Effects of snow cover variations on spatial patterns of carbon emissions in northern forests
Winter snow regulates terrestrial carbon (C) cycling by modifying micro-climate, but a comprehensive assessment and understanding at the process level is missing in ecosystem models. Here, we investigate the effect of winter snow variation, i.e., the timing and duration, on C fluxes from northern forests at two different spatial scales using eddy covariance measurements and ecosystem model simulations.
We found that flux site observational ecosystem respiration (Reco) and net ecosystem C exchange (NEE) were correlated with fluctuations of soil temperature controlled by variation in snow depth, but not correlated with the variation of air temperature. Winter Reco and NEE ranged from 0.03-1.9 gC·m-2·day-1 according to site observations, and the Biome-BGC modeled winter NEE for northern forest ecosystems was approximately 0.96 Petagram (1 Pg = 1015 gram) of C per year during the period between 1982 and 2009. A statistically significant change in model simulated winter Reco was noticed in Eurasia, with a decreasing rate at 0.61-1.21 gC·m-2·yr-1 in Siberia. Both increasing and decreasing trends of winter NEE were observed in North America, which resulted in an altered C emission pattern with a relatively stable total winter Reco. Across the entire study area, the winter Reco decreased 0.21 gC·m-2·yr-1, which accounts for approximately 20% of the enhanced annual C sink in the northern forest area. In total, reduced snow cover increased soil freezing potential, and temporarily suppressed the winter C efflux by 0.83 Pg of C during entire the 27-year study period.