Growth-size scaling relationships of woody plant species differ from predictions of the Metabolic Ecology Model

The Metabolic Ecology Model predicts that tree diameter (D) growth (dD/dt) scales with D-1/3. Using data on diameter growth and height-diameter relationships for 56 and 40 woody species, respectively, from forests throughout New Zealand, we tested one prediction and two assumptions of this model: (i) the exponent of the growth-diameter scaling relationship equals 1/3 and is invariant among species and growth forms, (ii) small and large individuals are invariant in their exponents and (iii) tree height scales with D-2/3. We found virtually no support for any prediction or assumption: growth-diameter scaling exponents varied substantially among species and growth forms, correlated positively with species' maximum height, and shifted significantly with increasing individual size. Tree height did not scale invariantly with diameter. Based on a quantitative test, violation of these assumptions alone could not explain the model's poor fit to our data, possibly reflecting multiple, unsound assumptions, as well as unaccounted-for variation that should be incorporated.