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

Mass transformations were estimated in burns in the clearings of three colo nist lots near Altamira, Para, Brazil. In each lot, two groupings of six 60 -m(2) plots were established in sites where the forest had been recently fe lled; plots were arranged as rays in a star-shaped pattern, with pre- and p ost-burn measurements made in alternate rays. Pre- and post-burn above-grou nd biomass was estimated by cutting and weighing the felled vegetation in 1 5 pre-burn and 18 post-burn plots (three pre-burn plots could not be weighe d before one of the colonists burned the clearing) and by line intersect sa mpling (LIS) done along the axis of each of the 36 plots. Because of the hi gh variability of the initial biomass present in the plots, volume data fro m LIS were more reliable for assessing change in the biomass of material ov er 10 cm in diameter (because this technique permits measuring the same tre es before, and after, burning); other quantities relied on data from direct weighing. The best estimate of the mean pre-burn above-ground biomass at t he site is 263 metric tons per hectare (t ha(-1)); considering available me asurements of the proportion of below-ground biomass elsewhere in Amazonia, the total dry weight biomass at the Altamira site corresponds to approxima te 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). Assum ing a carbon content of 75% for charcoal, 1.3% of the pre-burn aboveground carbon stock was converted to charcoal, substantially less than is generall y 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 i n their clearings, or 43% if these trees are excluded from the analysis. Th ese values are substantially higher than the 27.6% measured in an earlier s tudy near Manaus. However, most of the difference between results at the tw o 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 pie ces burn more thoroughly than larger ones. At Altamira, the large percentag e of above-ground carbon in vines (12.0%) is less typical of Amazonian fore sts than the lower percentage at Manaus (3.1%). The lower overall burning e fficiency found at Manaus is, therefore, believed to be more typical of Ama zonian burning. High variability indicates a need for further studies in ma ny localities, and for perfecting less laborious indirect methods. Both a h igh biomass and low percentage of charcoal formation suggest significant po tential contribution of forest burning to global climate changes from CO2 a nd trace gases. (C) 1999 Elsevier Science B.V. All rights reserved.