Modeling aboveground biomass components and volume-to-weight conversion ratios for loblolly pine trees

Abstract

With large taper and biomass measurement data sets from destructively sampled loblolly pine trees, we developed weight-to-volume ratio equations and biomass allocation equations based on tree dbh and height. The weight-to-volume ratio equations were fitted simultaneously with stem green weight with bark and stem outside-bark volume equations or stem wood dry weight and stem inside-bark volume equations, using a weighted nonlinear seemingly unrelated regression approach to account for the inherent correlation among the equations and the heteroscedasticity. The biomass component proportions were modeled with Dirichlet, fractional multinomial logit, and log-ratio regression approaches, which guarantee the constraint of all nonnegative proportions summing to 1. Both weight-to-volume ratios and biomass component proportions varied with tree size. The Dirichlet regression was superior to the fractional multinomial logit and log-ratio regression approaches.