Carbon budget of the Harvard Forest Long‐Term Ecological Research site: pattern, process, and response to global change

TLDR: In this article, the authors brought together hundreds of thousands of C-cycle observations at the Harvard Forest in central Massachusetts, USA, a mid-latitude landscape dominated by 80-120-yr-old closed-canopy forests, to answer four questions: where and how much carbon is presently stored in dominant forest types, what are current rates of C accrual and loss, what biotic and abiotic factors contribute to variability in these rates, and how has climate change affected the forest's C cycle.

Abstract: 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-yr-old closed-canopy forests. These data answered four questions: (1) where and how much C is presently stored in dominant forest types; (2) what are current rates of C accrual and loss; (3) what biotic and abiotic factors contribute to variability in these rates; and (4) how has climate change affected 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 680–750 g C m 2 yr ; belowground NPP contributed 38–47% of the total, but with large uncertainty. Mineral soil C measured in the same inventory plots in 1992 and 2013 was 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 . Net ecosystem production (NEP) in hardwood stands averaged ~300 g C m 2 yr . NEP in hemlock-dominated forests averaged ~450 g C m 2 yr 1 until infestation by the hemlock woolly adelgid turned these stands into a net C source. Since 2000, NPP has increased by 26%. For the period 1992–2015, NEP increased 93%. The increase in mean annual temperature and growing season length alone accounted for ~30% of the increase in productivity. Interannual variations in GPP and NEP were also correlated with increases in red oak biomass, forest leaf area, and canopy-scale lightuse efficiency. 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. 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 biotic and abiotic disturbances increases. Manuscript received 23 January 2020; accepted 22 May 2020. Corresponding Editor: Yude Pan. 16 Corresponding Author. E-mail: aabarker@fas.harvard.edu Article e01423; page 1 Ecological Monographs, 90(4), 2020, e01423 © 2020 by the Ecological Society of America