Stand density and local climate drive allocation of GPP to aboveground woody biomass.

The partitioning of photosynthate among various forest carbon pools is a key process regulating long-term carbon sequestration, with allocation to aboveground woody biomass carbon (AGBC) in particular playing an outsized role in the global carbon cycle due to its slow residence time. However, directly estimating the fraction of gross primary productivity (GPP) that goes to AGBC has historically been difficult and time-consuming, leaving us with persistent uncertainties. We used an extensive dataset of tree-ring chronologies co-located at flux towers to assess the coupling between AGBC and GPP, calculate the fraction of fixed carbon that is allocated to AGBC, and understand the drivers of variability in this fraction. We found that annual AGBC and GPP were rarely correlated, and that annual AGBC represented only a small fraction (c. 9%) of fixed carbon. This fraction varied considerably across sites and was driven by differences in stand density and site climate. Annual AGBC was suppressed by c. 30% during drought and remained below average for years afterward. These results imply that assumptions of relatively stationary allocation of GPP to woody biomass and other plant tissues could lead to systematic biases in modeled carbon accumulation in different plant pools and thus in carbon residence time.