Climate synchronises shrub growth across a high-arctic archipelago: contrasting implications of summer and winter warming

Climate change is most pronounced at high latitudes, where plant and animal populations are often strongly influenced by environmental fluctuations related to climate and weather. Environmental conditions can co-fluctuate over large distances and thereby synchronise primary production in space. However, large-scale studies of such spatiotemporal patterns remain rare in the Arctic, where short time-series and poor spatial replication have characterised the data available on both biotic and abiotic parameters. Here, we use dendrochronological tools to measure ring growth of a dominant dwarf shrub, the polar willowSalix polaris, previously found to reliably trace community-level vascular plant biomass production. We investigated climate drivers of vegetation growth and their role in the synchronisation of primary production across the rapidly warming archipelago of Svalbard (n = 8 sites, composed of 17 sub-sites, 0.06-293 km apart). We found contrasting effects of summer versus winter weather on ring growth and its spatial synchrony. Although an overall positive effect of summer temperature caused spatially synchronous growth, negative impacts of winter rain-on-snow events occurred only locally, potentially counteracting such synchrony. However, the anticipated increase in both summer temperature and spatial extent of rain-on-snow events, causing basal ice encapsulation of the vegetation, could change the relative importance of seasons for spatiotemporal dynamics of shrub growth. Because these shrub ring growth chronologies reflect annual fluctuations in total vascular plant biomass, fuelling the bottom-up controlled food-web, these results have large implications for our understanding of how climate change shapes tundra ecosystem productivity in time and space.