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.