Conclusions
Since at least the late 19th century, summer temperatures have been the main climatic driver of growth of the evergreen dwarf shrub Cassiope tetragona at the Low Arctic research site near Qeqertarsuaq on the southern tip of Disko Island, western Greenland. Winter temperatures have been a co-driver of shrub growth. In recent and past warm decades, however, shrub growth has diverged from summer temperatures. For some individual shrubs, apparently mostly those growing at positions with lower soil moisture availability as determined by micro-topography, early summer precipitation has become the main driver during the recent climatic period (1984-2013), while over half of the shrubs studied became insensitive to summer temperatures. The correspondence between climatic drivers, C. tetragona growth and branch initiation frequency, and satellite-observed vegetation productivity (NDVI), suggests that the climate-growth observations for C. tetragona at least partly apply to the entire shrub-dominated vegetation in the research area. Winter warming events are likely the most prevalent cause of branch mortality in C. tetragona . Increased branching after years with poor growth and cooler than average summers, seems related to stress and may enable the shrub to overcome damage, for example, caused by winter warming events.
These findings suggest that erect dwarf-shrub tundra in the Low Arctic will likely become decreasingly temperature- and increasingly moisture-limited under future warming and that, although winter warming supports shrub growth, increased frequency of extreme winter warming events will likely lead to increased branch mortality and damage the dwarf shrub tundra. The observed divergence between summer temperatures and shrub growth during warm phases and the contrasting, potentially balancing effects of warmer winter temperatures may offer a mechanistic explanation for the still widespread stability of Arctic tundra vegetation.