Abstract
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 (15N) 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 15N 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 15N than the evergreen shrubs in the relatively productive tall shrub site, and vice versa in the relatively infertile low birch site. The greater 15N acquisition by birch in the more fertile site, together with the absence of a deepened snow effect on 15N 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.
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Acknowledgments
We gratefully acknowledge Meghan Laidlaw, Ian McCormick, Linda Cameron, Christy Barbeau, Andy Kritsch, Tamara Hansen, Tyanna Steinwand and Ryan Mercredi for their assistance in the field and laboratory. We thank Donie Bret-Harte, Michelle Mack, Kate Buckeridge, Tara Zamin, Shelley Arnott, Melissa Lafrenière, Ryan Danby and several anonymous reviewers who provided thoughtful insights. We are thankful for assistance from Steve Matthews in the Division of Environment and Natural Resources, NWT Government, and from the North Slave Metis Association. NSERC (P.G.), NSF (P.G.) and DIAND-NSTP (M.V.), provided funding for this research.
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Vankoughnett, M.R., Grogan, P. Nitrogen isotope tracer acquisition in low and tall birch tundra plant communities: a 2 year test of the snow–shrub hypothesis. Biogeochemistry 118, 291–306 (2014). https://doi.org/10.1007/s10533-013-9930-5
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DOI: https://doi.org/10.1007/s10533-013-9930-5