Professor Boston University Boston, MA, United States
Abstract: Across temperate North America, carbon dioxide uptake by terrestrial ecosystems offsets approximately 20% of annual carbon (C) emissions, but this C sink may be threatened by climate change. The northeastern U.S. is projected to experience up to a 2.2-4.4 degrees C increase in air temperature by the year 2099 compared to the 1986–2015 base period, which is expected to coincide with up to a 95% decrease in the spatial extent of winter snowpack compared to 1951-2005. Studies have found that warmer growing season temperatures lead to greater rates of net C sequestration in forests. However, a smaller snowpack can induce soil freeze/thaw cycles (FTCs) in areas that experience below-freezing winter air temperatures, which damage tree roots, decrease rates of nutrient uptake, and reduce C sequestration in the aboveground biomass of trees up to 40%. It is unknown how the combination of increased growing season temperatures and winter soil FTCs impacts rates of net C sequestration in temperate forest ecosystems. We hypothesized that growing season warming stimulates higher net C sequestration in temperate forests, but winter FTCs reduce these gains. We tested this hypothesis using the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire. This experiment includes two reference plots, two plots that are warmed 5 degrees C during the growing season with buried heating cables, and two plots that are warmed during the growing season and experience soil FTCs by removal of snow in winter. We used dendrometer bands from 2012-2022 to measure monthly radial growth of trees and using allometric equations scaled up radial growth to ecosystem level C in tree aboveground biomass over time. We found that growing season warming increases tree growth and net C sequestration, but this is offset by damage caused by smaller snowpack and greater frequency of winter soil FTCs. Because Earth system models that include climate-carbon cycle feedbacks currently focus on growing season climate conditions, our results suggest that these models may overestimate the potential for temperate forests with seasonal snow cover to sequester C in aboveground biomass. While warmer growing seasons increase net forest C sequestration, the negative impact of winter FTCs on C storage is important to consider when predicting the future C sink capacity of northeastern forests. Understanding how climate change across the seasons impacts net C sequestration in temperate forests will help us improve climate models and inform policies including decarbonization strategies.
