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

Background/Question/Methods

Long term deposition of nitrogen is increasing in the Colorado Front Range concurrently with changes in the forms and amounts of precipitation.  Both of these factors impact the diversity and composition of plant communities. Alpine systems are considered particularly sensitive to these inputs due to historically low levels of nutrients. How exactly alpine communities respond to nutrient and moisture additions depends in part of the mechanisms structuring initial community composition. Biotic, competitive- facilitative interactions and small scale environmental heterogeneity are both influential factors, but their relative importance across the alpine landscape remains unclear.

            Tree Islands act as snow fences in the alpine, thereby creating resource islands on their leeward sides.  By quantifying resource inputs and measuring how different plant communities respond to these long term additions provides an opportunity to gain a better understanding of primary structuring mechanisms.  An analysis of this impact across an elevational gradient (a surrogate for a climate gradient) provides additional insights into how resource heterogeneity affects local community structure.

Results/Conclusions

Results of species richness from individual plots outside the influence of tree islands (unmanipulated) and within the influence of tree islands (manipulated) revealed insightful patters. Independent of plot location, species richness decreases with increasing soil moisture and available nitrogen content. Drier, paired manipulated-unmanipulated communities show greater variability in both moisture and nutrient content compared with wetter, paired communities.  When plots outside and inside tree island influence are summed, species diversity of combined pairs increased in those plots with high resource variability and low mean resource availability.   These findings provide a mechanistic interpretation for the overall decline in local species richness observed with elevation in the alpine.  The findings also suggest that the largest changes due to N deposition will occur in the relatively dry meadow, upland alpine areas.