Biomass Decline in Amazonian Forest Fragments

Abstract

The report by William F. Laurance et al. about the destruction of Brazilian rain-forest trees along the edges of clearings (1) contains important information about the tree loss in the edge effect of such clearings, but their experimental design seems to artificially inflate the effect they observed, which may call into question some of their results. Laurance et al. estimated biomass in sections of forest on the basis of tree diameter at breast height (DBH), which is a commonly accepted measurement. DBH was used to calculate dry aboveground biomass (AGBM) by an allometric model based on trees from the local forest. Trees of less than 10 cm DBH were excluded from the analysis. Although I do not have firsthand experience with tropical forests, my experience in temperate forests suggests that this method of measuring biomass introduces a bias toward measuring losses while undersampling the gains that partly offset the losses. The floor of a dense, closed forest is often dominated by the trunks of large trees. Few saplings and little foliage occur near the forest floor because of light limitation. The trunks are tall and most of the limbs occur high in the canopy, where light levels are highest. The similar geometry of the different individuals facilitates estimation of biomass on the basis of a simple measurement such as DBH. When a clearing is made or an individual tree falls, however, the characteristics of the trees in the vicinity change. First, many seedlings spring up in the area that has opened up, and quickly grow up into saplings. These young trees are rapidly accumulating biomass, but would not be counted by the techniques used in this paper until they reached a size of 10 cm DBH. Given the fact that 15 mm/year increase in DBH was considered an outlier, we may assume that 10 mm/year is close to the maximum growth rate exhibited by trees in the area. Over the 10 to 17 years of the study (1), the maximum size one could expect the fastest growing new growth to reach would be 10 to 17 cm diameter at ground level, and even smaller at breast height. This small size class is precisely the class that was excluded from the study, which effectively eliminates one of the largest potential sources for new biomass accumulation. In addition, the branches of some types of trees on the edges of clearings proliferate and grow toward the light, resulting in a change in the geometry of the tree and an increase in biomass well above the ground. This effect would also have been completely missed by measuring DBH. The effect reported by Laurance et al. appears real in terms of the loss of biomass in the large, old trees, but, given their techniques, it would not seem valid to use these data as a realistic estimate of the actual net loss in biomass or as a fair indicator of the level of biomass collapse.