The regional variation of aboveground live biomass in old‐growth Amazonian forests

TLDR

The biomass of tropical forests is a key component of the global carbon cycle, yet its absolute magnitude and environmental determinants remain poorly understood. The study synthesizes and interpolates basal area and aboveground live biomass of old‑growth lowland tropical forests across South America using 227 forest plots. The authors analyzed biomass using basal area and mean wood density across these plots, interpolating the results across the continent. Basal area is largely landscape‑dependent but stable regionally in moist forests, while mean wood density is lower in dynamic western Amazonia and higher in slow‑growing eastern Amazonia; together these patterns produce peak biomass of ~350 Mg dw ha⁻¹ in central Amazonia and the Guyanas, declining to 200–250 Mg dw ha⁻¹ at margins, yielding an estimated 93 ± 23 Pg C of intact Amazonian aboveground live biomass.

Abstract

Abstract The biomass of tropical forests plays an important role in the global carbon cycle, both as a dynamic reservoir of carbon, and as a source of carbon dioxide to the atmosphere in areas undergoing deforestation. However, the absolute magnitude and environmental determinants of tropical forest biomass are still poorly understood. Here, we present a new synthesis and interpolation of the basal area and aboveground live biomass of old‐growth lowland tropical forests across South America, based on data from 227 forest plots, many previously unpublished. Forest biomass was analyzed in terms of two uncorrelated factors: basal area and mean wood density. Basal area is strongly affected by local landscape factors, but is relatively invariant at regional scale in moist tropical forests, and declines significantly at the dry periphery of the forest zone. Mean wood density is inversely correlated with forest dynamics, being lower in the dynamic forests of western Amazonia and high in the slow‐growing forests of eastern Amazonia. The combination of these two factors results in biomass being highest in the moderately seasonal, slow growing forests of central Amazonia and the Guyanas (up to 350 Mg dry weight ha −1 ) and declining to 200–250 Mg dry weight ha −1 at the western, southern and eastern margins. Overall, we estimate the total aboveground live biomass of intact Amazonian rainforests (area 5.76 × 10 6 km 2 in 2000) to be 93±23 Pg C, taking into account lianas and small trees. Including dead biomass and belowground biomass would increase this value by approximately 10% and 21%, respectively, but the spatial variation of these additional terms still needs to be quantified.