Padrões fitogeográficos de Florestas Estacionais Deciduais na bacia do Rio São Francisco: análise florístico-estrutural e suas relações com o ambiente

Abstract

Understanding the factors that affect the structure and flora in forest communities is essential. This is especially true when these efforts are focused on meso and macro-scale studies and on vegetation types that lack more in depth information such as Seasonally Dry Topical Forests (SDTF). In this study we evaluated the floristic-structural similarity and spatial distribution of tree species related to soil variables and relief in four sites of SDTF located along the São Francisco River Basin (Chapters 1 and 2). We also sought the floristic relationships between 47 SDTF sites, in a geographical range extending from Rio Grande do Norte to Sao Paulo state, thus analyzing the relationships between these forests with climatic variables and altitude (Chapter 3 ). We observed differences in the flora (between sites and between the adult stratum and natural regeneration), structure and the environment among the four communities investigated. Paracatu and Peruaçu were the sites with highest richness, diversity and density. The areas with the highest basal area were Coribe and Paracatu. As for the environmental aspects we observed that soils with greater availability of nutrients were found in Coribe and Paracatu, while Arcos Peruaçu and were the poorest. The variations in the environmental characteristics, flora and structure may be related to environmental heterogeneity of forest fragments which is the result of both geographical variations among the sites and due to contact with other vegetation matrices. Climate variations, mainly related to rainfall, also put considerable weight on the distinction between them. In addition, land use history may also have influenced the structural differences presented among sites. The low floristic, structural and environmental similarity between sites and strata revealed high beta diversity between the four fragments, stressing the importance of biodiversity conservation of SDTF. After comparing the 47 sites the observed trend was a distribution of sites along a latitudinal gradient, differentiated according to the water sheds in which they are in. The areas located in the São Francisco River basin share species with other basins, which would be an indication of possible connectivity routes between sites. Regarding the relation of the environmental variables with sites we observed that altitude, precipitation (seasonality and in the driest and wettest quarter) and temperature (seasonality and annual average) were correlated with the first and third axis of the of Non-metric Multidimensional Scaling (NMDS) ordination analysis. This shows that altitude and climatic variables exerted strong influence on the distribution of the 47 sites, with these two axes explaining more than half of the model variation. Thus, the large environmental variation, observed in the altitudinal and climatic variables, which proved to be closely related to variations in space, especially with the latitudinal gradient, exerted great influence on site distribution. The most floristically similar sites have similar environments, possibly resulting from equivalent habitats, supporting the hypothesis proposed in this chapter that the theory of ecological niches can be applied to explain the phytogeographic patterns of deciduous forests.