Mean annual air temperatures for the northeastern U.S. are projected to increase 3 to 5 degrees Celsius over the next century, leading to warmer growing season soil temperatures, reduced snowpack, and increased frequency of soil freeze/thaw cycles (FTCs) in winter. Warmer soils have been shown to stimulate rates of net nitrogen mineralization, nitrogen uptake, and carbon storage by trees. However, wintertime soil FTCs have been shown to damage tree roots, which impairs the ability of trees to take up nitrogen, water, and carbon early in the following growing season. Compared to our understanding of growing season effects on northern hardwood forests, we know little about the combined effects of changes in the growing season and winter climate, which are important to determine since they may be additive, synergistic, or cancel each other out. We established the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire in 2012 to determine the combined effects of winter and growing season climate on the functioning of northern hardwood forests. We established six plots (11 X 14 m) in a red maple (Acer rubrum) dominated forest.
Results/Conclusions
Two plots are warmed 5 degrees Celsius above ambient temperatures throughout the growing season with buried heating cables. Two others are also warmed 5 degrees Celsius in the growing season and have snow removed during winter to induce soil FTCs. Two plots are not treated and serve as references for the experiment. For five years, we measured rates of aboveground productivity using litterfall baskets (2013-2017) and dendrometer bands (2014-2018). We find that growing season soil warming stimulates aboveground tree growth and carbon sequestration up to 50%, but these warming induced increases are offset by as much as two thirds by soil FTCs in winter. Together, these results demonstrate that warmer growing seasons are likely to have strong positive impacts on rates of carbon sequestration in northern hardwood forests, but these effects will be offset by the negative effects of winter climate on the health of trees. Together, these results demonstrate the importance of evaluating the effects of climate change across seasons on the functioning of forest ecosystems.