Shrub Expansion: Is deeper snow a mechanism for enhanced nitrogen uptake by shrubs?

Background/Question/Methods Increasing shrub biomass and dominance in arctic tundra vegetation could have profound implications for regional climate, carbon balance, and biogeochemical cycling. Shrub expansion may be promoted by a positive feedback between snow depth and shrubs. Winter processes may be important in promoting shrub growth, because dense tall shrub areas accumulate relatively deep snow that is known to raise winter soil temperature minima and therefore may promote nitrogen mobilization from organic matter. However, it has not yet been demonstrated that shrubs can take up this winter-mobilized nitrogen more than the rest of the tundra plant community, and therefore that their growth would benefit from this feedback. The objective of this research was to investigate whether annual nitrogen uptake by birch shrubs (Betula glandulosa) was enhanced in tall birch-dominated plots, and whether this preferential birch uptake was larger with deeper snow cover. Inorganic ¹⁵N tracer was added to control plots and experimentally manipulated snow (~1m) birch hummock tundra plots, and to tall birch-dominated plots in the Canadian low arctic. We predicted that increased snowcover would facilitate nitrogen turnover between the soil microbial pool and the soil solution, and therefore increase tracer accumulation in birch shrubs under deepened snow. In addition, we predicted that due to the combined effects of deepened snow and preferential birch uptake of inorganic ¹⁵N, shrubs in the tall birch-dominated plots would acquire more tracer than those located in the control plots.
Results/Conclusions Two years after tracer addition Betula glandulosa and Vaccinium vitis-idaea located in tall shrub-dominated tundra had greater ¹⁵N enhancement pool sizes of new shoot biomass than shrubs located in the control birch hummock plots. In addition, we found that deepened snow had no effect on birch plant nitrogen uptake and shoot biomass production.  Together these results indicate that nitrogen cycling in the winter is strongly influenced by vegetation and that deepened snow has a limited short term effect. Our study suggests that future projected increases in the depth and duration of snow may have little effect on nitrogen uptake by shrubs or other plant species, and therefore shrub expansion may be driven by factors other than interactions with snow.