The global carbon balance of forests based on flux towers and forest age data

Flux towers worldwide record significant carbon sinks in forests. Still, most sites are situated over young or middle-aged stands at, or close to, the peak of carbon uptake along their successional cycle. As a result, upscaling forest Net Ecosystem Productivity (NEP) — the difference between gross primary production (GPP) and ecosystem respiration (ER) — from existing sites to the global scale has been challenging, in contrast to the successful upscaling of GPP and the associated surface energy fluxes. In this study, we combine a recently released forest age map 1, empirical NEP-age relationships and environmental predictors, and machine-learning algorithms to generate gridded estimates of forest NEP for the period circa 2010. The quantified global forest NEP is 6.4 ± 2.0 PgC year-1. After subtracting carbon losses not measured by flux towers, including fire emissions, harvested biomass, fluvial leaching, and non-CO2 gases released to the atmosphere, we obtain a global net carbon balance of forests remaining forests (NBP) close to 3.0 ± 2.0 PgC year-1. This value is consistent with those based on forest inventories 2 and atmospheric inversions 3. We report a carbon sink of 1.5 ± 0.5 PgC year-1 for temperate forests, a value twice as large as that inferred from inventories, largely because inventories may underestimate soil carbon accumulation. Boreal forests show a small source of -0.1 ± 0.3 PgC year-1. In contrast, tropical forests not subject to degradation and deforestation have an NBP sink of +1.6 ± 1.1 PgC year-1, larger than what is inferred from biomass inventories. Half of the tropical forest sinks are located in dry forests and woodlands, which have smaller productivity than humid forests but accumulate carbon from a younger age due to frequent fire disturbances. Our results bring a new constraint on the global carbon budget, highlighting the importance of accounting for forest age in upscaling local flux tower data.