Effects of warmer growing season temperatures and a reduced winter snowpack on water and carbon uptake in a northern hardwood forest
Mean annual air temperatures for the northeastern U.S. are projected to increase 3 to 5 °C by the year 2100, which could have positive effects on northern hardwood forests through increased water and carbon uptake by trees during the growing season. Climate models also project that warmer air temperatures during the winter will be associated with a smaller and less persistent winter snowpack and increased frequency of soil freeze/thaw cycles, which may offset the positive effects of warming by damaging roots. In recent years, considerable progress has been made in our understanding of how warmer soil temperatures in the growing season and colder soils in winter affect plant dynamics and ecosystem functioning. Warmer soils have been shown to stimulate rates of transpiration and photosynthesis by trees. Soil freezing in winter has been shown to damage tree roots in northern hardwood forests and impair their ability to take up water and carbon early in the next growing season. However, the combined effects of warmer soils in the growing season and colder soils in winter on the ability of northern forest ecosystems to take up water and carbon are unknown.
We conducted an experiment in a northern hardwood forest at Hubbard Brook Experimental Forest in New Hampshire to determine the combined effects of winter and growing season climate on water and carbon uptake by red maple trees. We established six plots (11 X 14 m) in a red maple (Acer rubrum) dominated forest. Two plots are warmed 5 degrees Celsius throughout the growing season. Two others are warmed 5 degrees Celsius in the growing season and have snow removed during winter to induce soil freeze/thaw cycles. Two additional plots serve as references for the experiment.
The results of this experiment show that warmer soil temperatures lead to greater rates of sap flow (i.e. transpiration) in the early growing season, but these increases are offset by changes to sap flow caused by soil freeze/thaw cycles in winter. Surprisingly, experimental plots with both warming in the growing season and soil freeze/thaw cycles in winter had significantly greater rates of leaf-level photosynthesis compared to plots that were warmed in the growing season, but did not experience changes in freeze/thaw cycles in winter. Our results demonstrate complex interactions between climate change and tree physiology and highlight the importance of examining climate change across seasons in northern latitude forests.