Tropical forest burning in Brazilian Amazonia: Measurement of biomass loading, burning efficiency and charcoal formation at Altamira, Para

Abstract

Mass transformations were estimated in bums in the clearings of three colonist lots near Altamira, Para, Brazil. In each lot, two groupings of six 60-m2 plots were established in sites where the forest had been recently felled; plots were arranged as rays in a star-shaped pattern, with pre- and post-burn measurements made in alternate rays. Pre- and post-burn above-ground biomass was estimated by cutting and weighing the felled vegetation in 15 pre-burn and 18 post-burn plots (three pre-burn plots could not be weighed before one of the colonists burned the clearing) and by line intersect sampling (LIS) done along the axis of each of the 36 plots. Because of the high variability of the initial biomass present in the plots, volume data from LIS were more reliable for assessing change in the biomass of material over 10 cm in diameter (because this technique permits measuring the same trees before, and after, burning); other quantities relied on data from direct weighing. The best estimate of the mean pre-burn above-ground biomass at the site is 263 metric tons per hectare (tha-1); considering available measurements of the proportion of below-ground biomass elsewhere in Amazonia, the total dry weight biomass at the Altamira site corresponds to ≃322 t ha-1. Assuming 50% carbon (C) content for biomass, the above-ground biomass at Altamira represents a carbon stock of 130 t ha-1. Assuming a carbon content of 75% for charcoal, 1.3% of the pre-burn aboveground carbon stock was converted to charcoal, substantially less than is generally assumed in global carbon models. Measurements at Altamira imply a 42% reduction of above-ground carbon pools if calculated along with the scattered trees that farmers leave standing in their clearings, or 43% if these trees are excluded from the analysis. These values are substantially higher than the 27,6% measured in an earlier study near Manaus. However, most of the difference between results at the two sites is explained by differences in the distribution of initial biomass among the fractions, especially greater quantities of vines and of litter (including dead wood <5 cm in diameter) than at Manaus. Smaller diameter pieces bum more thoroughly than larger ones. At Altamira, the large percentage of above-ground carbon in vines (12.0%) is less typical of Amazonian forests than the lower percentage at Manaus (3.1%). The lower overall burning efficiency found at Manaus is, therefore, believed to be more typical of Amazonian burning. High variability indicates a need for further studies in many localities, and for perfecting less laborious indirect methods. Both a high biomass and low percentage of charcoal formation suggest significant potential contribution of forest burning to global climate changes from CO2 and trace gases.