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 six 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;
(iv) is climate change affecting the forest’s C cycle; (v) how do rates of C
accrual from regrowth compare to C cycle modifications imposed by
global change experiments; (vi) are C stocks and accrual rates at the
Harvard Forest representative of the surrounding ecoregion? 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. Inventory
data suggest no 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). The C sink in regrowing biomass equaled or
exceeded C cycle modifications imposed by global change experiments.
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. Data and 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.