Succession of tropical rain forest along a gradient of agricultural intensification: Patterns, mechanisms and implications for conservation.

Abstract

Land use affects the pattern and pace of secondary succession, yet few studies have directly compared post-agricultural succession across a range of agricultural strategies in the neotropics. This study compared succession of tropical forest along a gradient of agricultural intensification, represented, in order, by agroforestry, swidden cultivation, cattle ranching, and input-intensive monoculture in Guatemala and Nicaragua. This gradient is characterized by a decrease in crop diversity, increased frequency of land use, application of commercial fertilizers and pesticides, and reduction of forest in the landscape. The first hypothesis stated that the pace of succession, measured by growth and accumulation of woody plants, vines, and large herbaceous plants, decreases with agricultural intensification. In Nicaragua, succession was fastest after swidden, intermediate after agroforestry and pasture, and slowest after monoculture. In a parallel study in Guatemala, succession was faster after agroforestry and swidden relative to pasture and monoculture. These results generally support the first hypothesis. However, species composition was dominated by pioneer species in all agricultural practices. To explain these patterns, the second hypothesis stated that availability of regeneration mechanisms decreases along the intensification gradient. First, seed dispersal of 12 vertebrate-dispersed tree species decreased with intensification. Seed dispersal was highest in agroforestry, a result likely due to higher frugivorous bird richness in agroforestry than in other practices, and greater seed source availability on these farms. Second, germination of the same 12 species was species-specific. Shade-tolerant species germinated in higher proportions in agroforestry, swidden, and monoculture than in pasture. Pioneer species germinated in highest proportions in monoculture. Third, post-dispersal seed removal (assumed to be predation) of three different tree species was higher in agroforestry, swidden, and pasture than in monoculture. Therefore, regeneration mechanisms were in opposition along the intensification gradient and depended on tree species. Seed removal was most limiting in agroforestry and swidden, all three mechanisms were limiting in pasture, and seed dispersal was most limiting in monoculture. Agricultural intensification inhibits early forest succession. Seed dispersal, germination, and seed removal should be targeted appropriately to facilitate forest restoration along the agricultural intensification gradient, particularly to promote recovery of species composition similar to mature forest.