Carbon budget of the Harvard Forest Long-Term Ecological Research site: pattern, process, and response to global change
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−1; 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−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 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 light-use 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.
Finzi, Adrien C.; Giasson, Marc André; Barker Plotkin, Audrey A.; Aber, John D.; Boose, Emery R.; Davidson, Eric A.; Dietze, Michael C.; Ellison, Aaron M.; Frey, Serita D.; Goldman, Evan; Keenan, Trevor F.; Melillo, Jerry M.; Munger, J. William; Nadelhoffer, Knute J.; Ollinger, Scott V.; Orwig, David A.; Pederson, Neil; Richardson, Andrew D.; Savage, Kathleen; Tang, Jianwu; Thompson, Jonathan R.; Williams, Christopher A.; Wofsy, Steven C.; Zhou, Zaixing; and Foster, David R., "Carbon budget of the Harvard Forest Long-Term Ecological Research site: pattern, process, and response to global change" (2020). Geography. 867.