Biomass variability in tropical American lowland rainforests

Abstract

Tropical forest accumulates one of the largest biomasses among terrestrial ecosystems, however its precise amount and patterns of spatial variation are still imperfectly known. We discuss these issues on the basis of available data on the most widespread type of tropical American lowland rainforest (terra firme forest), considering the estimates of live tree aboveground biomass (LTAB) and total aboveground biomass (TAGB), in old-growth stands in different regions, and suggest probable sources of their broad variation. Methodological shortcomings arising from sampling design and intensity, size of sample unit,and allometric equations used to calculate biomass from field data are firstly considered. TAGB estimates based on 0.25 to one hectare plots ranged throughout the region from 160 to 435 Mg ha-1, while estimates of LTAB range from 167 to 419 Mg ha-1. With smaller plots, the range extends from 115 to 864 Mg ha-1. Structural differences concerning biomass distribution among two other life-forms: palms and woody lianas, and its allocation among plant structures, also show broad variation, contributing to the richness and variety of rainforest types. Amounts and patterns of vertical variability of root biomass are still much less known, and the scarcity of field data makes difficult to disclose either general patterns or determining factors. The available data suggest that belowground biomass reaches at least about 20% of the aboveground counterpart. Distribution of fine roots illustrates the contrasted patterns and show how they are exploiting different soil horizons. Conclusions stress the large variability in structural features among tropical American lowland rainforests. Apart from variation due to methodological procedures, there are real differences in biomass among old-growth forest types, which are evident at all spatial scales, from the single plot to the whole area of this biome.