Invasive plant species are thought to be especially capable of range shifts or expansion in response to anthropogenic climate change due to high dispersal and colonization abilities. The highly invasive annual grass, Bromus tectorum, has had a profound impact throughout the Great Basin Desert. Its presence and impact has been limited at higher elevations in the eastern Sierra Nevada, potentially due to limitations imposed by extreme wintertime conditions. However, climate models project an upward elevation shift of climate regimes in the Sierra Nevada that could favor B. tectorum expansion. This research specifically examined the effects of experimental snow depth manipulations and interannual climate variability over five years on  B. tectorum population dynamics at high elevation (2175 m) at the edge of the eastern Sierra Nevada. We used periodic matrix population models to examine how the response at particular life history stages contributed to the overall population-level response.
Results/Conclusions

Experimentally-increased snow depth had an effect on phenology and biomass, but had no detectable effect on individual fecundity. Instead an experimentally-increased snowpack inhibited population growth in one year by reducing seedling establishment and early survival. A similar negative effect of increased snow was observed two years later. However, a strong negative effect on B. tectorum was also associated with a naturally low-snow winter, when seedling emergence was reduced by 86 percent. Thus, we observed negative effects associated with both experimentally-increased and naturally-decreased snowpacks. Across five years, winters with naturally greater snow cover and a slower accumulation of degree-days coincided with higher B. tectorum seedling density and population growth. It is likely that the effect of snow at high elevation is nonlinear and differs from lower elevations where wintertime germination can be favorable.