Effects of changes in the winter snowpack on water and carbon fluxes in a temperate hardwood forest

Climate models predict winter warming in the northeastern United States of approximately 4 °C and a significant decrease in the depth and duration of the winter snow pack by the end of the 21st century. These changes may have significant long-term implications for ecosystem structure and function, and by extension biosphere-atmosphere interactions. In the absence of an insulating layer of snow, soils can freeze when air temperatures drop below freezing. In recent years, considerable progress has been made in our understanding of how soil microbial activity and nutrient cycling might respond to reduced snow cover and increased soil frost. However, the response of growing season plant dynamics to changes in the winter snowpack is less well understood, but may significantly impact regional carbon budgets. The objectives of this research were to determine the impacts of changes in the winter snowpack and increased soil frost on growing season plant dynamics such as sap flow, water use efficiency, and whole tree carbon gain. We conducted a snow-removal experiment in mixed stands of red maple (Acer rubrum) and red oak (Quercus rubra) at Harvard Forest in Petersham, MA. Snow was removed for the first 5 weeks of winter to mimic a later onset of the snowpack. Snow-removal induced soil frost and led to significantly lower soil temperatures in winter compared to the reference stands. The results of the first year of our multi-year study suggest that a later accumulation of snow and greater depth and duration of soil frost decreases rates sap flow and canopy carbon exchange in the early growing season, with a larger effect on red maple compared to red oak trees. Our results illustrate the importance of examining the impacts of changes in winter climate on plant dynamics during the growing season because there could be feedbacks to future climate.