Alpine plant community response to long-term moisture and nitrogen accumulation along an elevational gradient, Niwot Ridge, CO

The Colorado alpine is experiencing long-term changes in levels of nitrogen deposition and in annual moisture regime. These changes are known to impact the diversity and composition of plant communities, although exactly how remains somewhat unclear. Biotic, competitive- facilitative interactions and local environmental heterogeneity are both influential factors, as is the initial community and the relative degree of change.

Tree islands act as small-scale topographic barriers, allowing snow and associated nitrogen deposition to collect on their leeward sides, resulting in resource islands. We used these resource islands, in comparison with their surrounding environment, as a mimic some aspects of the current long-term change. We selected fifteen tree islands along an elevational gradient such that the leeward snowdrifts differed more from the surrounding snowpack with increased elevation. This effect is due primarily to increasing wind-scour of the surrounding, unprotected area. One by one meter paired plots were established behind and alongside of each tree island and assessed for resource availability and plant species cover. The side plots were used to represent baseline plant community and environmental conditions while the behind plots represented long-term resource additions and plant community response.

Comparisons of the plant community and resource availability in plots alongside and behind of each tree island, as well as the combined side-behind community and availability along the gradient provided some insight into the relative roles of biotic interactions and resource heterogeneity as structuring mechanisms. Soil moisture and soil inorganic nitrogen were closely related in all plots. The coefficient of variation in resource availability between the influenced and non-influenced plots increased with elevation, indicating a gradient of resource addition relative to baseline conditions. Among baseline plots, there was indication that diversity increased with decreasing resources. Pairwise community distances between the influenced and non-influenced plots at each site increased with the coefficient of variation in soil inorganic nitrogen (regression, p=0.027). Shannon-Wiener diversity for the combined influenced and non-influenced plots was also significantly correlated with the coefficient of variation for nitrogen availability (regression, p=0.020). No measurements of diversity, neither for combined plots nor in comparison, were correlated with mean resource availability. This indicates that resource (moisture and inorganic nitrogen) heterogeneity exerted a tighter control on alpine plant diversity than does total resource availability. In the Colorado alpine, long-term changes in the distribution of resources may prove to be highly influential.