Towards a functional and simplified allometry for estimating forest biomass

Aboveground tree biomass (M) can be estimated using a power function in the form of M = aD(b) where a and b are the scaling coefficient and scaling exponent, respectively, and D the tree breast-height diameter. Both a and b are reported to vary with species, site and age. However West et al. [West, G.B., Brown, J.H., Enquist, B.J., 1999. A general model for the structure and allometry of plant vascular systems. Nature 400, 6646671 suggested that M should scale against D with a universal exponent (b = 8/3), because the scaling exponent would depend on an optimal tree architecture. Moreover a should be related with the wood density (p) [Enquist, B.J., West. G.B., Charnov, E.L., Brown, J.H., 1999. Allometric scaling of production and life-history variation in vascular plants. Nature 401, 907-911]. We collected 49 datasets of different species (most from the literature) with individual data of diameter, mass and tree height. We analysed the height-diameter relationship and estimated b and a for each dataset, in order to test whether: (i) the scaling exponent may be considered universal or, conversely, dependent on species, tree stage or site and (ii) a was correlated with wood density. Analysis of the height diameter relationship for each species and site generally allowed a juvenile, an adult and a mature stage to be identified. b appeared to be related to tree stage but independent of species and site. The mean a value was also correlated with the wood density. We estimated tree biomass using different b exponents for each stage, deriving a from p. This approach was applied to a validation dataset and an average relative difference of 21.4% from the observed values was obtained. This would suggest that total forest aboveground biomass can be estimated by using functional allometry (i.e., universal b parameters), potentially avoiding any destructive tree sampling.