Nitrogen isotope tracer acquisition in low and tall birch tundra plant communities: a 2 year test of the snow-shrub hypothesis

Deciduous shrub density and landcover are increasing across many areas of the Arctic. Shrub growth may be promoted by a snow-shrub feedback whereby relatively tall shrubs accumulate deeper snow, raising winter soil temperature minima, increasing microbial activity, and enhancing soil solution nitrogen (N). Although there is good evidence for the above components of the hypothesis, it has not yet been determined if shrubs can access the elevated N pool generated by deepened snow. We added isotopic N tracer (N-15) in late summer to control and snowfenced low birch hummock tundra to test the influence of deepened snow on N cycling. Furthermore, tracer was added to tall birch tundra to compare N cycling in low and tall shrub ecosystems that have the same species composition. Experimentally deepened snow in low birch tundra did not significantly affect N-15 uptake by shrubs or any other species 2 years after the tracer addition. However, there were strong differences between the low and tall birch ecosystems, with the deciduous shrubs and graminoids accumulating more N-15 than the evergreen shrubs in the relatively productive tall shrub site, and vice versa in the relatively infertile low birch site. The greater N-15 acquisition by birch in the more fertile site, together with the absence of a deepened snow effect on N-15 acquisition by any species in the low birch hummock ecosystem, suggest that climate-change induced increases in birch shrub growth and expansion across the landscape will tend to occur most rapidly in and around existing tall birch shrub patches.