Declining winter snowpack offsets carbon storage enhancement from growing season warming in northern temperate forest ecosystems.

Northeastern US temperate forests are currently net carbon (C) sinks and play an important role offsetting anthropogenic C emissions, but projected climatic changes, including increased temperatures and decreased winter snowpack, may influence this C sink over the next century. Past studies show that growing season warming increases forest C storage through greater soil nutrient availability that contributes to greater rates of net photosynthesis, while reduced winter snowpack induces soil freeze/thaw cycles that reduce tree root vitality, nutrient uptake, and forest C storage. The year-round effects of climate change on this C sink are not well understood. We report here decade-long results from the Climate Change Across Seasons Experiment (CCASE) at the Hubbard Brook Experimental Forest, which determines the combined effects of growing season warming and a smaller winter snowpack on C storage in northern temperate forests. We found after a decade of treatments that growing season warming increases cumulative tree stem biomass C by 63%. However, winter soil freeze/thaw cycles offset half of this growing season warming effect. The amount of C stored in stem biomass of trees experiencing both growing season warming plus smaller winter snowpack is only 31% higher than the reference plots, but this difference is not significant. Our results suggest that current Earth system models are likely to overestimate the C sink capacity of northern temperate forests because they do not incorporate the negative impacts of a shrinking snowpack and increased frequency of soil freeze/thaw cycles on C uptake and storage by trees.