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

Terrestrial gross primary production and energy fluxes have been successfully scaled from ecosystem eddy covariance (EC) observations to the global scale. However, this has not been possible for net ecosystem productivity (NEP) because forest age is an important determinant of NEP and up to now there was no globally gridded forest age map available. Moreover, the age distribution of EC sites is not representative of global forests. Here we combine new maps of forest age with NEP-age relationships and environmental predictors using different machine-learning algorithms to produce gridded estimates of forest NEP. Globally, forest NEP when accounting for age is a sink of CO₂of ≈ 6.5 ± 0.5 Pg C a^-1. Removing from NEP the losses of carbon that are not observed at flux tower locations: fire emissions, harvested biomass, carbon leached to rivers, and non-CO₂gases released to the atmosphere, provides the net carbon balance of forests (NBP) being close to 4 ± 0.9 PgC a^-1. This new estimate matches independent forest inventories and atmospheric inversions results. Temperate forest NBP is a carbon sink of 1.7 ± 0.2 PgC a^-1, twice larger than inferred from biomass inventories by Pan et al ¹ . A possible explanation for this discrepancy is that Pan et al. underestimated soil carbon storage in these forests. Boreal forest NBP is a small sink of 0.3 ± 0.2 PgC a^-1. The NBP of intact tropical forests is estimated to be 1.8 ± 0.5 PgC a^-1, about 1/3 less than inferred from sparse inventory data. About half of the tropical NBP is located in dry tropical forests and woodlands, despite their smaller productivity than humid forests. This new global estimate of the forest carbon balance based on flux towers adds independent evidence on the location of the global land carbon sink.