Alpine community response to a long-term factorial nutrient and moisture addition experiment

Background/Question/Methods

            Both moisture and nutrient limitation are known to influence plant community composition and both variables are components of global environmental change. Alpine communities are thought to be particularly sensitive to changes in the timing of snowmelt. Previous studies have demonstrated changes in productivity and diversity in response to nitrogen, phosphorus and water additions in alpine environments. Responses to such environmental changes however, are not consistent across community type. Alpine plants tend to be long-lived and slow growing, indicating that longterm studies are necessary to capture changes.

            In 1993, a 60m long by 2.6m tall snowfence was established in the saddle area of Niwot Ridge. Sixteen 2x2m plots were established in each of the dry and moist meadow community types on the leeward side of the snowfence, as well as outside its influence. These plots were treated with nitrogen, phosphorus, nitrogen and phosphorus or were left as control. Finally, 16 treatment plots were established in a wet meadow community away from the snowfence. Fertilizer treatments have been applied roughly every other year since 1993 with 10g m^-2 N and 1g m^-2 P.  Vegetation information was collected from each plot in July and August 2012 using a 100 point-intercept method.

Results/Conclusions

            Results of a community similarity ordination (using vegdist and MetaMDS from the Vegan package in R) indicate that all plots and treatments combined show some distinction based on original meadow community type. More information is garnered by looking at the response of each meadow type individually. The snow addition completely transformed the dry meadow communities and appears to have overridden most fertilization effects. This was not the case for dry meadow communities without the snow addition where nitrogen addition communities were distinct from controls. Conversely, fertilization effects in the moist meadow appear to have had a greater influence on the plant communities than snow treatment. Removing nitrogen limitation resulted in distinct communities as compared to the controls regardless of snow, although the snow+N+P appears to have shifted the furthest. In the wet meadow, any nitrogen addition led to a distinct community from both the control and just phosphorus addition. Preliminary results indicated that soil moisture and inorganic nitrogen content match snow and nitrogen addition treatments.