Mean annual air temperatures for the northeastern U.S. are projected to increase 3 to 5 °C by the year 2100, which will increase the length of the growing season, but also reduce the depth and duration of winter snowpack and increase the frequency of soil freeze/thaw cycles (FTCs). Warmer soils have been shown to stimulate rates of soil respiration, but soil freezing in winter can increase, decrease, or have no effect on soil respiration. The combined effects of warmer soils in the growing season and FTCs in winter on above- and belowground carbon fluxes in northern forest ecosystems is unknown. 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 change on the functioning of northern hardwood forests. 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 with buried heating cables. Two others are warmed 5 degrees Celsius in the growing season and have snow removed during winter to induce soil FTCs. Two additional plots are not treated and serve as references for the experiment. We measured rates of carbon dioxide loss via stem and soil respiration over four years.
Results/Conclusions
We find that warmer soil temperatures lead to an approximately one-third increase in soil respiration during the first year of the experiment in both the plots that are warmed in the growing season and those that also experienced soil FTCs in winter. However, while rates of soil respiration have remained elevated in the warmed plots, winter FTC’s offset the stimulatory effects of warming in subsequent years. We also find that anomalously dry conditions in 2016 reduced the stimulatory effects of warming on soil respiration, but the warming effect rebounded in 2017. Aboveground, we find that stem respiration comprises a carbon loss equivalent to approximately 20% of the flux from soil respiration. Similar to soil respiration, we find that soil warming in the growing season increases rates of stem respiration. Our results highlight the importance of examining climate change across seasons when considering the response of carbon fluxes in northern temperate forests to climate change.