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ECOLOGICAL RELATIONSHIPS
BETWEEN YELLOW STARTHISTLE
AND CHEATGRASS
Larry L. Larson
Roger L. Sheley
community dynamics (Harper 1977). Hironaka (1990) proposed a successional pattern for rangelands favoring occupation by species that exhibit a winter annual strategy. He
suggests that community dynamics, following rangeland
deterioration, would favor later maturing winter annuals
that would produce greater numbers of seed and ultimately
dominate the site. The objective of this paper is to describe
preliminary results from life history research on yellow
starthistle and cheatgrass grown in association.
ABSTRACT
Ecological relationships between alien annual weed species are dominated by reproduction and resource allocation strategies. The reproductive strategy of cheatgrass
(Bromus tectorum) relies on a large number ofplants with
fewer large seeds per plant .and a concentrated seed drop.
In contrast, starthistle (Centaurea solstilialis) relies on
fewer plants, greater seed production, and two distinct
periods of seed drop. Annual fluctuations in community
dynamics between these two species are largely influenced
by their compatibility with annual climatic patterns.
LIFE IDSTORY STRATEGY
The study site was located 14 km west ofWalla Walla,
WA The area receives an annual precipitation of 229305 mm and is dominated by Walla Walla silt loam soil
(coarse-silty, mixed, mesic Typic Haploxeroll).
The study was initiated in June 1991 in a vegetation community dominated by cheatgrass and starthistle. Sampling
occurred at 2-week intervals and was continued through
two complete life cycles. The sampling strategy followed
the generalized model developed by Sager and Mortimer
(1976).
The soil seed bank reserve was determined by sifting
and separating cheatgrass and starthistle (plumed and
nonplumed) seed from 30 randomly located 686-mm3 soil
samples removed from the top 80 mm of the soil profile.
Density of mature individuals was determined by counting plants in 30 randomly located 2- by 5-dm plots. Density
of seedlings and established plants was determined by counting individuals in 5 percent and 50 percent of the plot area,
respectively, in 20 randomly placed 2- by 5-dm plots.
Cheatgrass seed production was determined by harvesting 20 mature individuals at each plot location. Seeds were
separated from the parent plants, and the seed production
estimate for the plot was adjusted to reflect mature cheatgrass density. Yellow starthistle seed production was determined by calculating the average number of seedheads
.per plant and randomly harvesting a single seedhead from
each of 10 individuals at each plot location. Seeds were separated into plumed and nonplumed categories, and the number of seeds per plot was determined.
Seed rain was estimated using wooden sticky traps (37
by 300 mm) coated with a smooth surface of lithium-based
grease and placed flush on the soil surface. Forty traps
were randomly placed within the study area. Seeds were
counted by species and type at each 2-week visit.
Analysis of variance was performed on each set of samples. Confidence intervals were calculated at the 5 percent
level of confidence. Life history data are presented on a
square-meter basis.
INTRODUCTION
The loss of native perennial vegetation on extensive
areas of North American rangelands has been followed
by the establishment of alien annual weed populations.
The grassland steppe of the Pacific Northwest and the
California Annual Grasslands, once dominated by native
perennial grasses such as bluebunch wheatgrass (Pseudoroegneria spicata [Pursh.] Love), are examples of rangelands where perennial vegetation loss has been followed
by cheatgrass (Bromus tectorum L.) domination (Mack
1981). Rangeland ecologists are becoming increasingly
concerned as yellow starthistle (Centaurea solstitialis L.),
a more recent alien annual, has begun to occupy cheatgrass range, resulting in further land use deterioration
(Hironaka 1990; Roche and Roche 1988; Sheley and others
1992).
The ecological relationships and community dynamics
that permit rangeland domination by annual weeds are
complex. It is generally recognized that organisms are
capable of budgeting energy or resources in order to complete their life cycles successfully (Radosevich and Holt
1984). The amount, timing, and juxtaposition of photosynthate allocated to root, shoot, leaf, and reproductive
effort, and the amount of time spent in dormancy, maintenance, and growth are important attributes that govern
plant species success.
Differential resource capture and allocation form plant
strategies and are closely linked with species survival and
Paper presented at the Symposium on Ecology, Management, and Restoration of Interm,ountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Larry L. Larson is Associate Professor and Roger L. Sheley is Graduate
Research Assistant, Department of Rangeland Resources, Oregon State
.University, Corvallis, OR 97850.
