River and Human Legacies in Amazonian Floodplain Postagricultural Forests

Studies in postagricultural forests have suggested that the effect of land use overrides that of environmental factors in defining the present forest composition. This study demonstrates that the effect of land use history in the composition of postagricultural forests can be reduced by exposure to frequent disturbance. The study was undertaken in a rural landscape of the Peruvian Amazon floodplain with a history of land use and periodic river disturbance. The history of land use for the region was reconstructed using a set of remote sensing images dating from 1948. Trees above 2.5 cm DBH were identified and measured in 46 plots of 600 m2 in postagricultural forests older than 10 years. For each plot, I measured its distance to other landscape features (river, swamps, lowland forest, upland forests) and its elevation above sea level. In this region, elevation correlates to the probability of a site being exposed to flooding. Correlations of the explanatory variables to a nonmetric multidimensional scaling (NMDS) ordination were used to evaluate the relative effect of elevation, distance to landscape features, and time since abandonment in the canopy and understory composition of the plots. Correlations to the NMDS ordination show that both canopy and understory species compositions are better explained by flood exposure for older plots. It also indicates that the distance to forest patches affects the composition of postagricultural forests. In general, the results show that land use in the Amazonian floodplain has not erased the natural environmental gradients of the landscape.