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 .