The Ecological Importance of Winter in Temperate, Boreal, and Arctic Ecosystems in Times of Climate Change

Abstract

Climate warming is strongest in winter and in northern ecosystems. Ecological and biogeochemical impacts, however, depend mainly on soil temperatures. Climate warming can contrastingly affect winter soil conditions across northern biomes due to the crucial importance of snow cover: Increasing winter precipitation results in soil warming in the arctic, while midwinter snowmelt events can induce more severe soil frost in arctic and boreal ecosystems. Cold-temperate ecosystems are projected to experience increased soil frost due to strongly reduced snow cover no longer insulating the soil against still cold air temperatures. In cool-temperate ecosystems, warming eventually causes the complete loss of soil frost. Both pathways, soil warming and soil cooling, have important implications for ecology and biosphere-atmosphere feedbacks: In arctic and boreal ecosystems, increased decomposition and mineralization allow for enhanced primary production, but midwinter melting followed by frost and/or rain-on-snow events might counteract this trend. More variable surface temperatures can damage primary production, and colder soil temperatures, due to reduced snow cover, can significantly decrease decomposition in cold-temperate ecosystems. For cool-temperate ecosystems, wetter winters could result in nutrient leaching, and altered dormancy patterns could cause increased frost damage despite air warming. In summary, winter processes are clearly relevant for the biosphere-atmosphere feedback, and even the sign of this feedback, i.e., ecosystems acting as carbon sink or as carbon source, depends on winter processes in temperate, boreal, and arctic ecosystems. This review concludes that current knowledge is not sufficient to quantify this feedback with satisfactory certainty. Important processes and the key uncertainties are identified, e.g., synchronicity in above- versus belowground growing season; temporal hierarchies in ecological processes such as the role of root damage and root activity for decomposition of soil organic matter (“priming”); or shifts in plant species composition due to winter climate change determining primary production as well as litter quantity and decomposability. Evidently, sound projections of future ecosystem functioning and biotic feedbacks to climate change require a comprehensive understanding of winter ecological processes, which have so far too often been neglected.