What about winter? Integrating the missing season into climate change experiments in seasonally snow covered ecosystems

1. Ecosystem-scale climate warming experiments have provided insight into the future dynamics of terrestrial ecosystems. In seasonally snow covered regions, changes in winter soil temperatures are largely mediated through shifts in depth and duration of snowpack that have implications for ecosystem processes. Despite known effects of changes in snowpack dynamics on ecosystem processes in seasonally snow covered environments, the extent to which warming experiments address winter climate change is unclear. Identifying how previous warming studies have addressed changes in winter climate is important for understanding what is known about interacting effects of climate change across seasons and to highlight gaps in our understanding of ecosystem responses to climate change throughout the year. 2. We reviewed 64 published ecosystem-scale climate change manipulation experiments spanning three decades that were conducted in seasonally snow covered regions and simulated warmer growing season temperatures. 3. We found that only 20% of experiments in snow covered ecosystems explicitly manipulated the winter climate to reflect projected changes in winter soil temperatures mediated through changes in snowpack over the next several decades. Experiments that manipulated both winter and the growing season climate employed a variety of passive and active warming methods. Above-ground active methods most realistically simulated projected changes in soil temperature that will be mediated through changes in snow cover. Experiments with distinct winter treatments were distributed among tundra, forest, grassland and wetland ecosystems and together show that shifts in winter climate have a strong influence on, and in some cases offset, observed responses to experimental warming in the growing season alone or across the year. 4. By designing climate change experiments that examine the effects of winter and growing season climate change together, there is an opportunity to develop a better mechanistic understanding and more accurate projections of ecosystem responses to climate change that would not be evident from examining changes in either season alone.