COS 20-9
Alpine moist meadow response to regional gradients of nitrogen deposition in the Rocky Mountains
Human alteration of the nitrogen (N) cycle has resulted in a drastic change in availability of biologically active N. Alpine ecosystems are particularly susceptible to increased inputs of N, as higher elevations receive disproportionately more precipitation and therefore high rates of N input. Previous alpine research, using N addition experiments, has estimated threshold levels of N associated with changes in both community and ecosystem characteristics of dry meadows, one of the most common alpine communities in the southern Rocky Mountains. At present, however, little work has examined whether these thresholds apply to gradients of ambient N deposition, or in alternative alpine meadow communities, as dry meadows account for only 30% of the total land cover. The objective of our study was to examine effects of ambient N deposition in alpine moist meadow communities, and determine whether this gradient produces change in community and ecosystem characteristics in areas surpassing threshold levels of N addition. Ambient levels of N deposition in many federally protected lands in Colorado are approaching, or have exceeded, the current estimated critical loads of N for changes in species composition, and we therefore selected sites located on federally protected lands receiving levels of N deposition along a gradient.
Results/Conclusions
Our results show that, generally, sites receiving higher levels of N deposition have more indication of crossing ecosystem thresholds. The order of our sites receiving N inputs includes Arapaho National Forest (NF), Niwot Ridge Long Term Ecological Research site (Niwot), Rocky Mountain National Park (ROMO), Fraser Experimental Forest (Fraser), and Shoshone NF. Soil pH levels from each of these sites shows that Shoshone had the highest pH, followed by Fraser, Niwot, Arapaho and ROMO. Soil cation exchange capacity followed this exact pattern, with the highest buffering capacity in Shoshone and the lowest in ROMO, as well as soil water concentrations of nitrate. These findings suggest that increased N deposition, in these areas, is promoting nitrate mobility and subsequent leaching of base cations that producing lower soil pH. ROMO, however, shows more evidence for surpassing ecological thresholds than predicted based on N deposition rates alone, suggesting that differences in plant cover may play a contributing role in amplifying ecosystem responses to N deposition. This is further supported by high abundance of a moist meadow dominant grass (Deschampsia caespitosa), that actively promotes rapid rates of N cycling, and may provide community scale feedbacks producing our observed ecosystem scale response.