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 °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 could be larger than 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.