Plant community mediated responses of alpine ecosystems to anthropogenic nitrogen deposition
PhD: University of Colorado Boulder, 2017.
Human alteration of the nitrogen (N) cycle has resulted in a rapid increase in the availability of biologically active N. An unintended consequence of increased N availability is increased levels of N deposition into natural landscapes, including alpine ecosystems which are particularly susceptible to adverse environmental impacts. In my dissertation, I have examined alpine plant and soil responses to N deposition 1) across multiple spatial scales throughout the Southern Rocky Mountains, 2) among diverse plant communities associated with unique environmental conditions common in the alpine of this region, and 3) among ecosystem pools of N contributing to stabilization of N inputs within those communities. I used a gradient of N deposition in the southern Rocky Mountains to examine spatial variation in the responses of an alpine moist meadow to gradual changes in N deposition. I developed a response framework of expected ecosystem changes associated with increasing inputs, and found that plant metrics, such as tissue concentrations of nitrogen, were positively correlated with ambient N inputs. Soil biogeochemical responses to N were minimal along the gradient, suggesting that a reduction in N inputs may allow ecosystem recovery if plant feedbacks to N cycling are negligible. I next established a N fertilization study in three common plant communities to compare community responses to simulated N deposition and examined the applicability of the response framework throughout the alpine. I found that communities responded to inputs of N differently for both plant and soil responses, with the dry meadow community showing the strongest effects of N inputs and at the lowest levels of N input while the wet meadow community had few responses contributing to ecosystem change. Finally, I compared the fate of N inputs among ecosystem pools of N within alpine communities receiving elevated N. I found differences in N uptake among the communities, and that uptake increased with elevated N inputs. Even so, ecosystem uptake of N in plant and soil pools was very low (7-11%), supporting the conclusion that the alpine ecosystem does not retain N inputs and that increased N deposition is unlikely to be stabilized in the alpine.