Assessing the impacts of climate change on biodiversity and developing climate-wise conservation planning requires in-depth understanding of the key drivers of species distributions and assemblages. This is particularly important in Arctic environments which will face the most notable climatic changes worldwide. The search for main determinants of biodiversity patterns in high-latitude ecosystems has focused on growing season conditions, but there is increasing amount of evidence suggesting that wintertime conditions can be equally or even more important factors for Arctic biodiversity than summer conditions (1, 2). Yet, large uncertainties exist regarding the role of winter climate in controlling the Arctic ecosystems, caused by the paucity of on-the-ground data on the spatio-temporal variation of tundra microclimate.

To fil this knowledge gap, we performed in-depth examination of the characteristics and relationships of microclimate - tundra vegetation at three different spatial scales in northern Fennoscandia. We used a total of over 600 microloggers to collect in-situ, year-around measurements of soil thermal conditions, and combined them with observations of snow and other key environmental factors. These data were employed to test the influence of winter microclimate on distributions, species richness and community composition of vascular plants, mosses and lichens. Our results show that winter thermal conditions vary much more strongly than summer conditions over short distances, and this variability is significantly related to the changes in species composition. Model comparisons show that winter conditions outcompete the effects of summer temperatures in driving the distributions and community properties of Arctic species, especially in vascular plants and lichens.

Our results indicate that winter is indeed a critical period for tundra vegetation. Wintertime thermal conditions driven by uneven snow distribution are thus key components of cold environments enabling a regional coexistence of large number of species. As winter climate is projected to warm more rapidly than summers (3), it becomes highly critical to increase the understanding of ecosystem responses to changes in snow conditions and winter temperatures. In conclusion, Arctic biodiversity assessments that ignore the effects of winter microclimate are prone to provide biased projections for the future of Arctic biodiversity.

1. S. Bokhorst et al., Changing Arctic snow cover: A review of recent developments and assessment of future needs for observations, modelling, and impacts. Ambio 45, 516-537 (2016). 2. C. M. Williams, H. A. L. Henry, B. J. Sinclair, Cold truths: how winter drives responses of terrestrial organisms to climate change. Biol. Rev. 90, 214-235 (2015). 3. R. Bintanja, O. Andry, Towards a rain-dominated Arctic. Nature Clim. Change 7, 263-267 (2017).