BIOTROPICA 31(1): 2-16 1999 zyxwvuts Fire as a Recurrent Event in Tropical Forests of the Eastern Amazon: Effects on Forest Structure, Biomass, and Species Composition' Mark A. Cochrane2 lnstituto do Homem e Meio Ambiente da Amazonia (IMAZQN), Caixa Postal 1015, Belem, Para 66.000, Brazil and Mark D. Schulze Biology Department, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A. ABSTRACT zyxwvutsr 'I'hc effects of fire zyxwvutsr on forest structure and composition were studied in a severely firc-impacted landscape in the eastern Amazon. Extensive sampling of area forests was used to compare structure and compositional differences between burned and unburned forest stands. Hurned forests were extremely heterogeneous, with substantial variation in forest structure and fire damage recorded over distances of zyxwvutsrqpo <50 in. Unburned forest patches occurred within burned areas, but accounted for only six percent of the sample area. Canopy cover, living biomass, and living adult stem densities decreased with increasing fire intensiry/frequeiicy, and were as low as 10-30 percent of unburned forest values. Even light burns removed >70 percent of rhc sapling and vine populations. Pioneer abundance increased dramatically with burn intensity, with pioneers dominating the understory in severely dainaged areas. Species richness was inversely related to burn severity, hut no clear pattern of species selection was observed. Fire appears to be a cyclical event in the study region: <30 percent of the burned forest sample had been subjected to only one burn. Based on estimated solar radiation intensities, burning substantially increases fire susceptibility of forests. At least 50 percent of the total area of all burned forests is predicted to become flammable within 16 rainless days, as opposed to only 4 percent of the unburned forest. In heavily burned forest subjected to recurrent fires, 95 percent of thc area is predicted to become flammable in zyxwvut <9 rain-free days. As a recurrent disturbance phenomenon, fire shows unparalleled potential to impoverish and alter the forests of the eastern Amazon. RESUMO 0s impactos sobrc a estrutura e composigzo florestal foram estudados em uma irea severamente impactada pelo fog0 na Amazbnia oriental. Amostragcns extensivas de partes da floresta foram utilizadas para comparar as diferengas de estrutura c de composig8o em trechos queimados c nso-queimados. As florestas queimadas eram extremamente heterogeneas, com uma significativa variagzo na estrutura florestal, zy e os dams causados pelo fog0 foram registrados em intervalos de menos de 50 metros. 'Trechos de floresta intacta ocorreram cntre as ircab queimadas, mas constituiram apenas 6 porcento da Area amostrada. A cobertura do dossel, a biomassa viva, c as densidades de individuos adultos vivos diminuiram coin o aumento da frequEncialintensidade das ocorrhcias de fogo, chegando a constituir apenas 10 a 30 porcento dos valores relativos hs florestas intactas. Mesmo as queimadas menos intensas chegaram a remover mais de 70 porcento das populag6es de pldntulas e cipos. A abunddncia de espkcies pionciras aumentou dramaticamcnte, sendo que essas espkcies dominaram o sub-bosque nas ireas mais severamentc afetadas. A riqueza de espkies foi inversamente proporcional gravidade da queimada, mas 1150 se observou um padrzo claro de selegzo de espkcies. zyxwvu 0 fog0 parece scr uma ocorrencia ciclica na regizo em cstudo-menos de 30 porcento das amostras de florestas queimadas havia sofrido uma queimada. Com base em estimativas sobre a intensidade da radiagiio solar, as queimadas aumcntam substaiicialmente a susceptibilidadc das florestas ao fogo. Estima-se que pelo menos 50 porcento do total da irea de todas as florestas queimadas venha a sc tornar inflamivel, caso ocorra um periodo de 16 dias sem chuva, o que contrasta coin a cifra de apenas 4 porcento no caso das florestas intactas. Em florestas fortemente afetadas pelo fog0 em queimadas repetidas, estima-se que 95 porcento da irea se tornari inflamivel em menos de 9 dias sem chuva. Como uma perturbagzo recorrente, o fogo apresenta urn potencial sem paralelos para empobrecer c alterar as florestas da Amazbnia oriental. Keywords: .sb$s; succession; tropicul ruin forest. biomass; cyclical disturbance; eastern Amazon; $re; jre susceptibility; forest degrudution; jorest ecology; species ' Kcceived 14 March 1997; revision accepted 3 October 1997. Woods Hole Research Center (WHRC), PO. Box 296, Woods Hole, MA 02543, U.S.A 2