Savanna woody encroachment in the Cerrado-Amazon ecotone

Abstract

Purpose We investigated if, in transitional region between the South American savanna (Cerrado) and Amazon Biomes, the facilitation process help drive this development of forest trees in open-canopy and small-treed savannas (‘Typical Cerrado’) in Brazil.Methods In multiple permanent plots, we recorded all trees (focal plants) with a diameter at ground level > 10 cm, and focused on those species with the highest biomass and stem density. We identified and measured the height and diameter of all plants at least 5 cm tall within a 3 m range of these focal plants and derived structural and diversity parameters. We also collected soil samples and measured microenvironmental parameters, including litter layer, grass cover and soil surface temperature. Then we compared all environmental and vegetation variables with equivalent measurements made in open areas away from the focal plant crowns. We classified all species identified according to their preferred environment (forest, generalist and savanna).Results Focal plants alter the microenvironment under their crowns, reducing soil surface temperature and grass cover and increasing the litter layer, soil organic matter, and soil nutrient content. These conditions facilitate the increase in species richness, density, and aboveground biomass accumulation of juveniles and adults of key forest and generalist tree species.Conclusion By changing microenvironmental conditions these trees act as nurse plants, promoting the formation of islands of plant diversity. This process favors encroachment by woody plants typical of forests, leading to the woody encroachment of savanna areas.