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ROOT GROWTH STRATEGY
Table 1-Bummarized life history of cheatgrass and starthistle
(attribute/m 2)
Seeds of cheatgrass and yellow starthistle were pregerminated, and four seedlings were transplanted into circular polyvinyl chloride tubes. The tubes (800 mm in length)
were filled with sterilized 'A' horizon soil from the study
site and brought to field capacity. Tubes were arranged in
an environmental chamber (10 oc ~th 12-h daylength) as
a randomized complete block design with five blocks and
10 tubes for each species per block. Sampling took place
10, 14, 18, 22, 26, 30, 34, 38, 44, and 46 days after planting. Sampling consisted of extracting above- and belowground plant parts to determine depth of root penetration,
root length (Comair Corp., Melbourne, Australia), aboveand belowground biomass, and leaf area (Licor-3100 with
conveyer belt, Li-Cor, Inc., Lincoln, NE). Data were analyzed using the analysis of variance procedures found in
SPSSIPC+ (SPSS Inc.,Chicago, IL).
Attribute
Cheatgrass
Mature plants
Seed production
660
7,000
Seed rain
7,000
Seed bank
Fall seedlings
Spring seedlings
Mature plants
300
6,200
2,000
543
Starthlstle
180
21,000
(15,000 plumed)
{6,000 nonplumed)
13,000
(11,000 plumed)
{2,000 nonplumed)
3,000
7,500
5,000
seedlings by January, and by late April the established
seedling count was 3,000 seedlingslm2• Both species
showed significant rates of mortality through the winter
months. A second wave of mortality is anticipated for
starthistle this spring and summer. The level of mortality
will likely be determined by climatic events during that
time period.
LIFE ffiSTORY STRATEGY RESULTS
The study was conducted during a drought in an area
co-dominated by cheatgrass and yellow starthistle. The annual cycle for cheatgrass began with 660 mature plants/m2,
producing an average of 11 seeds/plant for a total of 7,000
seeds/m2 (27 g) (table 1). The seed was released in July with
little loss occurring between crop production and seed rain
deposition. The seed rain was added to a seed bank representing 4 percent of the annual seed crop. In contrast, the
starthistle population contained 180 mature plants/m2,
yielding an average of two seedheads or 120 seeds/plant for
a total of21,000 seedslm2 (31 g). The seed crop contained
70 percent plumed and 30 percent nonplumed seeds. The
plumed seed was released in August with 30 percent loss.
The nonplumed seed was released in December and had
65 percent loss. The seed rain joined a seed bank equivalent to 14 percent of the annual crop.
Seed bank reserves act as a stabilizing factor that serves
to ensure species survival (Radosevich and Holt 1984).
Our data suggest that starthistle contributes a greater portion of its reproductive effort into this seed pool than cheatgrass. In addition, two distinct patterns of reproductive
resource allocation exist between cheatgrass and yellow
starthistle. Cheatgrass produced fewer, heavier seeds (one
seed type), while starthistle produced more, lighter seeds
(two seed types). High seed production by starthistle may
provide increased probability of successful dispersal, safe
site occupation, and genotypic variation (Harper 1977).
Fewer heavier seeds may provide cheatgrass with an advantage by providing sufficient reserves to become well established before it requires independent assimilation, and
may allow emergence from greater soil depth (Grime and
Jeffrey 1965; Harper and others 1970). Cheatgrass and
starthistle have different patterns of seed release. Cheatgrass matured early, released its seed by midsummer, and
had minimal seed loss. In contrast, starthistle matured
later, had two periods of seed dispersal, but had greater
seed loss.
Seedlings of both species emerged in November. Approximately 90 percent of the cheatgrass seed rain had produced
seedlings by January. Mortality reduced the cheatgrass
population to 540 mature plants/m2 by late April. In comparison, 57 percent of the starthistle seed rain produced
ROOT GROWTH STRATEGY
RESULTS
Shoot weight, root weight, leaf area, and total root length
were similar for both species, and combined means are presented (table 2). Individual T-tests at each harvest failed
to show differences between species. The ability of cheatgrass to grow rapidly has been widely documented (Harris
1967; Hull1963; Svejcar 1990). Our results suggest that
starthistle shares this competitive characteristic. Hironaka
(1961) found similar results comparing the later maturing
medusahead with cheatgrass.
