Fire penetration in standing Amazon forests BRUCE WALKER NELSON 1 MARILANE NASCIMENTO IRMÃO 1 1 INPA--Instituto Nacional de Pesquisas da Amazônia Caixa Postal 478, 69.011-970 Manaus, AM, Brazil bnelson@internext.com.br Abstract. Wall-to-wall Landsat images of the Brazilian Amazon from INPE, combined with interviews of local peoples and field checking of vegetation form and composition, are employed to show that infrequent fire has been an important abiotic determinant of forest type - even in central locations far from the climatic transition to savanna and open woodland (cerrado and cerradão). Evergreen Amazon forest types affected by fire over the last century include: “igapó” forest on seasonally inundated sandy soils; white-sand or “campinarana” forest on seasonally waterlogged and well-drained sandy soils; montane forest on well-drained rocky soils; and bamboo-dominated forest on low-permeability 2:1 clay soils. Vine forests may also be fire affected. Extensive fire scars are found even where average annual rainfall is 3600 mm/yr. Keywords: Landsat, El Niño, bamboo, liana, white-sand, bracken fern, flooded forest. 1 Introduction Prior to the conflagrations said to have consumed as much as 9,200 km 2 of forest in the state of Roraima in early 1998 ( A Crítica, Manaus, 7 April 1998, p. C-3), few inhabitants of Amazonia would have given credit to the idea that mature standing Amazon forests, undisturbed by selective logging, are susceptible to fire penetration. In fact, previous studies of plant-available-water in deep soil profiles on the eastern fringe of the Amazon forest (Nepstad et al., 1995) had already shown that two consecutive drought years will cause leaf drop and consequent drying to the flammability threshold by fine fuels on the floor of mature undisturbed forest, while a normal dry season is sufficient to allow ground fires to penetrate secondary forests of that region. These experimental data by other workers imply that infrequent fires have affected some Amazonian forest types in the recent past. The hypothesis is here borne out by the detection of extensive, spectrally distinct vegetation types occupying recent fire scars throughout the Amazon basin. Evidence for extensive fire over the last century in standing forest is reported here for one type of white-sand forest of the upper Rio Negro, inundated forests of the middle Rio Negro and montane forests of the Serra Parima on the upper Orinoco. Additionally, two very extensive vegetation types are susceptible to fire and may be fire derived: 180,000 km 2 of bamboo- dominated forests in the southwest Amazon and liana forests covering 310,000 km 2 , predominantly in southern and eastern Amazonia. The cumulative effects of these fires, possibly aggravated by three or more mega-droughts during the last 1500 years (Meggers, 1994), have had a profound effect on Amazonian phytogeography. 1471 Anais IX Simpósio Brasileiro de Sensoriamento Remoto, Santos, Brasil, 11-18 setembro 1998, INPE, p. 1471-1482.