Increases in nitrogen (N) emissions and deposition have been occurring throughout the western U.S. over the last half-century as a result of industry, agriculture, and transportation. These increases have been associated with changes in plant community composition and invasive species spread, though not much work has been done in arid ecosystems. Here, we sought to evaluate whether increased N deposition would have an effect on species composition and ecosystem invasibility by the widespread invasive annual grass Bromus tectorum at high elevation in the western Great Basin Desert. We set up 54 paired plots, half of which were exposed to elevated N deposition (5 g m-2 yr-1 at the time of snowmelt for four years) and half acted as controls, in sites differing in disturbance history (grazed, burned, and grazed-burned). We monitored species composition each summer from 2008 to 2011 and then calculated and compared species richness, Shannon’s diversity (H’), Simpson’s dominance (D’), Simpson’s evenness (E1/D), B. tectorum dominance, and community similarity (with ANOSIM and SIMPER analyses) by N treatment and site.
Results/Conclusions
We found no evidence that increased N deposition will affect plant community composition in this ecosystem. However, species composition was different by site in all years (ANOSIM, p < 0.05), and the grazed-burned plots consistently had the highest levels of B. tectorum dominance (p ≤ 0.0003) and cover (p ≤ 0.0001). B. tectorum cover was inversely related to native forb species richness (r = -0.44; p<0.0001), H’(rs = -0.73; p<0.0001), D’ (rs = -0.75; p<0.0001), and E1/D’(rs = -0.49; p<0.0001). Although N deposition may not influence B. tectorum spread, our data suggest that high elevation communities are already experiencing invasion impacts. In the most disturbed sites, B. tectorum cover is approaching the threshold for increased fire risk, which could result in more significant impacts for the region.