Allometric equations for the small trees that dominate many forests recovering from disturbance, such as fire, are relatively rare, increasing the uncertainty of aboveground biomass (AGB) estimates in young regrowth forests. In this study we sampled 516 small trees (diameter <5 cm at 1.3 m height) from 105 species within Indonesian mixed forests to develop multispecies biomass allometric models and root to shoot ratio. Small trees were sampled across three ecosystems: peat swamp forests, heath forests and dryland forests. Several combinations of weighted log-linear regression analyses of tree diameter (D) and tree height (H) versus tree biomass were applied to identify the best model for estimating AGB. The allometric models developed in this study used a power function with diameter (D) and diameter and height ((DH)-H-2) as predictors and ecosystem as an additional predictor in the D model. Seemingly unrelated regression (SUR) was used to generate equations of biomass components incorporating additivity. For the additive biomass equation components, the allocation of biomass for these small trees was stem > leaves > branches. AGB estimates based on allometric equations developed for larger trees, sourced from the literature, overestimated small tree biomass by up to 25% when compared with AGB estimates from equations developed in this study. The allometric models of small trees and the root-to-shoot ratio values obtained in this study will improve biomass estimates for young regrowth forests of Indonesia and the tropical region in general.
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