Soil depth penetration was greater for starthistle than
cheatgrass (table 3). Starthistle soil depth penetration surpassed cheatgrass after 22 days and was nearly twice the
Table 2---summary of shoot weight, root weight, leaf area, and total
root length for yellow starthistle and cheatgrass
Shoot
weight/
plant
Root
weight/
plant
Leaf
areal
plant
Total root
length/
plant
mg
mg
crrl
em
10
14
18
22
26
30
34
38
42
46
1
2
4
8
13
28
45
107
143
328
T
1
3
3
5
13
23
42
57
138
0.3
.5
.9
1.6
3.2
5.8
9.1
16.6
22.2
41.3
41
131
257
317
437
970
1,233
1,652
2,017
3,858
LSD(0.05)
53
13
4.5
363
Days
from
planting
93
Table 3-Depth of soil penetration (mm) by cheatgrass and yellow starthlstie root systems1
Day
Cheatgrass
these species are found in association, community dominance will be dynamic. Shifts in community dominance
will fluctuate to reflect the interface of life strategies with
the prevailing edaphic and climatic conditions.
Starthistle
10
81
93
14
18
120
150
184
REFERENCES
22
150
165
209
259
Grime, J. P.; Jeffrey, D. W. 1965. Seedling establishment
in vertical gradients of sunlight. Journal ofEcology. 53:
621-642.
Harper, J. L.1977. The population biology ofplants.
London: Academic Press. 892 p.
Harris, G. A 1967. Some competitive relationships between
Agropyron spicatum and Bromus tectorum. Ecological
Monographs. 37: 89-111.
Hironaka, M. 1990. Range ecology as the basis for vegetation management. In: Roche, B. F., Jr.; Roche, C. T., eds.
Range weeds revisited. Misc. Publ. 0143. PnUman, WA:
Cooperative Extension, Washington State University.
85p.
Hironaka, M. 1961. The relative rate of root development
of cheatgrass and medusahead. Journal of Range Management. 14: 263-267.
Hull, A C., Jr. 1963. Competition and water requirements
of cheatgrass and wheatgrass in the greenhouse. Journal
of Range Management. 16: 199-204.
Mack, R. N.1981. Invasion ofBromus tectorum L. into
western North America: an ecological chronicle. Agroecosystems. 7: 145-165.
Radosevich, S. R.; Holt, J. S. 1984. Weed ecology, implications for vegetation management. New York: John Wiley
and Sons. 176 p.
Roche, C. T.; Roche, B. F., Jr. 1988. Distribution and
amount of four knapweed (Centaurea L.) species in east. ern Washington. Northwest Science. 62(5): 242-251.
Sager, G. R.; Mortimer, A M. 1976. An approach to the
study of the population dynamics of plants with special
reference to weeds. Annals of Applied Biology. 1: 1-47.
Sheley, R. L.; Larson, L. L.; Johnson, D. E. 1992. Germination and root dynamics of three range weeds and two
forage species. [Submitted to Weed Technology].
Svejcar, T. J.1990. Root length, leaf area, and biomass of
crested wheatgrass and cheatgrass seedlings. Journal of
Range Management. 43: 446-448.
Talbott, C. J. 1987. Distribution and ecological amplitude
of selected Centaurea species in eastern Washington.
Pullman, WA: Washington State University.123 p.
Thesis.
26
30
34
38
42
46
1LSD
114
324
427
265
521
335
382
567
715
767
404
(0.05) for any two row means. 88.
penetration of cheatgrass at the end of the 46-day experiment. Deeper soil penetration provides niche differentiation between these species and likely contributes significantly to the later maturing characteristic of starthistle.
Deep silt loam and loam with few coarse fragments are
the most common soils associated with starthistle domination (Talbott 1987). Under dry conditions, the early maturing cheatgrass would have an advantage over starthistle
by utilizing moisture and completing its life cycle ahead of
the later maturing species. This could limit the resources
available to starthistle and thereby limit viable seed production. Hironaka (1961) proposed the same scenario for
cheatgrass and the later maturing medusahead. Under
moderate and wet moisture conditions, starthistle would
have an advantage of continuing growth later, producing
more seed than cheatgrass, distributing seed through time,
and maximizing safe site occupancy.
High plant density can produce growth conditions that
simulate dry conditions (Radosevich and Holt 1984). Under
high-density conditions, rapid and deep soil penetration
may allow the avoidance of interspecific competition and
depleted soil moisture. This niche differentiation should
prove advantageous for starthistle when grown in dense
communities of cheatgrass. However, species plasticity
is one of the more powerful density reactive mechanisms
that contribute to the regulation of reproductive output by
a population (Harper 1977). Thus, additional research in
the area of density response needs to be conducted.
Our preliminary results suggest that cheatgrass and
starthistle have evolved complex life strategies. When
94