Effects of global change on high elevation populations of Bromus tectorum in the eastern Sierra Nevada, CA

Background/Question/Methods

Bromus tectorum L. is an exotic annual grass from Eurasia that has spread through much of the Great Basin Desert, displacing native plant communities and altering fire regimes.  At high elevations, B. tectorum is present in small populations, but agents of global change may facilitate its spread.  This research explores how climate change, land use change, and nitrogen (N) deposition might affect B. tectorum invasion in the eastern Sierra Nevada, CA.  To test effects of climate change, we manipulated snowpack with snow fences in 2010 and simulated a shift toward more spring rain events with water additions in 2009.   We tested effects of increased N deposition on B. tectorum biomass and fecundity in paired plots within each snow/rain zone, one acting as a control and the other receiving 5 g/m² N at the time of snowmelt.  Plots were set up in three dominant microhabitats: under Purshia tridentata shrubs, Artemisia tridentata shrubs, and in inter-shrub spaces.  To test effects of N deposition on biodiversity across the landscape, we compared species composition and B. tectorum dominance in paired control and +N plots in areas that have experienced different disturbance history (grazing, burning, and grazing + burning).

Results/Conclusions

We found differences in B. tectorum across the landscape, both by microhabitat and disturbance history.  However, experimental snow and +N treatments had little effect on B. tectorum invasiveness.  We did find a decrease in B. tectorum germination with increased snow in Purshia tridentata habitats, but no difference in biomass or fecundity by snow zone.  N additions had no effect on B. tectorum invasiveness in plots that received ambient levels of precipitation.  However, rain simulations increased B. tectorum biomass and fecundity.  Increased rain also affected B. tectorum response to N; we found an increase in B. tectorum biomass with increased N only in +rain plots.  Neither species richness nor B. tectorum dominance were affected by 3 years of N additions in any land use area.  We did find pronounced differences in B. tectorum dominance by land use history, with highest cover and biomass in grazed-burned areas.  Overall, our results suggest that a shift from snow to rain events, coupled with continued grazing disturbance, will likely facilitate B. tectorum spread in the eastern Sierra Nevada.  Increased N deposition may have little effect on B. tectorum invasion in the region, unless it is coupled with increased soil moisture.