The goal of this project is to use a combination of field surveys and mechanistic-hypothesisdriven greenhouse experiments to determine the influences of environment, plant community, and land management on cheatgrass invasion success. Specific objectives include the following:
Conduct comparative surveys along a latitudinal climatic gradient from central Nevada, where cheatgrass dominates much of the landscape, to the INL Site, collecting 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 and specifi cally test hypotheses about how site characteristics affect invasion success of cheatgrass
Use controlled-environment experiments that involve individual species and constructed communities to establish a mechanistic understanding of competition between cheatgrass and native species.
From 2007 to 2009, over 400 field sites were visited. Several plant community characteristics, signs of disturbance, and physical environment variables were measured. Soil samples were collected and analyzed for soil nutrients, texture, and seed bank. Most of the field sites were visited only once, enabling investigators to sample across a wide area and providing the maximum variation in most landscape and vegetation variables. The rest of the field sites were visited for multiple years, allowing investigators to examine effects of inter-annual variation on cheatgrass distribution. In this study the most important variables associated with cheatgrass presence were grass and forb cover, soil phosphorus (P), and mean temperatures in the fall and spring. The most important variables associated with cheatgrass abundance were shrub canopy and soil P. Shrub canopy, grass and forb cover, and biological soil crust may offer some biotic resistance to the initial stages of cheatgrass invasion. However, once cheatgrass is present on a site, shrub canopy, and grass and forb cover are positively associated with increasing abundance.
The comparative surveys have also led to a theory paper, in which an invasion triangle model is proposed that incorporates attributes of the potential invader, the biotic characteristics of the site, and the environmental conditions of the site, and introduces the influence of external factors. This model also incorporates external influences (such as grazing or fire). This model can be used qualitatively, as well as quantitatively, to examine the contributing factors and overall risk of invasion.
The researchers conducted an experiment examining how invasive and native grasses influence soil nutrients and how those changes influence subsequent plant growth. They found little support for the hypothesis that, as a group, invasive grasses like cheatgrass affect soil nutrients differently than native grasses do. Instead, the results strongly point to a more complex response with the effect dependent upon the species rather than simply a general trait of invasive species. However, alteration of soil nutrients may still be an important mechanism promoting invasion by individual species, but not a general trait of invasive grasses.
The researchers also investigated the role that soil-plant feedback mechanisms may play in re-enforcing invasions. Feedbacks that are positive or suppressive promote the same species returning while feedbacks that are negative or facilitative promote other species. In this study, every invasive grass induced feedbacks that promoted the same species in at least one soil type. Depending on soil type, cheatgrass specifically induced positive and suppressive feedbacks which promoted return by the same species.