Transpiration of Dominant Tree Species Varies in Response to Projected Changes in Climate: Implications for Composition and Water Balance of Temperate Forest Ecosystems

The climate is changing in many temperate forests with winter snowpack shrinking and an increasing frequency of growing season air temperatures exceeding long-term means. We examined the effects of these changes on growing season rates of transpiration (sap flow) in two snow removal experiments in New Hampshire and Massachusetts, USA. Snow was removed during early winter, resulting in greater depth and duration of soil freezing compared to untreated plots. We examined the dominant tree species at each site, Acer saccharum at Hubbard Brook, NH and Acer rubrum and Quercus rubra at Harvard Forest, MA. Trees responded to a smaller snowpack, but with distinct species-specific responses consistent with ecological traits. Snow removal decreased rates of sap flow per kPa vapor pressure deficit (VPD) in sugar maples in the early growing season and red maples throughout the growing season. In contrast, sap flow rates per kPa VPD increased for red oak with snow removal. Downscaled climate projections from the Coupled Model Intercomparison Project indicate increases in heat stress days at both sites by the end of the century, leading to increased rates of whole-season transpiration across all three tree species, which will be offset in red maples and increased in red oaks with a smaller snowpack and increased frequency of heat stress days. Results of this study demonstrate that the combined effects of changes in climate during the growing season and winter will impact transpiration differently among tree species, with implications for forest water balance and tree species composition in the northeastern USA