Influence of winter temperatures
Winter temperatures preceding the growing season were observed to be the main co-driver of C. tetragona growth at the site level and for 11/25 shrubs studied over the period 1893-2013. In recent decades, it was found to be a co-driver for five shrubs and the main climatic driver for two. Notably, a similar positive effect of warm winters on dwarf shrub growth was observed for the deciduous dwarf shrub Betula nana at an adjacent site on Disko Island . In their study, the positive effect of winter temperatures on growth was interpreted as a result of warmer winters and related spring air temperatures causing snow to melt out earlier, allowing soils to drain and warm faster. This then results in longer growing seasons, leading to increased growth. For C. tetragona at the study site this mechanism is less plausible, as early summer temperatures were the main climatic driver of only four shrubs (1893-2013) and of none in recent decades. This suggests that this species may not benefit from earlier snow melt-out, which is in line with the observation that C. tetragona growth benefits from warmer Julys, but not from longer growing seasons near its European southern distribution limit in Abisko, Sweden . An alternative explanation could be grounded on the fact that winter air temperatures are positively related to winter precipitation sums in Disko Island (Figs S7 and S8). In warmer winters, an increase in winter snow precipitation may result in a deeper snowpack and higher winter soil temperatures due to insulation. Moreover, in recent decades winter soil temperatures have been directly coupled with winter air temperatures in the area . Warmer winter soil temperatures may be linked to higher microbial activity in winter and result in greater nutrient availability in the subsequent growing season and hence support increased shrub growth. In addition, warm winters are negatively linked with sea ice extent in the Disko Bay area and earlier sea ice retreat leads to warmer summers . This may explain the positive correlation between (late) winter and summer temperatures at the site (Figs S7 and S8) and partly or alternatively explain the observed association between winter temperatures and C. tetragona growth. A similar possible link between sea-ice conditions prior to the growing season, summer temperatures, and C. tetragona growth has previously been suggested in northern Greenland .