Effects of global change on Bromus tectorum invasion in the eastern Sierra Nevada, California

Background/Question/Methods Bromus tectorum L., has displaced native shrub and bunchgrass communities throughout the Great Basin Desert. At higher elevations in the eastern Sierra Nevada, B. tectorum invasion has been slow and appears to be limited by temperature and a shorter growing season due to the deep winter snowpack. However, changing climatic patterns and edaphic conditions may facilitate increased invasion. Invasibility is likely to vary spatially by land use, disturbance history, and microhabitat. Our research objectives were: (1) to determine how B. tectorum will respond to the coupled effect of changing snowfall and increased nitrogen deposition, (2) to identify differences across microhabitats (i.e., under the shrubs Artemisia tridentata and Purshia tridentata, and in intershrub spaces), and (3) to identify differences due to disturbance history (grazing (GU), burning (UB), and their combination (GB)). To determine effect of precipitation changes, we used snow fences that created zones of increased and decreased snowpack. Paired plots were established within each snow treatment in the three microhabitats to simulate increased and ambient levels of N deposition. Impacts of N effects on invasibility were measured in three sites with different disturbance histories. Bromus tectorum growth and fecundity were measured throughout the 2008 growing season, as was native species composition and abundance. Results/Conclusions During the first year of treatment application, there were no apparent changes in species diversity or composition by snowpack or N treatment, though there was an effect of disturbance history. Species richness was greater in GU and UB than in GB plots. Correspondingly, B. tectorum was twice as common at the grazed-burned site than with grazing or burning alone. The number of B. tectorum spikelets per individual appeared to increase with both increased and decreased snowpack compared to ambient conditions (though the difference was not significant), while the density decreased. This resulted in similar propagule pressure across all snow treatments. B. tectorum response to N appears to be affected by microhabitat and disturbance history. These results suggest that B. tectorum may become more competitive under certain climatic and edaphic conditions, though responses will likely vary spatially across the landscape with disturbance history and microhabitat. The interaction of multiple agents of global change is likely to have much more complex effects than any single driver alone.