The hidden demography of the 21st century global forest carbon sink

Forests play a central role in the global carbon cycle, yet most assessments quantify only net carbon balance, obscuring the demographic processes that govern long-term forest carbon dynamics1-3, and reinforcing a prevailing paradigm that tropical forests dominate the global land carbon sink2-6. Here we assemble and harmonize approximately one million remeasured permanent forest plots to establish the first global, observation-based benchmark of intrinsic forest carbon turnover, decomposing net aboveground biomass change (2.23 Pg C yr⁻¹) into growth (7.74 Pg C yr⁻¹), recruitment (2.11 Pg C yr⁻¹), and mortality (-7.62 Pg C yr⁻¹). Across major tropical ecozones, intrinsic net change is near zero despite high growth rates, because mortality closely tracks growth, whereas temperate oceanic forests exhibit the largest net gains globally. Size-structured analyses further reveal that global forest carbon dynamics are dominated by demographic fluxes in small to intermediate size classes rather than by the largest trees. Although large trees store substantial biomass individually, their relatively low abundance limits their contribution to ecosystem-scale fluxes. Together, these results contribute a demographically explicit evaluation of fundamental hypotheses regarding tropical dominance, the role of large trees, and the relative importance of growth, regeneration, and mortality in shaping forest biomass dynamics.