Ontogeny partly explains the apparent heterogeneity of published biomass equations for Fagus sylvatica in central Europe

A set of robust biomass equations was developed for European beech (Fagus sylvatica), using a large database made of trees from three different European countries. Models were calibrated on the French control dataset, including a broad range of tree size, age and geographical conditions. Their independent validation on Belgian, German unfertilized, German and French fertilized stands gave very promising unbiased results for all of the main tree compartments. The basic fitted allometric equation (biomass = β × (d²h)^γ) allowed us to work with biologically meaningful parameters, where β encompasses both the form of the tree and the wood density, and γ is the allometric exponent that indicates the proportionality between the biomass and volume relative increments. The allometric parameters were found to vary with stand age (decreasing for the crown and increasing for the bole), and introducing these parameters to the equations significantly improved the performance of all aboveground biomass equations. This age effect was related to changes in both stem form and wood density for the trunk and to changes in hydraulic conductance for the crown. We argue that introducing stand age into the predictive parameters is the key for the accuracy of our equations. Management options and stand fertility stand for the low residual variations around this relationship. To confirm this result, we were able to homogenize most of the published biomass equations by accounting for the stand age given in each original paper. We noticed that our results were consistent with and matched the patterns that were observed for Eucalyptus (Eucalyptus). This could mean that species with similar wood properties and crown architecture may exhibit similar biomass equation structures.