Exploring intermediate richness peak: Contrasting local and regional patterns of vascular epiphytes in the Brazilian tropical montane cloud forests

Abstract

Altitudinal gradients can provide valuable insights into the distribution patterns of species richness at both local and regional scales. Typically, species richness peaks at intermediate elevations. This pattern can be attributed to factors such as climatic conditions, the mid-domain effect, and historical-evolutionary processes. Our research aimed to investigate the distribution patterns of vascular epiphytes along the regional elevation gradient in dwarf cloud forests of Serra da Mantiqueira (SM), Brazil. We hypothesised that species richness would peak at intermediate elevations correlating with the availability of water. To test this hypothesis, we sampled 18 plots, each measuring 20 × 20 m, in seven protected areas with varying elevation gradients. The lowest plot was at 1270 m, and the highest was at 2350 m. In each plot, we surveyed trees (phorophytes) with a circumference at breast height of 30 cm. Rarefaction analysis was conducted to compare species richness among plots, while beta diversity analysis was used to assess species composition changes along the elevation gradient and across the SM range. We employed a generalised linear model (GLM) with a Poisson distribution to examine the relationships between environmental variables and plant richness. Our findings suggest that there is no intermediate peak in species richness in any of the localities. However, regionally, the elevation range of 1500-1700 m exhibited the highest richness, particularly when compared to the higher plots, which are subject to low temperatures that act as environmental filters, limiting species richness despite higher precipitation. Furthermore, plots above 1980 m showed low turnover, indicating that these filters contribute to a more homogeneous composition. Species composition across the SM was influenced by climatic differences, and overall, geographical distance was not significant, as within the same locality exhibited low turnover. This study provides a foundation for understanding patterns of species richness and composition along altitudinal gradients in the context of global climate change.