Allometric models for aboveground biomass in small and multi-stemed trees in Rwanda

Abstract

Estimates of aboveground biomass (AGB) of trees are crucial for understanding how forest ecosystems influence the global carbon cycle and tropical forests are of particular importance. Allometric equations are used to estimate AGB but the allometric relationship differs between sites, species and size of the trees. Most developed equations focus on large trees with one dominate stem. This study focuses on estimating AGB in a population of small and multi-stemmed trees from species native to Rwanda. It aims to test the validity of available equations developed for larger trees and to develop new equations for accurate AGB estimates. Additional measurements of multiple stem diameter, stem count and crown area are tested. A total of 83 trees from 8 species were harvested, and their AGB was measured. The samples’ diameter at breast height ranged from 1.6-13.4 cm and 16 samples had multiple stems. Wood density, height, all diameters at 1.3 m and 0.3 m above ground and crown area were measured. AGB was compared to the estimates derived from several equations reported in the literature. New equations were developed through regression analysis. The results show that a commonly used pantropical equation also is valid for the smaller trees of this study. This equation performed almost as good as the model developed in this study, when using the same variables. Moreover, the equation, as well as the simplest equation of this study, is biased regarding trees with different number of stems. Additional measurements of multiple stem diameter or two measurements of the main stem, at different heights, as well as a stem count largely improves the estimates, in particular for multi-stemmed samples. Crown area also improves the estimates. New site- and species-specific equations are presented in this study.