Effects of changes in winter snowpack on above- and belowground carbon fluxes in a mixed-hardwood forest

Background/Question/Methods

In forests of the northeastern U.S., soil frost is a natural event that occurs when there is insufficient snowpack accumulation to insulate soils in winter. These forests typically have a continuous snowpack for much of the winter; however, winters with a late-developing or intermittent snowpack occasionally occur and may result in colder soil temperatures and a greater frequency and severity of soil frost. Climate models project a reduction in snowpack depth and duration by the end of the 21^st century in the northeastern U.S., which may have important implications for ecosystem carbon (C) fluxes and the ability of forests to mitigate climate change. The objectives of this research are to quantify the impacts of reduced winter snowpack and increased soil frost on stem CO₂ efflux and soil respiration. We are conducting a snow removal experiment in mixed stands of red oak (Quercus rubra) and red maple (Acer rubrum) trees at Harvard Forest in central Massachusetts. Snow was removed from three plots (13m x 13m) during the first five weeks of winter to mimic a later development of snowpack.

Results/Conclusions

During the first year of this multi-year study snow removal increased the depth and duration of soil frost, increased the frequency of freeze-thaw cycles, and impeded soil warming in the spring compared to the reference plots. A later development of snowpack and associated changes in soil frost dynamics tended to reduce rates of stem CO₂ efflux, with a larger effect on red maple compared to red oak trees. In contrast, these changes in winter climate tended to increase belowground losses of CO₂ from bulk soil respiration. Greater root + rhizosphere respiration, rather than changes in heterotrophic soil respiration, appears to be driving this increase in bulk soil respiration. This may be indicative of greater belowground C allocation in response to a later developing snowpack and increased soil frost. Furthermore, the response of stem CO₂ efflux to these changes in winter climate appear to vary by tree species, highlighting the importance of understanding species specific responses to environmental perturbations.