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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.
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