalue)
( I 'value)
(Pvalue)
Cover
N
I
0.31
0.74
0.02
0.04
0.69
0.82
0.05
Rye
I
0.14
0.19
0.32
0.21
0.82
0.36
0.39
Squirreltail
I
0.58
0.42
0.26
0.40
0.85
0.42
0.14
Wheatgrass
I
0.01
0.01
i
0.01
0.01
0.47
0.01
Rye*N
I
0.27
0.70
0.06
0.67
0.85
0.22
0.67
Squirreltail+N I
0.87
1.00
0.26
1.00
0.64
0.26
0.34
Wheatgrass+N l
0.31
0.75
0.03
0.04
0.72
0.84
0.05
Squirreltail+
Wheatgrass
I
0.58
0.42
0.37
0.40
0.87
0.41
0.14
Rye*
Wheatgrass
I
0.14
0.19
0.43
0.21
0.79
0.34
0.39
Rye+
I
W heatgrass+N
0.27
0.70
0.07
0.67
0.82
0.23
0.67
1.00
0.29
1.00
0.87
Squirreltail+ I
W heatgrass+N
1variances were heterogenious, therefore main effect were analyzed using t-tests
0.62
0.27
0.34
41
Without nitrogen
I With nitrogen
I
cn
5
CL
VJ
VJ
c;
u
S1
CS
C
Bluebu
S
witho ut wheatgrass
with wheatgrasn
Fioure 6. Interaction o f bluebunch wheatgrass and N on bluebunch
wheatgrass density at site 2 in 1997
I
r t
I' W ith nitrogen
W ithout nitrogen
A A Al
LSD=3.10
w ithout w heatgrasi
Figure 7.
w ith w h e n tg m a j
Interaction o f bluebunch wheatgrass and N on bluebunch
wheatgrass at site 2 in 1998
42
site I. Including bluebunch wheatgrass with bottlebrush squirreltail increased bottlebrush
squirreltail to 15.6 plants m'2. At site 2 including bottlebrush squirreltail among treatments
resulted in 22 plants m"2 in 1998 (P=0.02). N o other treatment affected bottlebrush
squirreltail density.
Cover and Biomass. Based on individual t-tests comparing main effects o f
treatments, no treatment affected percent cover at either site or year. Addition o f
bottlebrush squirreltail did not affect its biomass (0.02 to 0.35 g m"2) at site 2 in 1998
(P=0.003; L SD =I.2). Addition o f bottlebrush squirreltail plus N increased bottlebrush
squirreltail biomass from 0.35 to 2.9 g m"2 (P=0.04).
Annual Rye
Density, In 1997, seeding annual rye increased rye density from 0.0 plants m"2to
22.7 plants m"2 (P=0.0008) and from 0.6 plants m"2 to 11.3 m"2 (P=0.034; main effect) at
site I and site 2, respectively. Addition o f bluebunch wheatgrass at site I increased annual
rye from 22.7 plants m"2 to 60.4 plants m"2 (P=0.0008). No treatments affected annual rye
at site 2 in 1998.
Cover and Biomass. Based on individual t-tests comparing main effects o f
treatments, no treatment affected percent cover at either site or year. N o treatment affected
annual rye biomass in 1998.
43
Discussion
Previous research has indicated that spotted knapweed may excel in the presence o f
high N concentrations (Papova 1960; Story et al, 1989). Spotted knapweed has the earlyseral characteristics o f rapid growth rates, high seed production, high above ground
biomass, and enhanced establishment and growth in high nutrient environments.
Story et
al. (1989) found that increasing N significantly increased spotted knapweed biomass.
McLendon and Redente (1991) found that dominance o f a site by annuals in early stages of
secondary succession was related to high nutrient availability. In addition, McLendon and
Redente (1992) tested the supply o f N on plant community dynamics and found N
incorporation in perennial plant tissue was a primary mechanism in controlling the rate o f
succession. In our study, annual rye decreased spotted knapweed density and biomass,
when combined with bluebunch wheatgrass, at site 2 in 1998. In addition, spotted
knapweed cover was decreased 20% below all other treatments by seeding bottlebrush
squirreltail at site I in 1997; however, spotted knapweed was unaffected by bottlebrush
squirreltail at site 2. Although these responses could result from competition for various
factors (e.g. light, moisture, nutrients), annual rye and bottlebrush squirreltail may have
affected spotted knapweed by removing available N from the rooting zone. In a growth
chamber experiment, annual rye and bottlebrush squirreltail reduced available soil N below
that o f other treatments (Herron et al., 1998). This supports the hypothesis that annual rye
and bottlebrush squirreltail can sequester N and make it less available to spotted
knapweed.
44
Addition o f N and annual rye increased spotted knapweed density over that o f
annual rye at site 2 in 1998. Addition o f N may have negated the N depleted conditions
produced by annual rye. Species, such as annual rye with higher potential growth rates are
favored over those with slower growth rates under conditions o f high N availability
(Berendse et ah, 1987; Heil and Bruggink 1987; Aerts and Berendse 1988).
In many cases, including bluebunch wheatgrass enhanced spotted knapweed
establishment and growth, and adding N exacerbated the response. Whitford (1986) found
that both soil moisture and nutrients are often higher under a plant canopy than between
canopies or in vegetation gaps in dry environments. Many other researchers have found a
form o f facilitation, rather than competition, resulting in higher biomass under plant
canopies (Muller 1953; Halvorson and Patten 1975; Parker et ah, 1982). It appeared that
bluebunch wheatgrass enhanced spotted knapweed by altering the microenvironment to
provide increased safe-sites for spotted knapweed.
