Response of sub-Arctic tundra vegetation to wintertime warming events
Climate projections predict warming in arctic regions, however impacts on vegetation have not been thoroughly investigated. There is consensus that the arctic will “green-up” with warmer temperatures and longer growing seasons, however increased frequency and temperature of mid-winter warming events could instead be detrimental, reducing snowpack and thereby removing the insulating layer protecting plants from subsequent freezes. We hypothesized that 1) timing of snowpack reduction would have differential effects on vegetation; and 2) evergreen and deciduous species would show opposite responses to snowpack reductions in mid-winter vs. spring. To test these hypotheses, we conducted manual snow removals in January and April 2014 at a sub-arctic tundra site in Abisko, Sweden. Removals were conducted on 1x1m plots, each containing two deciduous (Betula nana, Vaccinium uliginosum) and two evergreen (Juniperus communis, Empetrum nigrum) sub-arctic shrub species. Stomatal conductance, hydraulic conductivity, and vegetation cover were assessed in July 2013 and 2014, before and after treatment implementation. Air and soil temperature and soil moisture were measured hourly.
The January removal caused significant reductions in minimum soil temperature relative to control and April removal plots. Species coverage did not change significantly between years or treatments; however snow removals significantly affected percentage of dead cover. Consistent with our hypothesis, evergreen species experienced greater damage from January than April removal. Both J. communis and E. nigrum showed significant damage from January removal, while J. communis damage actually decreased with the April treatment. B. nana suffered significant damage from both snow removal treatments, which was best explained by number of freeze-thaw days. V. uliginosum showed no change in damage with either treatment. These effects are mirrored in the stomatal conductance data, which declined across most species because the post-treatment summer was drier and warmer than pre-treatment. In J. communis, however, stomatal conductance declined more with January snow removal than in control. In B. nana only the control showed significantly reduced stomatal conductance in 2014. There were no significant differences in hydraulic conductivity between treatments or years. These results suggest that, while physiological changes and species composition shifts may take longer to manifest, snow reduction has clear detrimental effects that are differential among species and may have significant implications in the long-term.