Are Tropical Forests an Important Carbon Sink? Reanalysis of the Long-Term Plot Data

In a recent (1998) publication of Science, data from a large number of forest inventory plots were used to estimate biomass trends in old-growth tropical forests. Although no evidence was found of net biomass change in mature Paleotropical forests, old growth of the humid Neotropics was inferred to have been a substantial biomass carbon sink in recent decades. Methodological artifacts affected this analysis, however. Many humid Neotropical plots were measured strictly at breast height, where tropical trees frequently have buttresses and other protruberances. Because biomass allometric equations are based on above-buttress tree diameters, and because bole irregularities show disproportionately rapid radial increments, estimates of biomass and biomass increase must be based on above-buttress measurements. In addition, some plots were on recent floodplains, where forests undergo biomass accretion during primary succession. The data set includes 25 sites from the humid lowland Neotropics that were measured above buttresses with standard techniques and that were not on recent floodplains. Mean estimated biomass change for these sites was 0.3 Mg·ha−1·yr−1, with a 95% confidence interval including 0.0 (−0.3 to +0.9 Mg·ha−1·yr−1). While the Science study was a laudable attempt to address an important aspect of the global carbon budget, the underlying data do not indicate a significant biomass carbon sink in old-growth forests of the humid Neotropics.