Compatible taper-volume systems for major tropical species in Mexico

Abstract

A compatible system consisting of stem taper, merchantable-, stem-, branch-and stem and coarse branch tree volume equations was developed for 11 major tropical species in southeast Mexico. A database corresponding to 6187 felled trees was used to develop the system of equations. Iterative seemingly unrelated regression was used to fit the system in two steps: The stem volume, branch volume and stem and coarse branch tree volume equations were first fitted simultaneously and the common estimated parameters were substituted in the stem taper and merchantable volume equations. In the second step, the remaining parameters were estimated by fitting the stem taper and merchantable volume equations simultaneously. Weighted regression and a second order autoregressive model were used in the model fitting to account for heteroscedasticity and autocorrelation. To assess whether the compatible system differs between ecoregions for the same species, a system with the same set of parameters for all the ecoregions was compared with a system with a different set of parameters for each ecoregion. The results of the comparisons indicated the need to fit a different system for each ecoregion and species. The proposed equations can be used to estimate stem-, branch-and merchantable volume at any upper diameter and height, as well as stem and coarse branch tree volume. If classification of the products by merchantable sizes is not required, the stem and coarse branch volume equation is preferred, thereby simplifying the calculations and making it more suitable for practical purposes.