Variação do gradiente topográfico na diversidade de espécies arbórea-arbustivas em floresta tropical sazonalmente seca

Abstract

The great edaphic and climatic variability associated with the geomorphological variations of the relief are important factors to promote the floristic and phytophysiognomic variability of the vegetation in the Caatinga. These characteristics are complex, because the levels of aridity depend not only on rainfall patterns, but also on the geological type of the soil, which can guarantee higher levels of nutrients and water storage capacity. Starting from the premise that heterogeneous environments have different resources and abiotic conditions, the present work was developed with the objective of expanding the knowledge of biodiversity and functional strategies displayed by communities of tree-shrub species, associated with a context of topographic gradient in the seasonally dry rainforest. Different biotic properties were sampled, such as vegetation structure, taxonomic diversity (TD), functional diversity (FD) and 10 plant characteristics in three communities in each position of the topographic gradient: depression, slope and plateau, in Chapada do Araripe (state of Pernambuco). Different sets of environmental factors were also determined, which included topographic and edaphoclimatic aspects, and studied their relationship with the characteristics of the vegetation. Different sets of environmental factors were also determined, which included topographic and edaphoclimatic aspects, and studied their relationship with the characteristics of the vegetation. The heterogeneity of edaphoclimatic conditions was essentially predicted by the change in elevation in the Araripe region. The species richness in the Araripe region varied according to the topographic gradient in order of elevation: plateau, slope and depression. The main sets of environmental factors that explained the species richness were attributed to the results of the effects of two sets of variables 'climate-edaphic' (44% of the explication). The diversity of species in the region also coincided with peaks of tree biomass, heterogeneity in the height of the trees, and sometimes with the density of vegetation (NDVI). The elevation alone was able to predict changes in the FD and TD of dominant species, with a positive effect on both. However, the change in TD did not follow the variation in FD, differing in the way they were structured along the topographic gradient. The FD was greater, above all, on the slope, and on the sedimentary plateau than in the depression; while TD was higher in the sedimentary plateau, but did not differ between the slope and depression. The dominance of species characteristics was focused on resource acquisition strategies in depression (> leaves; <leaf contente; and <wood density); for features aimed at resource conservation, especially on the slope (<leaves; > leaf contente; and > wood density). The effects based on edaphoclimatic factors were tested in isolation, and part of the variation described here suggests that TD was more associated with 'soil-climate' relationships in the topographic gradient, while differences on FD were better related with the variation of the chemical attributes of the soil in the region. Programs to establish new areas of environmental protection must take into account the need to protect a proportion of each of these three levels of the topographic environmental gradient, in order to guarantee greater variability in plant communities, species survival and ecosystem functions.