Effects of soil fertility and land-use on forest succession in Amazônia

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This paper examines the role of soil fertility and land-use history on the rates of forest successional regrowth in five regions of the Amazon Basin. Sites are located in the Bragantina Region, Tomé Açú Region, Altamira Region and Marajó Region of the State of Pará, Brazil and in a region of the Colombian Vaupés. Methods used included vegetation inventories of successional and mature forests, soil sampling, ethnographic assessment of land-use histories, and land cover classification based on multi-temporal Landsat Thematic Mapper digital satellite data. The paper examines inter-regional differences, intra-regional differences, and Basin-wide differences in rates of forest regrowth. Inter-regional differences are best explained by the differences between areas in soil fertility, whereas intra-regional differences are best explained by the differential impact of land-use history on forest recovery. Basin-wide differences in rates of succession can best be captured by differences in tree height, and secondarily by differences in basal area. In inter-regional comparisons we found that ultisols, oxisols, and spodosols present similar rates of regrowth, but considerably slower rates when compared to alfisols. During the first 5–10 years of regrowth, alfisol areas have average stand height 1 m higher, and this difference doubles after 15 years of regrowth. In intra-regional comparisons, using one region as an example, we found that land-use differences are most able to explain differences. Areas that had been in swidden agriculture grew back at a rate of 1.5 m per year, as compared with 0.45 m per year for areas that had experienced mechanized land preparation, and 0.62 m per year for areas that had been in pasture. In Basin-wide comparisons we have been able to discriminate three distinct stages of secondary succession using structural criteria, across both soil and land-use types with height as the most predictive criteria of overall structural development. By comparing our five sites’ data with that of other investigators, the paper proposes Basin-wide patterns of regrowth for each of three structural stages of secondary succession that facilitate linking the field vegetation data to remotely-sensed data of land cover.