Widespread sink limitations on forest biomass growth in the Northern Hemisphere

Forests, which sequester atmospheric CO₂ in the form of biomass within long-term reservoirs, are critical to the global land carbon sink. Current land surface models assume a strong coupling between photosynthesis and plant biomass changes, with carbon supply (i.e., carbon assimilation though photosynthesis) has been considered the main driver of plant biomass growth (‘source limitation’). However, the potential for sink limitations to constrain plant biomass growth, where plant biomass change become decoupled from photosynthesis, has been raised and supported by free air CO₂ enrichment (FACE) experiment, inventory and tree ring evidence.

In this study, we relied on high spatial resolution satellite-based retrievals of above-ground biomass (AGB) and vegetation primary productivity (GPP), to quantify the extent of decoupling between plant photosynthesis and biomass growth at the ecosystem scales over the past decade in the Northern Hemisphere (from 35ºN to 90ºN). We found that the fraction of decoupled area in non-intact forest is 66 ± 9%, significantly higher than in the intact forest. Extensive decoupling was observed across Europe, Russia and Canada. This spatial pattern was verified using multiple satellite-derived and inventory-derived AGB, and GPP data from P model. To investigate the drivers of decoupling, we built a generalized additive model to predict spatial variations in decoupling fractions within non-intact forests. The model suggests that harvest and logging account for most of the decoupling in Europe, while wildfires in Siberia may promote a recovery of coupling due to rapid vegetation regrowth. More importantly, even in intact forests, 56 ± 13% still exhibited the decoupling signals. In western Russia, this decoupling appears to be driven by droughts, likely due to carbon allocation shifts to support metabolism and critical plant functions, thereby constraining biomass growth. In western Canada, decoupling was found in in old-growth, or dense intact forests, where high decomposition, competition, or mortality may result in stable or declining forest biomass over time. Our analysis provides a geographic overview of regions experienced sink limitations to forest biomass growth, as well as insights into the mechanisms regulating terrestrial carbon sequestration. These findings represent a critical step toward improving process-based models and enhancing predictions of terrestrial carbon dynamics under future climate change scenarios.