Increasing carbon storage capacity across global forest biomes

The carbon (C) sink in aboveground woody biomass (B) of mature forests remains one of the most uncertain components of the global C budget, with contrasting estimates from forest inventories, remote sensing, and ecosystem models, and an uncertain contribution from environmental change. The self-thinning relationship—describing the decline in tree density as mean tree size increases—encapsulates the carrying capacity for B and varies across forest types and environments. Assessing its temporal stability enables the separation of C uptake driven by changes in forest area or recovery from past disturbance from changes in the carrying capacity of mature forest biomass, potentially induced by environmental change.

Here, we compiled 105,763 inventories in natural forests spanning all major forest biomes worldwide to quantify temporal changes in the self-thinning relationship at the global scale. We detected a gradual and pervasive upward shift of the tree density–size relationship across all biomes, suggesting a thickening of mature forests and a partial relaxation of self-thinning constraints. The most pronounced thickening occurred in forests located in warm, dry climates and in regions with low nitrogen deposition and high soil phosphorus availability, whereas forests characterized by high soil C:N ratios and elevated organic carbon content showed the weakest responses. The observed shift in the self-thinning relationship implies a global C sink of 1.9 Pg C yr^-1 [95% confidence interval: 1.77-2.07 Pg C yr^-1], highlighting the changing carrying capacity of aboveground biomass stocks in mature forests as a key mechanism underlying the persistent terrestrial C sink.