Human Disturbances and Woody Flora Diversity Along a Caatinga–Brejo de Altitude Gradient in Northeastern Brazil

Abstract

Tropical dry forests are among the ecosystems most threatened by human activities worldwide. The Caatinga, a seasonally dry tropical forest restricted to northeastern Brazil, exemplifies this scenario, as local human populations strongly depend on forest resources. In some areas within the Caatinga, increasing elevation promotes the formation of isolated humid montane forests locally known as Brejos de Altitude. In this study, we aimed to investigate patterns of taxonomic and phylogenetic alpha (α) and beta (β) diversity of woody plants along a Caatinga–Brejo de Altitude gradient, considering the effects of acute and chronic anthropogenic disturbances. Floristic data and chronic disturbance measurements were conducted in 25 10-m² plots. Acute disturbances were identified through field observations and satellite imagery analyses using QGIS and Fragstat. Taxonomic and phylogenetic diversity metrics were calculated at both α- and β-diversity scales. The influence of acute and chronic disturbances and altitude on α- and β-diversity metrics was evaluated using multimodel inference. We found that taxonomic and phylogenetic α-diversity were not influenced by the evaluated predictors. In contrast, elevation and proportion of agricultural land affected taxonomic β-diversity, promoting species replacement along the gradient. Phylogenetic β-diversity was also influenced by elevation, agricultural land use, and chronic disturbance, although these variables exerted contrasting effects. We conclude that β-diversity is more sensitive to disturbance and elevation, which promoted species replacement and changes in the composition of woody flora along the Caatinga–Brejo de Altitude gradient.