Variation in wood density determines spatial patterns inAmazonian forest biomass

TLDR

Uncertainty in biomass estimates limits carbon‑flux models, and previous comparisons of Amazonian aboveground biomass have been constrained by sparse data for western forests and site‑specific estimation methods, while regional variation in stand‑level wood specific gravity had not been considered. The study aims to determine how species composition (wood specific gravity) and forest structure (basal area) jointly influence aboveground biomass variation in Amazonian forests. The authors analyze data from 56 mature forest plots across Amazonia to assess these effects. The analysis shows that central and eastern Amazon forests have higher stand‑level wood specific gravity and aboveground biomass, with basal area explaining 52–63 % of AGB variation and wood specific gravity accounting for 30–45 %, indicating that regional species composition drives a broad AGB gradient across Amazonia.

Abstract

Abstract Uncertainty in biomass estimates is one of the greatest limitations to models of carbon flux in tropical forests. Previous comparisons of field‐based estimates of the aboveground biomass (AGB) of trees greater than 10 cm diameter within Amazonia have been limited by the paucity of data for western Amazon forests, and the use of site‐specific methods to estimate biomass from inventory data. In addition, the role of regional variation in stand‐level wood specific gravity has not previously been considered. Using data from 56 mature forest plots across Amazonia, we consider the relative roles of species composition (wood specific gravity) and forest structure (basal area) in determining variation in AGB. Mean stand‐level wood specific gravity, on a per stem basis, is 15.8% higher in forests in central and eastern, compared with northwestern Amazonia. This pattern is due to the higher diversity and abundance of taxa with high specific gravity values in central and eastern Amazonia, and the greater diversity and abundance of taxa with low specific gravity values in western Amazonia. For two estimates of AGB derived using different allometric equations, basal area explains 51.7% and 63.4%, and stand‐level specific gravity 45.4% and 29.7%, of the total variation in AGB. The variation in specific gravity is important because it determines the regional scale, spatial pattern of AGB. When weighting by specific gravity is included, central and eastern Amazon forests have significantly higher AGB than stands in northwest or southwest Amazonia. The regional‐scale pattern of species composition therefore defines a broad gradient of AGB across Amazonia.