Elevated nitrogen effects on Bromus tectorum dominance and native plant diversity in an arid montane ecosystem

Questions: Dominance of the widespread fire-altering invasive grass, Bromus tectorum, is markedly reduced at upper elevations in the Great Basin Desert. Here, we evaluated whether increased anthropogenic nitrogen (N) deposition would have an effect on species composition and ecosystem invasibility by B.tectorum at high elevations, and whether B.tectorum cover was associated with decreased native plant diversity. Location: Sagebrush steppe of the eastern Sierra Nevada, CA, US, at the western edge of the Great Basin Desert. Methods: We set up 54 paired plots, half of which were exposed to elevated N deposition (50kg center dot ha(-1)center dot yr(-1) at the time of snowmelt for 4yr) and half acted as controls, in areas differing in disturbance history (grazed, burned and grazed-burned). We monitored species composition each summer from 2008 to 2011 and then compared species richness, Shannon's diversity (H'), Simpson's dominance (D'), Simpson's evenness (E-1/D), B.tectorum dominance and community similarity (with ANOSIM and SIMPER analyses) by N treatment and disturbance history. Results: Species composition differed by disturbance history in all years (ANOSIM, P<0.05), and the grazed-burned plots consistently had the highest levels of B.tectorum dominance (P0.0003) and cover (P0.0001). Bromus tectorum cover was inversely related to native forb species richness (r=-0.44, P<0.0001), H'(r(s)=-0.73, P<0.0001), -ln(D') (r(s)=-0.75, P<0.0001) and E-1/D'(r(s)=-0.49, P<0.0001). We found no evidence that increased N deposition would affect native plant diversity after 4yr in this arid montane ecosystem, but the possibility of longer-term effects cannot be eliminated. Conclusions: Results suggest that high-elevation plant communities are already experiencing invasion impacts even though changes to the fire cycle have not yet occurred. In the most disturbed areas, B.tectorum cover is approaching the threshold for increased fire risk, which could result in more severe impacts at high elevations.