Soil and topographic variation as a key factor driving the distribution of tree flora in the Amazonia/Cerrado transition

Abstract

We tested the hypothesis that topographic/soil gradient and ecological succession are the main factors driving the high tree diversity in a protected area in Amazonia/Cerrado transition. The gradient is a slope composed by savanna-like vegetation of rocky cerrado on the top, typical and dense savanna cerrado in the middle, and cerradão ecotonal forest in the bottom. The ecological succession is a result of Cerrado encroachment when protected from fire. The cerradão and dense cerrado were associated to higher values of clay, silt, Mg and organic matter, whereas the typical and rocky cerrado were the opposite, with highest sand, altitude and Al. The PCA analysis revealed 73% of the influence of such habitat conditions in the species distribution along the slope. The most important determinants of species distribution are the topography and soil properties, specially texture, organic matter and concentrations of Mg. The species richness was higher in the dense and typical cerrado (intermediate slope), probably due to the mutual influence of the top flora of rocky cerrado and the lower gradient flora of cerradão. The influences of topography and soil texture is probably related to the water availability, where the sandy soil and full drainage in the top of the slope provide less water supply to the vegetation (rocky cerrado) compared to the bottom (cerradão). Patches of fire-protected Cerrado encroachment are common in Bacaba Park, with forest and savanna species coexisting. The substitutions of species across space and time, corroborates our hypothesis about the successional and topographic/edaphic variations acting as the main determinant of high tree diversity in the protected area.