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 .