Additive biomass equations based on complete weighing of sample trees for open eucalypt forest species in south-eastern Australia

Abstract

The aboveground biomass and carbon stock of less commercialized Eucalyptus open forests of south-eastern Australia are not well-known due to a lack of accurate biomass equations for even the prevalent species in these forests. To improve biomass and carbon estimates of these forests, a total of 245 trees of 11 eucalypt species with DBH (diameter overbark at the breast height of 1.3m above ground level) up to 139cm and height up to 39m were destructively sampled across three broad study sites in north-central Victoria. Trees were sampled as the four aboveground components of lower stem, upper stem, crown, and dead attached stem and branch materials. All four components were weighed completely in the field. To determine the dry to fresh weight ratio for each of the four components, 90 of these trees were further subsampled for moisture content. General linear models were used to relate dry to fresh weight ratio to site, species and tree size and the parameters of the models were estimated through generalized estimating equations. The model-estimated dry to fresh weight ratios were used to convert fresh weight to dry weight for all biomass components of fresh weight sample trees. Using the derived dry weight data, two systems of nonlinear additive biomass equations for stem, crown and total tree biomass were developed for each of the 11 species, with DBH as the independent variable in one system and the combined variable of DBH and tree height in another. Residual error variances and approximate confidence bands containing about 90% of the observed data about the mean curve of predicted biomass were estimated and derived for all biomass components of each species. In addition to the species-specific equations, two sets of site-specific biomass equations were developed using pooled biomass data from each site regardless of species. These systems of additive biomass equations will improve the accuracy of biomass and carbon estimation of open eucalypt forests in south-eastern Australia.