Dissolved Organic Carbon Mobilization Across a Climate Transect of Mesic Boreal Forests Is Explained by Air Temperature and Snowpack Duration

The mobilization of soil dissolved organic carbon (DOC) is driven by biogeochemical and hydrological processes operating at different temporal and spatial scales. In seasonally snow-covered ecosystems, the snowpack holds both biogeochemical and hydrological significance as insulator of the soil during winter and reservoir of a large proportion of the annual precipitation that is released during spring melt. This four-year study was conducted within three maritime balsam fir forests spanning 47°N to 53°N where mean annual precipitation (1074 mm to 1340 mm) and mean annual temperature (0 °C to 5 °C) increase with decreasing latitude. All forests are consistently snow-covered throughout winter with snowpack depths sufficient to protect soils from extreme freezing. However, there is a decrease in the amount of snowfall (462 to 393 cm), and a decrease in the length of the snowpack season (160 to 109 days) with decreasing latitude, analogous to the changing winter conditions. Mean annual DOC fluxes decreased with latitude and exhibited no relationship with interannual precipitation, but a positive relationship with interannual air temperature. To interpret this result, a series of additional hydrometeorological indices were ranked. Of these, air temperature and snowpack duration best described interannual variability in DOC flux and were highly correlated. Therefore, air temperature may indirectly affect DOC mobilization through a direct control on snowpack season length. Both warmer years and warmer sites have shorter snowpack seasons and mobilize more soil DOC, suggesting that boreal forest soils are larger sources of DOC to mineral soil and/or aquatic ecosystems under warmer winter temperatures via changes to snowpack dynamics.