Variation of the interspecific forest mass-density relationship along gradients of leaf area and global radiation

Abstract. Stand mass scales as the -1/3 exponent of plant density for large compilations of plant communities on a continental or global scale, being the slope of the regression line in a log–log plot, where the intercept is a normalization constant reflecting the assumption of a constant rate of energy use by the species and environments involved. Here the normalization constant is replaced by a light absorption function, enabling to investigate how the interspecific mass–density relationship varies along spatial, largely latitudinal gradients of leaf area and the sum of global radiation over the growing season for relatively undisturbed forests. The test of the model for globally distributed forest communities shows the highest explained variance when both gradients are included in the light absorption function, meaning that the exponent is determined not only by the rate but also the sum of energy use over the growing season. The exponent of tree density converges to 1/2 instead of the expected 1/3 value based on the -1/3 exponent value in the bivariate biomass–density relationship. The 1/2 value corresponds with the so-called self-thinning rule that applies to the self-thinning line constructed as the upper boundary of mass–density points for monospecific even-aged plant stands, where gradients in energy use can be neglected. The results demonstrate the appropriateness of introducing a light absorption function in the bivariate mass–density relationship, suggesting a thermodynamic interpretation that may be of interest to other plants and even animals when gradients in energy use similarly affect the intercept and slope of the interspecific mass–density relationship.