Towards rainy high Arctic winters: How experimental icing and summer warming affect tundra plant phenology, productivity and reproduction

The Arctic is warming rapidly and much faster in winter than in summer. Warm spells in winter lead to more frequent extreme rain-on-snow events that alter snowpack conditions and can encapsulate plants in 'basal ice' ('icing') for months. Yet, how icing affects plant communities, especially over multiple winters and under warmer summers, remains largely unstudied. We investigated winter icing and summer warming effects on vascular plants' productivity, reproduction and phenology in mesic dwarf shrub heath, an important reindeer habitat in high Arctic Svalbard, where winter temperatures have been rising particularly fast. In a full-factorial field experiment, rain-on-snow and resultant icing were simulated in five consecutive winters, and each followed by experimentally increased summer temperatures. Vascular plant responses at the community level, with particular attention to the dominant dwarf shrub Salix polaris, were assessed throughout each subsequent growing season. Icing alone increased community-level primary productivity, but only late in the growing season and reduced inflorescence production. Accordingly, S. polaris showed delayed early leaf phenophases, but accelerated subsequent development, resulting in smaller, thinner leaves. This compensatory growth response apparently occurred at the cost of delayed seed maturation. The phenological delay was associated with icing-induced delays in spring soil warm-up, possibly favouring resource allocation to primary productivity over reproduction. Experimental summer warming (on average 0.8 degrees C) largely counteracted the effects of icing, enhancing community productivity throughout the growing season, offsetting S. polaris leaf size reductions and turning around its delayed phenophases, including seed dispersal. Effect sizes of icing and warming combined could be larger than those under warming alone. Yet, summer warming did not negate the reduction in community inflorescence production caused by icing. Synthesis. Extreme rain-on-snow events encapsulating plants in ice can influence high Arctic plant communities in mesic habitats to similar extents as-the better-studied-summer warming. Nevertheless, the absence of magnified icing effects over the years indicates community resistance to icing, particularly under warmer summers, which contrasts with earlier documented ice-induced die-offs in communities dominated by evergreen shrubs. As warm spells during winter become the rule rather than exception, we call for similar experiments in coordinated circumpolar monitoring programmes across the tundra biome.