Carbon Budget at the Harvard Forest 1992-2015

How, where, and why carbon (C) moves into and out of an ecosystem through time are long-standing questions in biogeochemistry. Here, we bring together hundreds of thousands of C-cycle observations at the Harvard Forest in central Massachusetts, USA, a mid-latitude landscape dominated by 80–120-year-old closed-canopy forests. These data answered four questions: (i) where and how much C is presently stored in dominant forest types; (ii) what are current rates of C accrual or loss; (iii) what biotic and abiotic factors contribute to variability in these rates; and (iv) is climate change affecting the forest’s C cycle? Harvard Forest is an active C sink resulting from forest regrowth following land abandonment. Soil and tree biomass comprise nearly equal portions of existing C stocks. Net primary production (NPP) averaged 750–970 g C m-2 yr-1; belowground NPP contributed 30–60% of the total. Mineral soil C measured in the same inventory plots in 1992 and 2013 were too heterogeneous to detect change in soil-C pools; however, radiocarbon data suggest a small but persistent sink of 10–30 g C m-2 yr-1. Net ecosystem production (NEP) in hardwood stands averaged ~300 g C m-2 yr-1. NEP in hemlock-dominated forests averaged ~450 g C m-2 yr-1 prior to infestation by the hemlock woolly adelgid (HWA) in 2013, and then became a net C source. Stand dynamics and climate change in the last three decades enhanced the C sink in hardwood stands; NPP increased 26% between 2000–2014 (p = 0.02) and NEP increased 93% between 1992–2015 (p = 0.13). Compared to long-term global change experiments at the Harvard Forest, the C sink in regrowing biomass equaled or exceeded C cycle modifications imposed by soil warming, N saturation, and hemlock removal. Median forest biomass in the surrounding ecoregion was only 78% of that at the Harvard Forest due to higher timber harvesting rates across the region. Results of this synthesis and comparison to simulation models suggest that forests across the region are likely to accrue C for decades to come but may be disrupted if the frequency or severity of disturbance increases.