Shrub expansion in the low Arctic: The influence of snow and vegetation feedbacks on nitrogen cycling
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Climate change has coincided with expansion of deciduous shrub species in the Arctic. Increased deciduous vegetation in the tundra could have profound implications on regional climate, carbon balance, and biogeochemical cycling of nutrients. Winter biological processes may be a mechanism explaining shrub expansion in the Arctic. Tall shrubs accumulate relatively deep snowcover, raising winter soil temperature minima, enhancing microbial activity and promoting nitrogen mobilization that may then be taken up by shrubs. However, it has yet to be determined if shrubs can acquire winter-mobilized nitrogen, and if so, whether they acquire it early in the spring, or over the growing season. The purpose of this study was to test if increased snow alone or the combination of vegetation-type and snow depth affect nitrogen cycling and plant uptake. To test this, inorganic 15nitrogen tracer was added to control and experimentally deepened snow plots (using snowfences) in low birch hummock tundra, and to tall birch-dominated plots near Daring Lake, N.W.T. in the Canadian low Arctic. The first study (Chapter 2) characterizes soil 15nitrogen cycling over a single winter to investigate if experimentally deepened snow in low birch hummock ecosystems enhances nutrient availability to plants in the early spring. In addition, 15nitrogen cycling in low birch hummock and tall birch ecosystems were compared to characterize the combined impacts of vegetation-type and snow depth on nutrient availability to plants by early spring. The second study (Chapter 3) investigated the longer term fate of added 15nitrogen to determine if 15nitrogen acquisition and allocation differs among plant species over a two year period. Together, the results indicate that nitrogen cycling in the low birch hummock tundra was not significantly affected by deeper snow over short (after one winter) or longer terms (two years). By contrast, nitrogen availability in early spring, and birch shrub 15nitrogen uptake after two years were enhanced in the tall birch as compared to the low birch hummock ecosystem. These results suggest that the combination of vegetation-type and snow depth effects in the tall birch ecosystem could be a mechanism contributing to tundra to shrubland transitions across the Arctic.