Minimizing Risk of Cheatgrass Invasion and Dominance at the INL Site

Minimizing Risk of Cheatgrass Invasion and Dominance at the Idaho National Laboratory Site


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 central Nevada.   


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).  The 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 Plain 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 of cheatgrass invasion success.   


Comparative surveys

We have conducted comparative surveys along a latitudinal climatic gradient from central Nevada, where cheatgrass dominates much of the landscape, to the INL Site.  We have established 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 using a stratified random design.  We sampled intensively at the INL Site; at sites near the 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 collected information ranging in scale from microscopic (soil nutrients) 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 plant-soil feedback, 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 was undertaken to test the interactions of precipitation timing and community composition in determining invasion success. 


A plant-soil feedback experiment was undertaken to evaluate what changes in soil properties are induced by native and invasive grasses and how those changes influence subsequent plant performance.  Alteration of soil nutrient dynamics is recently garnering more attention as both a cause and effect of plant invasion.  The ability of non-native plants to alter soil nutrient dynamics in a significantly different manner than native plants may facilitate invasion by some non-native plants.  If alterations to native soil nutrient dynamics are large enough, compound over time, or intensify with growing populations, then shifts in large scale nutrient cycling may be induced.  This project examines the effects of 7 native and non-native co-occurring grasses.  We are using this data for two separate manuscripts.  One manuscript intensively examines the effect of the study species on nutrient dynamics using a nutrient budget approach.  A nutrient budget approach such as this provides a comprehensive examination of the effects of plants on soil nutrient dynamics.  The second manuscript examines the actual plant-soil feedback and includes changes in soil biological properties and effects on subsequent plant growth.     


Accomplishments through 2009


Comparative surveys

GIS data collected in 2006 were used to help identify potential sampling points.  For our sites at the INL Site, we selected areas with a diversity of vegetation type and fire history.  In June 2007, we visited the INL Site and sampled our first 100 sites.  In May and June 2008, 300 more sites were visited.  Our 2008 field sites were located at the INL Site, and in central and northern Nevada.  Our 2009 field sites were located at and immediately around the INL Site.  We measured several plant community characteristics, signs of disturbance and physical environment variables.  Soil samples were collected and analyzed for soil nutrients, texture, and seed bank.  Seed bank evaluation requires several months of cold storage and several more months of germination time in the greenhouse.  Climate data were collected from National Oceanic and Atmospheric Administration weather stations.  Fire history and stocking rates were gathered from published BLM maps and maps provided by Stoller. 


Most of our field sites were only visited once enabling us to sample across a wide area and providing the maximum variation in most landscape and vegetation variables.  The remaining field sites were visited for multiple years.  This allows examination of the effects of inter-annual variation on cheatgrass distribution. 


Controlled greenhouse studies

Mesocosms - 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, Achnatherum hymenoides or Elymus elemoides), late-season native species (Pseudoroegneria spicata, Achnatherum 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% normal precipitation for Reno, NV) or ambient total precipitation (equal to the amount of precipitation received through the growing season in Reno, NV).  Finally, this ‘precipitation’ was either all distributed evenly through the course of the experiment (watered uniformly once per week) or 50% of the total precipitation amount was distributed evenly and the other 50% 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 6 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. 


Plant-Soil Feedback - In 2009, we conducted an experiment to elucidate the changes in soil properties that are induced by invasive and native grasses and to examine how those changes influence subsequent plant growth.  The grasses that we used were invasive species: Aegilops triuncialis, Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae and native species: Elymus elymoides, Pseudoroegneria spicata, and Vulpia microstachys.  We collected 2 soil types from natural sage steppe areas outside Reno, NV, for use in the experiment.  We measured soil properties before and after the first generation of plant growth.  For the second generation of plant growth, we used a factorial design where every plant species was grown in its own soil and soil cultured by every other species.  Second generation plant performance was monitored.  The soil properties that were measured were both physical (soil nutrients and pH) and biological (fungus and bacteria). 




Comparative surveys

In 2009, we finished our field data collection and are finishing with our seed bank germination period before data analysis can proceed in earnest.  The comparative surveys have led to a theory paper regarding invasion tentatively titled “A Triangle Model for Evaluating Species Invasion,” which is currently in internal review.  In the theory paper, we present an invasion triangle model that encapsulates the complexity of invasion in an intuitively appealing and straightforward framework.  The invasion triangle model is an adaptation of the plant pathology disease triangle.  The invasion triangle model incorporates attributes of the potential invader, the biotic characteristics of the site, and environmental conditions of the site, as well as introduces the influence of external factors.  This model can be used qualitatively as well as quantitatively to examine the contributing factors and overall risk of invasion.  We describe in detail each side of the invasion triangle model and external influences and discuss how popular invasion hypotheses are incorporated into the model.  We also use published data to illustrate the practicality of the invasion triangle model.  Adoption of the framework provided by the invasion triangle model will be beneficial for furthering understanding, focusing research, and directing management regarding invasive species.  We will use the invasion triangle as a framework for examining our data on cheatgrass invasion; however data analysis has not proceeded far enough to allow discussion of preliminary results.  

Controlled greenhouse studies


Mesocosms - We have processed the above-ground biomass samples collected in December 2007.  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.


Plant-Soil Feedback - The experiment is complete and data are currently being analyzed.  An examination of our results regarding changes in nutrient dynamics indicates differences between natives and non-natives for all of the soil nutrients included in the project.  We investigated different mechanisms involved with nutrient dynamics including plant uptake and changes in soil content.  We found plant uptake was higher by invasive grasses for P, Ca, Mg and Mn.  Native grasses increased available mineral N and K in the soil more than non-natives.  However, a more striking pattern emerged when species were evaluated individually, with most aspects of nutrient dynamics being affected in a species specific way.  In other words, not all natives as a group and not all non-natives as a group affect nutrient dynamics similarly.  For example, the effect of native Vulpia microstachys on nutrient dynamics was often more similar to non-natives than the other natives, and the effect of non-native Agropyron cristatum on nutrient dynamics was often more similar to natives than other non-natives.  These results elucidate a more complex dynamic than our simple hypothesis predicts.  Our hypothesis that non-native grasses should alter all nutrient dynamics with greater magnitude than native grasses might have been naïve as there has been little success finding a characteristic shared by invasive plants (Alpert et al. 2000). Evaluating the species-specific effects of both native and non-native plants on soil nutrient dynamics will elucidate one mechanism that influences both the ability of some species to invade and the potential effects of some invasions.  The ability of a species to alter the abiotic features of their ecosystem has been termed ecosystem engineering (Jones et al. 1994).  If a non-native plant species has the ability to ecosystem engineer (i.e., alter nutrient dynamics) enough to alter population dynamics (Cuddington et al. 2009), that ecosystem engineering ability may contribute to their invasion potential.  The different capabilities of plants for nutrient uptake and soil nutrient mining can affect ecosystem level nutrient cycling (Ehrenfeld 2003).  Data analysis regarding the entire plant-soil feedback has not progressed far enough to discuss preliminary results.   


Plans for Continuation


This project will continue through 2010.  By the end of 2009 all of our data were collected for all our projects except the last germination period of the seed bank from the field collected soils; data analysis and writing are ongoing.   


Publications, Theses, and Reports


We anticipate several peer reviewed publications and conference proceedings on varied topics (such as, but not limited to: Triangle Model for Evaluating Species Invasion, Plant-Soil Feedback in Native and Non-native grasses, Nutrient Dynamics under Great Basin Grasses, What Makes a Site Invasible by Cheatgrass, 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 2010.