Interactive effects of winter weather variation and nitrogen deposition on alpine moist meadow ecosystem processes in the Rocky Mountains

Abstract:

Alpine climate change in the Rocky Mountains has been linked to changes in precipitation patterns between summer and winter periods, and in total amounts of accumulation over the year. Annual variation in alpine snowpack can have important effects on concentrations and amounts of nitrogen (N) deposition entering the alpine from the atmosphere, as between one third to one half of N deposition occurs in association with precipitation, and high elevations primarily receive precipitation in the form of winter snow. Variation in snowpack further affects the amount and timing of water available to vegetation during the growing season, which can have large implications for alpine ecosystem responses in association with N deposition. To examine the potential interactive effects of variation in winter weather and N deposition, we established five sites along an ambient gradient of N deposition in the Rocky Mountains and collected measurements of N cycling between 2012 and 2014. This time frame included a year with low snow pack (2012), a year with average snow pack (2013), and a year with high snow pack (2014) among sites in the study, and allowed for us to examine candidate dynamic climatic drivers that may create variation in ecosystem processes associated with N. We found that soil water nitrate concentrations following snow melt were highly different for 4 sites along the N deposition gradient between 2013 and 2014. Growing season resin extractable N, however, was unaffected by inter-annual changes in winter precipitation. One possible explanation for no change in resin N may be associated with high inter-annual variation in plant community composition. There were significant differences in the species composition between 2012 and 2013, as well as shifts in the concentrations of N found in dominant plant species tissue. Our results suggest that plants will be important controls on biogeochemical responses of alpine moist meadows under variation in winter precipitation.