Understanding the Interactive Effects of Nitrogen Deposition, Global Warming, and
Increased Snowfall on the Encroachment of Woody Shrubs into the Alpine Tundra

Isabel W. Ashton, Jane G. Smith, Marko Spasojevic, and Katharine N. Suding
Department of Ecology and Evolutionary Biology, University of California,
Irvine, 321 Stenihaus Hall, Irvine, CA

Alpine ecosystems are highly sensitive to nitrogen (N) deposition, recent warming temperatures, and changes in snowpack. Alpine plants are particularly threatened by local and regional extinction as they are replaced by species from lower elevations. Concurrent with global warming and increasing N availability, there has been an increase in the abundance and distribution of woody shrubs, particularly Salix spp., in alpine zones and positive feedbacks among temperature, snowpack, and nutrient availability are predicted to accelerate this shrub encroachment; however the exact mechanisms by which warmer temperatures favor the spread of shrubs over herbaceous plants remains unclear. In 2006, we began a factorial experiment involving the manipulation of air temperature, soil temperature, and snowpack within the moist meadow alpine at Niwot Ridge LTER. Growing season temperatures have been increased 1-2° C above ambient using open-topped chambers, N has been added at a rate of 8 g m-2 yr-1and snow fences have increased snow pack and maintained warmer winter soil temperatures. To test the hypothesis that the survival, growth, and reproduction of Salix is favored by N, warmer temperatures, and increased snow pack we have monitored Salix survival and growth and compared this to the tundra community. Our preliminary data show that Salix survival is greater in warmed plots, suggesting that increased global
temperatures may drive woody encroachment into the alpine. There is a strong interactive effect of N and snow, where Salix survival increases with added N, but decreases with added N and snow. It is critical to understand such interactive effects of N deposition and other global changes in order to predict and prevent further conversion of alpine tundra to shrublands.