Date of Award


Document Type


Degree Name

Master of Science (MS) in Biology




"Invasive annual grasses cause ecological and economic damage across the western US and are a focal point in restoration and management plans. Managing invasion resistance of natural areas is critical to maintaining their biodiversity and function. Climate change is expected to shift species interactions, including invasive annual grasses, and may alter invasion resistance. One component of invasion resistance is biological soil crust (biocrust), which has been shown to limit Bromus tectorum (cheatgrass) in arid regions. However, biocrust has been less studied in semiarid regions and its effects on other invasive annual grasses, such as Ventenata dubia, have never been studied. The goals of my project were to I) determine how the relationship between invasive annual grasses and biocrust vary across a precipitation gradient and among different invasive annual grass species, 2) determine how patterns of biocrust and Ventenata dubia change over time (3 years), and 3) determine the effects of trampling on biocrust composition and species invasion over 3 years. I surveyed biocrust and invasive annual grasses in 168 plots across 21 sites along a precipitation gradient from arid to semiarid. I also resurveyed vascular plant species and biocrust across a series of transects that were located across a gradient from high to low V. dubia abundance three years after they were established. Finally, I resurveyed vascular plants and biocrust in a series of experimentally trampled and control plots after three years. Biocrust was surveyed using morphological and color groups. I found that the dominant invasive annual grass along the precipitation gradient in eastern Washington shifted from B. tectorum in arid regions to V. dubia in semiarid regions. Biocrust percent cover and lichen richness was negatively correlated with both invasive annual grasses across all sites. Soil depth and topography also affect distribution of invasive annual grasses. Looking at long-term patterns of V. dubia in a semiarid prairie, I found that biocrust abundance or composition did not affect V. dubia. Additionally, trampling applied 3 years ago appeared to have no impact on V. dubia or biocrust abundance. V. dubia abundance over 3 years of sampling was strongly correlated with spring water depth in vernal pond areas. My results can be used to help land managers better understand the relationship between biocrust and invasive annual grass, particularly in the face of climate change which may create more arid climates in the Inland Northwest. Additionally, land managers can focus management of V. dubia in locations where excess spring precipitation persists"--Leaves iv-v.