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not be out of the ordinary to see or hear a tree crashing to the ground on an afternoon walk through the forest. From 4 to 6% of any Amazon forest will be studded with canopy openings (light gaps) formed by treefalls. Recently formed light gaps have a ground layer of tree and vine seedlings. As a light gap patch ages, it enters the building phase during which it develops into a densely stocked patch of pole-sized trees. The patch reaches a mature phase when it contains a mix of large trees, poles, and seedlings. With a practiced eye, it is possible to walk through the rain forest and detect these light gaps and identify the building and mature phase patches—testimonies to past disturbances and to the dynamic nature of these ecosystems. Importantly, this type of small-scale disturbance is more of a subsidy than a stress to the plant community, because the resources critical for growth—light, water, and nutrients—are more readily available in treefall gaps than in the undisturbed forest understory.
Another natural disturbance that has no doubt been an important part of past Amazon disturbances is fire. For example, in the Upper Rio Negro region of Amazonia, charcoal is widespread and abundant in the soil. Radiocarbon dating of soil charcoal samples (Sanford et al., 1985) indicates that numerous fires have occurred during the past 6,000 years in this area. The radiocarbon dates correspond well with what are believed to have been dry episodes during recent Amazon history. The presence of abundant charcoal in Rio Negro soils is not an anomoly. Amazon researchers from EMBRAPA (Empresa Brasileira de Pesquisa Agropecuária) and INPA (Instituto Nacional de Pesquisas da Amazonia) in Brazil concur that charcoal is common in the soils of the central and eastern Amazon. Indeed, it appears to be much more difficult to find sites that do not have charcoal than to find sites that do.
Given the prevalence of fire throughout the history of the Amazon forest, how does Amazon vegetation respond to fire disturbance? Our studies of forest succession following forest cutting and burning disturbances at San Carlos de Rio Negro, Venezuela (Uhl and Jordan, 1984) provide an indication of regrowth potential following fire. We found that forest reforms quickly on burned sites. This is because many Amazonian-tree species have the ability to sprout after damage, and although fires do kill many stems outright, a pool of individuals survives burning and quickly sprouts. In addition, Amazon forests have a rich seed bank (from 500 to 1,000 seeds of successional woody species per square meter), and a portion of these seeds survive burning, germinate, and become established. The ability of some rain forest species to survive fire disturbances, e.g., when seeds are buried and thus protected and later sprout, may be the result of natural selection (i.e., fire may have been a selecting agent for these characteristics).
DISTURBANCE BY HUMANS IN AMAZONIA
Human-induced disturbances, such as slash-and-burn agriculture and the conversion of forest to pasture, are generally more intense than the natural disturbances just discussed. Nature’s disturbances are over in an instant (a treefall, a wildfire), whereas humans prolong the disturbance period in their efforts to wrest some benefit from the land.