of Cheatgrass Invasion and Dominance at the Idaho National
Predicting plant community susceptibility to invasion by
introduced species and determining mechanisms of resistance are
fundamental concerns of ecology and ecosystem management. In the
Great Basin, the invasive annual cheatgrass (Bromus tectorum)
was introduced in the late 1800s and by the 1990s has grown to
dominate more than 3 million acres, with another 14 million
acres heavily infested and 60 million acres considered at risk
for potential domination (Pellant and Hall 1994). However, the
eastern portion of the Snake River Plain, including the INL
Site, has largely escaped the cheatgrass dominance found in the
western portions of the Snake River Plain and in northern and
There are several characteristics of the eastern Snake River
Plain that might contribute to the relatively minor extent of
cheatgrass invasion. The maintained cover of native species may
make the vegetation of the INL Site resistant to invasion
(Anderson and Inouye 2001). INL Site has a markedly different
landscape disturbance history than more heavily cheatgrass
invaded sites. Climate variables, such as colder winter
temperatures and more late spring precipitation on the eastern
Snake River Pains also differ from most cheatgrass dominated
areas. The relatively minor extent of cheatgrass invasion at the
INL Site in comparison with surrounding areas provides an
exciting and unique opportunity to identify environmental
conditions, community characteristics or management practices
conferring ecosystem resistance to invasion.
The goal of this project is to use a combination of ﬁeld
surveys and mechanistic hypothesis driven greenhouse experiments
to tease out the inﬂuences of environment, plant community, and
land management on cheatgrass invasion success.
Comparative Surveys. We are conducting
comparative surveys along a latitudinal climatic gradient from
central Nevada, where cheatgrass dominated much of the
landscape, to INL Site. We are establishing sampling plots at
several hundred locations along this ‘mega-transect’ taking care
to adequately sample sites with different types of disturbance
legacies, management histories, vegetation composition,
temperature and precipitation regimes. We will continue to
sample intensively at the INL Site; at sites near INL Site which
are climatically similar but with different land use and
disturbance histories; and at sites in both northern and central
Nevada with a range of disturbance, community composition and
climatic variables. We are collecting information ranging in
scale from microscopic (soil nutrients and microbes) to
community (vegetation and animal) to landscape (climate and land
use patterns) to parameterize a structural equation model (SEM)
(Grace 2006) and speciﬁcally test hypotheses about how site
characteristics affect invasion success of cheatgrass.
SEM is a powerful statistical way to infer causality: speciﬁcally,
we are using it to determine why cheatgrass is more abundant in
certain locations and less in others. An additional beneﬁt of
SEM is that we can include variables based on ‘expert opinion’
rather than relying on strictly empirical data. This means we
can include a wealth of invaluable information that would not be
otherwise useable in a more traditional quantitative model.
Controlled Greenhouse Studies. We are using
controlled-environment experiments that involve individual
species and constructed communities to establish a mechanistic
understanding of competition between cheatgrass and native
species. We are investigating competitive relationships, effects
of diversity, density and disturbance and response to variation
in water regime (timing and pulse size). Preliminary
single-species trials indicate that cheatgrass and perennial
species differ in their abilities to respond to water pulses
depending on size and frequency of water events, and that
moisture at the right time in the life cycles of cheatgrass
could promote high competitive ability and possible invasion (K.
Allcock, unpublished data). A mesocosm experiment is currently
underway to test the interactions of precipitation timing and
community composition in determining invasion success.
Comparative Surveys. The GIS
data collected in 2006 was used to help identify potential
sampling points. For our sites at INL Site, we selected
areas with a diversity of vegetation type and ﬁ re history.
In June 2007, we visited INL Site and sampled our ﬁrst 100
sites. We measured several plant community characteristics,
signs of disturbance and physical environment variables.
Soil samples were collected and analyzed for soil nutrients,
texture, seed bank and soil food web dynamics. In October
2007, we returned to INL Site and inserted resin capsules
into the soil. These capsules will collect soil nutrients
over the winter. We will collect the resin capsules when we
return to INL Site in spring 2008. It is our hope that these
resin capsules will decrease the amount of lab work required
to characterize soil nutrients as well as provide a time
integrated measure of soil nutrient availability.
In November 2007, over 150 ﬁeld sites in
Nevada were identiﬁed, visited and resin capsules inserted.
Our Nevada sites are in two areas, one is located outside
Midas in northwestern Nevada and the other about 40 miles
north of Austin in the central part of the state. These
sites offer a huge variation in land use patterns, ﬁre
history, vegetation types and climate variables.
The data collected is being processed and used for model
building and method reﬁnement.
Controlled Greenhouse Studies.
