Effects of Experimental Icing and Summer Warming on the Polar Willow Salix polaris on Svalbard
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- Institutt for biologi 
The effects of climate change are particularly strong in the Arctic, where temperatures are increasing rapidly. While a clear trend of a more productive and greener Arctic in response to summer warming has been confirmed by several studies, recent assessments suggest that the Arctic is browning. In some regions, more frequent extreme winter rain-on-snow (ROS) events, resulting in ground ice encapsulating the vegetation are predicted with climate warming. In this study from high Arctic Svalbard (78°N), I investigate how the dwarf shrub Salix polaris, an important food source for herbivores, is affected by a warmer climate in a full-factorial field experiment. Warmer summer temperatures were induced using open top chambers, while heavy winter ROS was simulated by experimentally applying ROS and thereby encasing the vegetation in about 13 cm solid ice. No impacts of experimental icing on S. polaris annual stem length, leaf biomass or flower production were found, possibly reflecting the robustness of the shrub. However, S. polaris responded to increased summer temperatures by decreasing its flower production and producing shorter annual stem length. The leaves also got heavier per surface area (reduced specific leaf area), but not larger (area or length), due to warming. Surprisingly, the total productivity of the plant community was found to be enhanced by winter icing, and not to be affected by summer warming, indicated by their effects on peak season Normalised Difference Vegetation Index. My results on S. polaris indicate that summer warming and winter icing may pull in different directions, but their relative importance is still unknown.