It was not surprising that bluebunch wheatgrass and bottlebrush squirreltail density
were increased with the addition o f N, where they were seeded. Conversely, addition o f N
did not affect spotted knapweed. In our study, about 31 kg ha"1 o f N were applied. We
believe that low application only enhanced newly seeded species with young root systems
by remaining in the upper portion o f the soil profile. In addition, the winter-annual,
cheatgrass, may have had a temporal advantage in sequestering N. They may have captured
the applied N and preempted its use by the deeper rooted, spotted knapweed (Sheley et ah,
1993). These results differ from previous research o f Story et ah (1989) and Carter et ah
(1999); however, neither study included newly seeded species simultaneously with N
application.
45
The most important finding o f this study is that bottlebrush squirreltail and annual
rye had some negative effects on spotted knapweed without affecting bluebunch
wheatgrass. I f the impacts on spotted knapweed persist, they may confer an advantage to
bluebunch wheatgrass in the future.
Therefore, we believe that using either cover crops that sequester N or mid-seral
species to facilitate the establishment o f late-seral species when revegetating weed infested
rangeland has merit. Our results suggest that further work on altering nutrient availability
to direct succession away from weed dominance is warranted.
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54
A PPEN D IC ES
A ppendix A
SPO TTED K N A PW EED AND B LU EBU N CH W B EA TG B A SS D EN SITY , C O V ER ,
AND B IO M A SS M EANS G EN ERA TED F O R T R E A T M E N T M A IN E FFEC T S.
Table 7. Spotted knapweed means for density, cover, and biomass generated for treatment main effects.
Site I___________________ ________________________________—------- Site_2-------------------1997___________________
Source
df
1997
_______________1998
Density
Cover
Density
Cover
Density
Cover
Biomass
(P la n ts m '1)
(P ercen t)
(P lants m"'2)
(Percent)
(P lants m '2)
(P ercen t)
(G ra in s m 1)
N
I
109.53
0.25
60.18
0.22
168.42
0.30
118.51
Rye
I
101.23
0.20
60.53
0.16
121.55
0.30
109.35
Squirreltail
I
96.33
0.19
55.61
0.16
121.01
0.28
112.09
Wheatgrass
I
110.24
0.30
51.50
0.16
116.71
0 29
105.84
Table 8. Bluebimch wheatgrass means for density, cover, and biomass generated for treatment main effects.
Cover
Density
Cover
Density
Cover
Biomass
Density
(Percent)
(G ra m s m '1)
(P lants m"1)
(P la n ts m"’ )
(P la n ts m"2)
(P ercen t)
(P ercen t)
0.01
0.22
15.54
N
0.25
4.16
22.70
0.03
I
0.01
0.16
13.35
Rye
0.01
3.42
24.62
0.01
I
0.01
0.16
0.01
Squirreltail
0.01
3 85
16.46
14.86
I
0.01
0.16
28.20
0.04
I
0.01
5.78
37.31
Source
Wheatgrass
If
58
Appendix B
RESPONSE SURFACES FOR THE PREDICTION OF BLUEBUNCH
W HEATGRASS BIOMASS
59
F igure 8.
Response surface using bluebunch w heatgrass and spotted knapw eed
atgrass biomass (g plant"1) (Wbb)
density to predict bluebunch w heatgrass biomass - control tre a tm e n t
Wbb=O.37(1+0.225(Nbb+0.17*Nsk))',-0.627
60
F igure 9.
Response surface using bluebunch w heatgrass and spotted knapw eed
density to predict bluebunch w heatgrass biomass - an n u al rye
(g plant"1) (Wbb)
treatm ent.
J=
61
APPENDIX C
RESPONSE SURFACES FOR THE PREDICTION OF SPOTTED KNAPWEED
BIOM ASS
62
Figure 10.
CM
W sk=O.106(1+0.225(Nsk+0.3758*Nbb))'
5/1
€5
O
C-
(g plant"1) (Wsk)
ted knapweed bio
5
GQ
Response surface using bluebunch wheatgrass and spotted
knapweed density to predict spotted knapweed biomass - control
treatment.
63
Figure 11.
Response surface using bluebunch w heatgrass and spotted knapw eed
density to pred ict spotted knapw eed biomass - b o ttleb ru sh sqm rreltail
tre a tm e n t
eg
Cl
64
Figure 12.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed biomass - annual rye treatment.
W sk=0.0057(1+0.225(Nsk+0.02 t Nbb))'0137
eg
CL
(g plant"1) (Wsk)
tted knapweed bio
r;
65
F igure 13.
Response surface using b luebunch w heatgrass and spotted knapw eed
density to predict spotted knapw eed biomass - nitrogen tre a tm e n t
66
Figure 14.
Response surface using bluebunch wheatgrass and spotted knapweed
density to predict spotted knapweed biomass - phosphorus treatment.
Wsk=O. 195(l+0.225(Nsk+0.5368*Nbb)) - 1.304
on
S
.2
3
S .
CO
"3
CJ
a
>
CL
CS
C
■3S 3eC"3
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M
ONTANA STATE UNIVERSITY - BOZEM
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