Get reliable science

Understanding the underlying factors influencing exotic plant abundance and dominance is important to developing effective management and restoration strategies. We measured foliar cover of all vascular plants at 131 plots across The Nature Conservancy's Zumwalt Prairie Preserve in northeastern Oregon to evaluate the historical, biotic and environmental factors that best explained exotic plant abundance, distribution, and dominance. A total of 37 exotic species were encountered, the majority of which (62%) were forbs. However, the most abundant exotics were either annual grasses (Bromus spp., Ventenata dubia) or introduced perennial grasses (e.g. Poa pratensis, Thinopyrum intermedium). Exotic species abundance varied with past land use, environmental gradients and plant association. Seeded forage species such as T. intermedium and P. pratensis dominated abandoned cultivated fields. Bromus spp. were widely distributed across the landscape, but was associated primarily with Festuca idahoensis- Pseudoroegneria spicata associations. Ventenata dubia was the most abundant exotic species in the driest sites (Danthonia unispicata dominated sites). Results show that patterns of exotic abundance are the result of historical, environmental and biotic factors. Additionally, the relatively poorly studied and acknowledged weed, V. dubia, is a major invader of Pacific Northwest Bunchgrass communities. Moreover, results suggest that V. dubia is functionally different than annual Bromus spp., occupies different ecological niches, and is invading via different mechanisms. Thus, despite being an annual grass like Bromus spp., weed management approaches likely differ and species-specific approaches must be developed for this species. We then discuss implications of our findings for restoration and management of bunchgrass rangelands in the interior Pacific Northwest.