Invasive annual grasses, like Ventenata dubia and Bromus tectorum, are major ecological and economical threats to the western United States. While B. tectorum is well established, V. dubia is a relatively new invader. Recently, V. dubia abundance has appeared to dramatically increase, potentially displacing other plants and biological soil crust (BSC). V. dubia prefers shallow, rocky, periodically inundated soils, coinciding with BSC habitat. Our goal was to document the change in V. dubia abundance from 2009-2012 and its relationship with BSC communities and B. tectorum abundance. Our study was located at the Mima mound prairie at Turnbull National Wildlife Refuge in Cheney, WA, which encompasses a range of microsites and soil depth gradients. Mima mounds are hemispherical mounds of soil; in eastern Washington they are underlain by basalt bedrock and alluvial gravel substrates, which affects intermound soil depth. We hypothesized that V. dubia abundance has increased since 2009, but that B. tectorum abundance has not changed. We also hypothesized that V. dubia is primarily growing on basalt intermounds, where the BSC community is found. To test these hypotheses we surveyed vascular plant and BSC communities in 225 1-m2 plots on basalt intermounds, basalt mounds, and alluvial mounds.
Results/Conclusions
As expected, V. dubia abundance increased 10-fold on Mima mounds of both substrate types since 2009, with a greater increase on basalt mounds. B. tectorum abundance did not significantly change in this time period on either substrate type. In 2012, V. dubia stem counts and percent cover were significantly higher in basalt intermound plots than in basalt and alluvial mound plots, similar to the BSC community distribution, which is also more developed on intermounds. In comparison, B. tectorum abundance is relatively low on basalt intermounds and high on Mima mounds. It is unclear how V. dubia is affecting B. tectorum abundance, which appears unaffected by its presence. Before V. dubia, basalt intermound areas were dominated by BSC with little vascular plant growth, however, it appears that V. dubia has been able to open this niche, potentially leading to a drastic decrease in BSC abundance and diversity in the Inland Northwest.