Winter warming events damage sub-Arctic vegetation: consistent evidence from an experimental manipulation and a natural event

1. The Arctic is experiencing considerable change in climate, particularly in winter, and a greater frequency of extreme climatic events is expected. However, the impacts of winter climate change and extreme events have received far less attention than the impacts of season-long summer warming. Here we report findings from observations following a natural event and from experimental studies to show that short (<10 days) extreme winter warming events can cause major damage to sub-Arctic plant communities at landscape scales. 2. In the landscape observations, impacts were assessed following an extreme winter warming event that occurred in December 2007 in northern Scandinavia. During this event, temperatures rose up to 7 °C resulting in loss of snow cover and exposure of vegetation to firstly warm and then returning cold temperatures. 3. In the following summer, extensive areas of damaged dwarf shrub vegetation could be observed. Ground observations showed damaged areas to have a 16 times greater frequency of dead shoots of the dominant shrub Empetrum hermaphroditum, resulting in 87% less summer growth compared to neighbouring undamaged areas. The landscape scale extent of this damage was confirmed by satellite-derived Normalized Differential Vegetation Index values that showed a considerable 26% reduction (comparing July 2007 with July 2008 values) over an area of 1424 km². This reduction indicates a significant decline in either leaf area or photosynthetic capacity or efficiency at the landscape scale. 4. Strikingly similar damage was also observed in a field manipulation experiment using heating lamps and soil warming cables to simulate such extreme events in sub-Arctic heathland over two winters. Here, an up to 21 times greater frequency of dead shoots and 47% less shoot growth of E. hermaphroditum was observed in plots exposed to simulated winter warming events compared to unmanipulated controls. 5. Synthesis. These combined landscape observations and experimental findings provide compelling evidence that winter warming events can cause considerable damage to sub-Arctic vegetation. With increasing winter temperatures predicted, any increase in such damage may have major consequences for productivity and diversity of these sub-Arctic ecosystems, in contrast to the greening of parts of the Arctic currently attributed to summer warming.