Equations for predicting the biomass of Acacia cyclops and Acacia saligna in the western and eastern Cape regions of South Africa Part 1 : Tree-level models

Abstract

A forest inventory was carried out in coastal areas infested by the alien tree species Acacia cyclops and A. saligna on the West Coast, Agulhas and Eastern Cape Plains. Sample trees were felled to construct prediction equations, which were subsequently used to obtain regression estimates of wood mass for knee height diameter classes below 2.5 em, between 2.5 and 5.0 em and above 5.0 em respectively. Various regression models were tested. The single biomass component was regressed on log-transformed diameter, log-transformed commercial stem length and log-transformed biomass. The results were compared with a 10-parameter equation, which used the linear and quadratic term of diameter at knee height (DKH), and commercial stem length as well as four interaction variables as predictors. For practical reasons, it was recommended to use basal area and squared basal area as predictors and to ignore commercial stem length as independent variable. This produced a tariff-type estimator for biomass components. In addition, equations were developed to estimate leaf biomass from DKH and to evaluate the relationship between leafbiomass and wood biomass