Seedling establishment of invasive Bromus tectorum and native Elymus multisetus in sagebrush steppe ecosystems

Background/Question/Methods Sagebrush-steppe ecosystems of western North America are influenced increasingly by the annual invasive Eurasian grass Bromus tectorum L. (cheatgrass). This invasive grass increases fine fuel accumulation and initiates a grass/fire cycle that causes shifts from perennial grass/shrub to annual grass dominance. These ecosystem changes negatively affect establishment of native perennial plant species which are becoming locally uncommon. We examined factors that influence ecosystem resistance to invasion by B. tectorum and resilience following disturbance in sagebrush-steppe. We evaluated differences in resource availability (soil water and nitrogen) and seedling establishment of B. tectorum and Elymus multisetus, a native perennial grass, in an intact sagebrush-steppe ecosystem with a native perennial grass understory, and an adjacent invaded sagebrush ecosystem with a B. tectorum understory.  We used a factorial design that included the two species and four microsite types (under shrub; shrub removal; interspace with herbaceous cover and bare-ground interspace). At each of three sites within the two communities, four randomly selected replicates (40 x 40 cm plots) of each microsite type were seeded with 100 B. tectorum seeds and four were seeded with 100 E. multisetus seeds in October 2004 and 2005. Plots were censused in June 2005 and 2006.
Results/Conclusions Bromus tectorum establishment, biomass and seed production were lowest in interspaces dominated by perennial bunchgrasses, and highest in microsites previously occupied by the invasive grass (P < 0.05).  Elymus multisetus establishment was higher in bare-interspaces and undershrub microsites, but overall seedling survival was higher in the native than the invaded community (P < 0.05) where competition with B. tectorum and litter accumulation appeared to constrain E. multisetus establishment. Our results suggest that environmental conditions that increase resistance of native communities to B. tectorum appear to be linked to strong resource competition and high occupation of the physical space by native perennial grasses. In the invaded community B. tectorum appears to create feedback mechanisms that facilitate its own establishment while reducing the likehood of native perennial grass establishment.