Spatial distribution of tropical wetlands in Central Brazil as influenced by geological and geomorphological settings

Abstract

Tropical wetlands in Central Brazil are located in savanna areas and are made up of more terrestrial-type wetlands – campo limpo úmido (grassland-type savanna), campo sujo úmido (shrub-type savanna), mata galeria (riverine forest); as well as more aquatic-type wetlands – vereda (valley-side marsh with palm groves of Mauritia flexuosa), várzea (backswamp), lake, and river. They are regulated by a seasonal climatic regime characterized by a wet-rainy season from October to March and then followed by a dry season. Underground water is abundant and rivers frequently overflow during the rainy season. Many of these wetlands which are protected by law are significant regulators of water quality. In order to predict tropical wetland function and draw up environmental management policies, it is important to understand how abiotic and biotic factors influence tropical wetland origin and spatial distribution. In this large-scale study we set out to investigate the influence that geological and geormorphological settings, i.e. geological substrates and geomorphological units, have on the spatial distribution of tropical wetlands in Central Brazil. Two watersheds, the Caiapó and Piracanjuba, were selected in order to carry out the study. They present different types of rock and unconsolidated surface materials. Planation surface, escarpment and steep slope, gentle slope, and aggradation are the dominant geomorphological units in the watersheds. Principal component analysis was conducted in order to determine the influence of the selected abiotic variables on the spatial distribution of tropical wetlands. The study showed that the presence of sedimentary and low-grade metamorphic rocks and planation surface strongly influence the spatial distribution of the more terrestrial-type wetlands. Geological and geomorphological settings explain about 60% of the variability in the spatial distribution of these wetland types. No clear effect of the abiotic variables selected was observed on the more aquatic-type wetlands. An association between stratified layers or planar geological surfaces, groundwater discharge zones, and slope breaks is suggested to explain the influence of the geological and geomorphological settings on the wetlands under review. The study demonstrated the importance of considering abiotic factors, not usually included in classification schemes, to further understand the spatial distribution of tropical wetlands.