AimTo understand: (1) how temperature, precipitation and stand age control the above-ground biomass carbon density (BCDa) of mature forests and its macroecology patterns across latitudes; (2) the age threshold for old-growth forests at a global scale.

LocationGlobal forests.

MethodsWe compiled a database (897 sites) of mature forests between 80 and 1200 years old. The site data include latitude, longitude, mean annual temperature, mean annual precipitation, forest type, stand age, BCDa, living biomass (above- and below-ground biomass) carbon density and total (living plus dead) biomass carbon density. Based on the site data, we performed regression analyses to show how BCDa changes with climate and forest stand age.

ResultsAt a global scale, the highest BCDa of mature forests occurred mainly in the mid-latitude regions where mean annual temperatures were 8-10 degrees C and mean annual precipitation was between 1000 and 2500mm. The average BCDa of forests in the stand age class of 450-500 years was higher than those in the other stand age classes. For forests between 80 and 450 years old, which form the majority of mature forests, carbon accumulation was faster in dead biomass than in living biomass.

Main conclusionsThe highest BCDa of mature forests is located in mid-latitude regions with cool temperatures and moderate precipitation. The age threshold for old-growth forests at a global scale should be 450-500 years, which is much older than the previously documented age of 100-200 years. This older age threshold for old-growth forests is probably one of the primary reasons why recent works have concluded that old-growth forests are still carbon sinks.