A seminal publication in Ecological Monographs (Finzi et al. 90(4), 2020, e01423) on long-term studies of carbon cycling at the Harvard Forest of western Massachusetts presents a comprehensive carbon budget and evidence that this 80-120 year old hardwood forest is gaining carbon at an average rate of about 3 Mg ha-1 yr-1. My former research associate, Kathleen Savage, and I contributed two decades of soil respiration measurements to this study. Working with Susan Trumbore and Julia Gaudinski, we also provided radiocarbon evidence of a small (0.2 Mg ha-1 yr-1) soil sink for carbon.
During the same period, we also conducted studies on carbon cycling at the 160 year old spruce-hemlock forest of Howland Maine. A summary of the Howland carbon budget synthesis is the subject of the Davidson et al. oral presentation for the 2020 annual meeting of the American Geophysical Union. Like its Harvard Forest neighbor to the south, the Howland Forest is also gaining carbon, with a 3-decade average rate of about 2 Mg ha-1 yr-1. Most was found in accumulating live tree biomass, with a small fraction possibly accumulating in the soil or as dead wood (see figure below).
In both forests, there was large inter-annual variation in net ecosystem productivity (NEP) that resulted in rendering not statistically significant a trend of increasing NEP since the early 1990s. However, the trending rate of increase at the Howland forest was about 7 g m-2 yr-1, whereas it was less than 2 g m-2 yr-1at the Howland Forest. Lack of a clearer and stronger CO2 fertilization effect at the Howland Forest is puzzling – it could be that there are confounding effects of other global change processes that are cancelling the fertilization effect of increasing atmospheric CO2, or perhaps the spruce and hemlock species that dominate this site are not responsive to CO2 fertilization.