Temperature-growth decoupling
The shoot length growth response to summer temperatures ofCassiope tetragona at the site level over the entire period
studied (1893-2013) can be classified as highly sensitive (∆AIC = 49.63;
cf. ). However, in recent decades (1984-2013) this sensitivity can be
categorized as low (∆AIC = 5.78) and it was completely absent for over
half of the individuals studied, as shown by the analyses on individual
shrub level. While late summer (July-August) temperatures were the most
limiting factor for growth at the site level over the period 1984-2013,
early summer precipitation was identified as an important co-driver of
growth at the site level and as the most limiting factor for 20 percent
of the individuals studied here. A similar divergence between growing
season temperatures and evergreen dwarf shrub growth has recently been
demonstrated in the French Alps for the species Rhododendron
ferrugineum .
The observed lower sensitivity of shrub growth to summer temperature in
recent decades was probably caused by the on average warmer summers
during this period (7.13 °C in 1984-2013 vs. 6.40 °C in 1893-1983,
p<0.001; Fig. S6). This, in combination with the observed
non-linear growth response to summer temperatures, with a smaller, lack
of, or even negative response to mean summer temperatures exceeding a
threshold of approx. 7 °C (Fig. S5). Although the shrubs were capable of
maintaining high levels of shoot length growth in summers with mean
temperatures exceeding this threshold, other factors than summer
temperature became more limiting for the majority of the shrubs studied.
Interestingly, reported a weakening in temperature-growth relationships
in Betula nana and Salix glauca shrubs in more continental
Greenland, 272 km southeast of Disko Island, when summer (there:
May-August) temperatures passed a threshold of approx. 7 °C. A similar
non-linear response of C. tetragona shoot length growth was
observed over a gradient of growing degree-day sums
(GDD5; cumulative daily mean temperatures above 5 °C)
across three sites, two in High Arctic Svalbard, one in Sub-Arctic
Sweden . It is, therefore, likely that this evergreen dwarf shrub
species will not be able to respond with increased growth rates to
warmer summers in the Low and Sub-Arctic under future global warming.
This may lower its competitiveness, especially for light, in relation to
taller deciduous shrub species, which are migrating into tundra
ecosystems . However, growth of such species has been shown to be less
sensitive to warming at drier sites, than at moister sites . The dwarf
shrub C. tetragona , on the other hand, is restricted to
relatively dry soils and adapted to dry conditions with its xeromorphic
leaves . Therefore, it may remain competitive in drier habitats.
In this study, a positive influence of summer precipitation on growth ofC. tetragona was detected for some of the shrub individuals
studied here over the period 1984-2013. Summer precipitation was
observed to be a co-driver of C. tetragona growth for one of four
sites on Ellesmere Island in High Arctic Canada . However, a similar
influence had not yet been observed for this species at other sites. Not
at relatively warm tundra sites in the Sub-Arctic (; ), nor in the High
Arctic , including the northernmost polar desert in northern Greenland .
In addition, C. tetragona growth did not respond to
experimentally doubled summer precipitation in High Arctic Svalbard .
The reason for a lack of detection of a summer precipitation signal in
other Arctic sites could be (micro)site-specific. Soil moisture
availability may be largely decoupled from summer precipitation and
instead determined by downslope meltwater run-off, active layer thaw,
and low evapotranspiration rates in the High Arctic . At Arctic-alpine
sites with late snow cover, where the species is found , snowmelt likely
provides sufficiently high soil moisture levels throughout the growing
season. In addition, at other sites, climate-growth relationships have
not been assessed at the individual level and sensitivity to
precipitation may not only be site-specific, but also micro-site
specific, due to, for example greater soil moisture availability in
small depressions or snow accumulation behind rocks. Remarkably, two of
the shrubs studied here (DI15 and DI16) grew near rocks and their growth
remained sensitive to summer temperatures in recent decades (Fig. S4).
Furthermore, climate-growth links have mostly solely been studied over
longer periods, and did not focus on the recent warm period. However,
growth of the deciduous dwarf shrub Betula nana in the
northeastern Siberian tundra was shown to be sensitive to summer
precipitation in warm, but not in cool summers . In addition, shoot
length growth of C. fastigiata was detected to be sensitive to
moisture availability in the tree-line ecotone and growth ofJuniperus indica shrubs was found to be driven by early growing
season precipitation , both in the central Himalayas of Nepal. In
addition, low amounts of summer precipitation as a co-determinant of
drought conditions has been shown to indirectly drive declining
deciduous shrub growth at drier sites, while summer temperature drives
increasing deciduous shrub growth at wetter sites in the Arctic .