Towards rainy Arctic winters: experimental icing impacts tundra plant productivity and reproduction

Abstract The Arctic is the most rapidly warming region on Earth, but our understanding of ecosystem impacts is still poor. For instance, warming occurs more than twice as fast in winter than in summer, yet long-term tundra vegetation studies have almost exclusively focused on effects of the latter. In winter, more frequent extreme warm spells and associated rain-on-snow events can dramatically alter snowpack conditions and even encapsulate the vegetation in ‘basal ice’ for several months. Such icing effects on plant phenology, productivity and reproduction remain largely unexplored. We performed a novel five-year-long field experiment in which we simulated every winter a rain-on-snow event resulting in ice encasement of the vegetation, and assessed vascular plant responses in each subsequent growing season. We also tested whether these responses could be modified by summer warming (open top chambers). Icing delayed the dominant shrub’s phenology, in particular early leaf development and seed maturation, increased community-level primary production in the second half of the growing season, but also reduced flower production. These delays and allocation trade-offs were associated with a delay in sub-surface soil thawing and soil warming in spring/summer, conditions known to influence plant root activity and nutrient availability at high latitudes. Interestingly, summer warming mitigated effects of icing alone and the combined treatment showed advanced phenology, increased primary production across the growing season and reduced the negative effects on reproduction. In general, effect sizes of icing were as large as those observed for experimental summer warming alone. The community-level increase in primary production following icing is in sharp contrast with the recently proposed ‘arctic browning’ effect of extreme winter warm spells in evergreen shrub-dominated tundra communities. Yet, the negative effect on reproductive traits can become critical for species viability if these events become chronic. As we are heading towards a rain-dominated Arctic, our findings highlight the need for coordinated monitoring of the effects of warmer and rainier winters across tundra plant communities and growth forms across the Arctic.