In late 2006 and early 2007, we established a series of
two-species plant communities in 50-gallon barrels on the
University of Nevada Reno. These communities were comprised
of combinations of early-season native species (Poa
secunda, Achnatherium hymenoides or Elymus
elemoides), late-season native species (Pseudoroegneria
spicata, Acnatherium thurberii or Hesperostipa comata),
or one of each group. All plants were collected from the
wild and transplanted to our constructed communities. One
fourth of the barrels were not planted with any perennial
species. All barrels were seeded with cheatgrass at a rate
of 2000 seeds per m2. Each of these communities
(early, late, mixed, or no perennials) was then subjected to
either elevated total precipitation (150 percent normal
precipitation for Reno, Nevada) or ambient total
precipitation (equal to the amount of precipitation received
through the growing season in Reno, Nevada). Finally, this
‘precipitation’ was either all distributed evenly through
the course of the experiment (watered uniformly once per
week) or 50 percent of the total precipitation amount was
distributed evenly and the other 50 percent was applied in
three randomly-timed ‘storm events’ in which barrels
received 1/6 of the total allotted water volume for that
treatment over the course of three days. We had six
replicates of each community type, water amount, and water
distribution combination, giving a total of 96 barrels.
Substantial mortality of transplanted perennials in the
constructed communities in early 2007 meant that many plants
had to be replaced at the beginning of the 2007 growing
season (March-April 2007), so we delayed implementation of
our experimental treatments until June 2007 in order to
allow the replaced plants to establish. Watering treatments
continued through November 2007, and ﬁnal harvest occurred
in December 2007. At the time of harvest we recorded density
of cheatgrass, and clipped above-ground biomass, sorted by
species. Samples are currently being oven-dried and weighed.
Comparative Survey. We only
have data from 100 of the anticipated 500+ sites in the
comparative survey and are still processing the samples and
data. Thus, preliminary results are not yet available.
Controlled Greenhouse Studies.
We are processing the above-ground biomass samples collected
in December 2007. While the data are not yet ready to
analyze, it appears that the ambient-amount,
irregular-distribution watering regime caused some stress to
both cheatgrass and perennial transplants, with fewer
cheatgrass plants germinating and emerging, and several
perennial transplants dying. The higher-precipitation
treatments fared better. Emergence of cheatgrass in the
high-precipitation, irregular-distribution treatment was
initially low, but increased dramatically after the ﬁrst
‘storm event’. There did not appear to be any obvious visual
effect of the planted species on cheatgrass density or
biomass. There was no effect of planted species on soil
water content (as measured by time domain reﬂectometry, [TDR])
in the top 10 cm of soil, and minimal effect of the watering
treatments on surface soil water content 24 hours after the
water pulses were applied.
Plans for Continuation
This project will continue through 2010. We
will continue collecting ﬁeld data for the comparative
survey at INL Site and our other ﬁeld sites in 2008 and
2009. A select number of sites at INL Site will be followed
year to year; however, most sites for the comparative survey
will only be visited once. SEMs require a large number of
data points in order for the algorithms used to identify
reliable parameter values (Tanaka 1987), and we plan on
sampling approximately 500 sites through the course of this
Publications, Theses, Reports, etc.
We anticipate several peer reviewed
publication and conference proceedings on varied topics
(such as, but not limited to: the effects of soil microbial
community on cheatgrass success, the effects of soil surface
morphology on cheatgrass germination and the effects of
varied precipitation regime on cheatgrass competitive
ability), in addition to the Ph.D. dissertation to be
completed by Lora Perkins in 2009.
Investigators and Affiliations
Lora Perkins, PhD student,
Department of Natural Resources and Environmental Science,
University of Nevada Reno, Nevada
Robert S. Nowak, Professor,
Department of Natural Resources and Environmental Science,
University of Nevada, Reno, Nevada
Kimberly G. Allcock,
Postdoctoral Associate, Department of Natural Resources and
Environmental Science, University of Nevada, Reno, Nevada
U.S. Department of Energy Idaho
Nevada Arid Rangeland Initiative and the
Nevada Agricultural Experiment Station
Anderson, J., and R. Inouye. 2001.
Landscape scale changes in species abundance and biodiversity of a
sagebrush steppe over 45 years. Ecological Monographs 71:531-556.
Grace, J.B. 2006. Structural Equation
Modeling and Natural Systems. Cambridge University Press, NY.
Pellant, M. and C. Hall. 1994.
Distribution of two exotic grasses on intermountain rangelands:
status in 1992. p. 109-112 In: S.B. Monsen and S. G. Kitchen
(compilers). Proceedings—ecology and management of annual
rangelands. General Technical Report INT-GTR-313, Ogden, UT, USDA
Forest Service, Intermountain Research Station.
Tanaka, J.S. 1987. How big is big
enough? Sample size and goodness-of-ﬁt in structural equation models
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