Winter seasonality with low amounts of incoming radiation, frost and snow cover characterize large areas at mid- and high latitudes, especially in the arctic and in mountain ranges. Because of these adverse conditions, it is often assumed that ecosystem processes, such as plant photosynthesis and nutrient uptake and microbial activities, cease or at best diminish to marginal rates of summer values. On the other hand, snow is a good thermal insulator and a sufficiently thick snow cover might enable temperature-limited processes to continue in winter, especially belowground. Changes in winter precipitation may changes this, but so far ecosystem processes in winter are little understood because low temperatures and snow complicate investigating them in situ. We performed a snow-removal experiment on an ombrotrophic bog in the Swiss Jura mountains (1036 m.a.s.l.) to compare above- and belowground ecosystem processes with and without snow cover during winter and the subsequent growing season. The presence of snow in winter (ca. 0.7m) strongly reduced the maximum potential photosynthesic capacity (Amax) of Eriophorum vaginatum as well as the total microbial biomass compared to spring and summer values. Amax of Sphagnum magellanicum and uptake of 15N-labelled ammonium-nitrate by vascular plants on the other hand were almost as high or higher in winter as in summer. Snow removal increased the number of freeze-thaw cycles in mid-winter but increased the minimum soil temperature in late-winter before ambient snow-melt. This strongly negatively affected all measured ecosystem processes. Plant 15N-uptake, Amax of Eriophorum and total microbial biomass returned to or exceeded control values soon before or after snowmelt, while Sphagnum Amax and length growth, as well as the structure of the microbial community showed clear carry-over effects of the reduced winter snow cover into next summer. Alltogether our data indicate that peatlands are active in winter and that a continuous snow cover is crucial for ecosystem processes both in winter and in the subsequent summer.