Abandoned pastures in eastern Amazonia. I. Patterns of plant succession

Vegetation composition, structure, and biomass accumulation were studied on 13 forest sites that had been cut and burned, used as cattle pasture, and then abandoned in the E Amazon near Paragominas, Para, Brazil. Forest regenerated vigorously on sites of previously light use. Above-ground biomass accumulation averaged 10 t ha^-1 y^-1 or 80 t after 8 yr (c25% of mature forest levels). Tree species richness was also high (c20 per 100 m²) and almost all species also occurred in native forest. Moderately grazed pastures also developed forest but biomass accumulation was only 5 t ha^-1 y^-1. Tree species richness was also lower than on light-use sites and there were fewer forest trees. Abandoned pastures subjected to heavy use had the least distinct patterns of succession. The 8-yr-old site was dominated by grasses and forbs with <1 tree 100 m^-2 and an above-ground biomass accumulation of 0.6 t ha^-1 y^-1, a value c6% of that found on light-use sites. Light-use sites had significantly higher biomass and species richness than either moderate- or heavy-use sites. Site age was a good predictor of above-ground biomass accumulation on light- and moderate-use sites, but not on heavy-use sites. Only where land has been used too intensively for long-periods is reforestation uncertain, but probably <10% of the pasture land in N Para has degraded to this level. Re-growth forest, regardless of pasture-use history, will not necessarily have the same characteristics of physiognomy or species composition as that originally occupying the site. As burning becomes more prevalent in eastern Amazonia, abandoned sites may not develop into forest and irreversible degradation of the regional ecosystem must be contemplated. (See also 90L/00365). -from Authors