In recent decades, human activities have increased the amount of biologically available nitrogen (N) in the biosphere. The resulting increase in N availability has broadly affected ecosystems, through increased productivity, changes in species composition, altered nutrient cycles, and increases in invasion by exotic plant species, especially in systems that were historically low in N. California serpentine grasslands are a rare ecosystem type dominated by endemic species, including several threatened and endangered plants and animals. However, in recent decades invasion by exotic grasses has increased, presumably due to increased N deposition. While the exotic species have been effectively managed with grazing by cattle thus far, the effects of future increases in N deposition and cattle grazing are unclear. We examined the interactive effects of cattle grazing and N deposition on plant community composition and invasion resistance in a serpentine grassland to determine the efficacy of current management strategies under current and projected N levels.
Results/Conclusions
As predicted, grazing led to decreases in net primary productivity, higher native richness, and reduced exotic cover. However, the effect of grazing differed under ambient and elevated N levels. Under ambient N, grazing effectively maintained native species richness, with grazed sites containing two more species on average than ungrazed sites. However, under increased N deposition, grazing was ineffective at maintaining native richness, as there was no significant difference between native richness in grazed and ungrazed sites. Grazing intensity was also positively related to declines in exotic species cover, but grazers were less effective at reducing exotic species cover under increased N deposition. Our findings suggest that there are interactive effects between grazing and N deposition and that the current grazing regime may not be an effective management strategy with future increases in N deposition.