Can we set a global threshold age to define mature forests?

Globally mature forests appear to be increasing in biomass density. There is disagreement whether these increases are the result of increases in CO2 concentrations or a legacy effect of previous land-use. Recently, it was suggested that a threshold of 450 years should be used to define mature forests and that many forests increasing in biomass may be younger than this. However, the study making these suggestions failed to account for interactions between forest age and climate. Here we revisit the issue to identify: (1) how climate and forest age control global forest biomass density and (2) whether we can set a threshold age for mature forests. Using data from previously published studies we modelled the impacts of forest age and climate on biomass density using linear mixed effects models. We examined the potential biases in the dataset by comparing how representative it was of global mature forests in terms of its distribution, the climate space it occupied and the ages of the forests used. Biomass density increased with forest age, mean annual temperature and annual precipitation. Importantly the effect of forest age increased with increasing temperature, but the effect of precipitation decreased with increasing temperatures. The dataset was biased towards Northern hemisphere forests in relatively dry, cold climates. The dataset was also clearly biased towards forests <250 years of age. Our analysis suggests that there is not a single threshold age for forest maturity. Since climate interacts with forest age to determine biomass density a threshold age at which they reach equilibrium can only be determined locally. We caution against using biomass as the only determinant of forest maturity since this ignores forest biodiversity which often takes longer to recover. Future study of the influence of climate on forest biomass should aim to use the data currently being generated by long-term monitoring networks and satellite based observations.