Claire Tortorelli1, Meg Krawchuk1 and Becky K. Kerns2
1College of Forestry, Oregon State University, Corvallis, Oregon, USA.
2US Forest Service, Pacific Northwest Research Station, Corvallis, Oregon, USA.
In North America’s Intermountain West, a recently introduced annual grass, Ventenata dubia, threatens to increase fuel accumulation and fire activity, potentially disrupting ecosystem function where it dominates previously uninvaded semiarid ecosystems. Its recent and rapid expansion into agricultural and natural areas has raised concern amongst land managers and ecologists since little is known regarding Ventenata invasion potential, the extent of its ecological impacts, or how these differ from other invasive annual grasses in the region. We investigated environmental drivers of Ventenata invasion and impacts on plant communities across the Blue Mountain Ecoregion in the Pacific Northwest of the United States to help address this knowledge gap. We sampled Ventenata abundance, plant community composition and structure, and environmental variables including canopy cover, soil and topographical characteristics, and burn severity from 110 plots within and surrounding seven recently burned areas across the ecoregion. To evaluate the relationship between environmental variables and Ventenata, and to compare plant community composition and structure between burned and unburned areas, we used linear models and non-parametric analyses including nonmetric multidimensional scaling and multi-response permutation procedures.
Our results show that Ventenata is associated with a wide range of abiotic environments and myriad plant community types including forests, shrub-steppe, and meadows, suggesting that Ventenata has widespread biotic and abiotic tolerances. Such broad tolerances are often linked to a high invasion potential. Of these environments and plant community types, we found Ventenata to most heavily invade basalt parent materials, elevations between 1300 m and 1600 m, and stiff sagebrush (Artemisia rigida) plant communities that were not heavily invaded by other exotic annual grasses. Our results highlight these ecosystems as being at a high risk for invasion and suggest that Ventenata is invading previously uninvaded communities, thus exacerbating landscape-scale invasion impacts. Although Ventenata was not associated with large shifts in community composition between invaded and uninvaded communities, burning in concert with invasion had severe impacts on community structure in stiff sagebrush ecosystems, where, five years post fire, nearly all prominent shrub species were absent and Ventenata dominated. Invasion may facilitate the spread of wildfires in fire-sensitive stiff sagebrush communities, potentially transforming unique shrub-steppe ecosystems into annual grasslands. The replacement of keystone shrub species with dense annual grasses could have severe implications for hydrologic cycling, habitat quality, and soil stability in these ecosystems that historically demonstrated resistance to invasion and disturbance. Further research is required to fully understand the ecological impacts of Ventenata invasion, but results from this study provide insight into current impacts on plant community composition and structure which could be used to prioritize sensitive ecosystems for management, helping to mitigate potential ecological losses.