The global relationship between forest productivity and biomass

Aim We aim to determine the empirical relationship between above-ground forest productivity and biomass. There are theoretical reasons to assume a relationship between forest structure and function, as both may be influenced by similar ecological factors such as moisture supply. Also, dynamic global vegetation model simulations imply that any increase in forest productivity driven by climate change will result in increases in biomass and therefore carbon storage. However, few studies have explored the strength and form of the relationship between forest productivity and biomass, whether in space or time. Location Global scale. Methods We collated a large data set of above-ground biomass (AGB) and above-ground net primary productivity (ANPP) and tested the extent to which spatial variation in forest biomass across the Earth can be predicted from forest productivity. Results The global ANPP-AGB relationship differs fundamentally from the strongly positive, linear relationship reported in earlier analyses, which mostly lacked tropical sites. AGB begins to peak at c. 15-20 Mg ha(-1) year(-1) ANPP, plateaus at ANPP > 20-25 Mg ha(-1) year(-1), and may actually decline at higher levels of production. Main conclusions High turnover rates in high-productivity forests may limit AGB by promoting the dominance of species with a low wood density. Predicted increases in ANPP will not necessarily favour increases in forest carbon storage, especially if changes in productivity are accompanied by compositional shifts.