Estrutura do sub-bosque em fragmentos ripários na floresta atlântica do extremo norte, Pernambuco, Brasil

Abstract

Deleterious changes in fragments of forest communities have been often associated to biotic and abiotic changes related to area and edge effects. The understory is an important component of forest communities and exhibits a clear response to the process of fragmentation, due to its susceptibility to such effects. In the Northern section of the Atlantic Forest in the State of Pernambuco, Brazil, forest vegetation remains as fragments, mainly located in bottom valleys, in which riparian sites often occur. This study aimed at improving the knowledge on the structural organization of the understory and on effects of area and edges on the floristic composition of forest understory in riparian forests. Firstly, a floristic and structural survey of the understory was carried out, and its similarity to the arboreal component was assessed. Understory species were classified as follows: transient and understory typical (tall and short). Individuals were grouped into height classes and, for each species, absolute and relative descriptors of density and frequency were estimated per class. Based on these parameters, we calculated an estimative of natural regeneration per height class, and a total regeneration index for each species. Secondly, with basis on species-area relationship and edge effects, we studied the structure of edge-interior understory vegetation in six fragments, three large (>100 ha) and three small (<100 ha). Meteorological stations were set up at the edge and interior of each fragment for assembling air temperature and humidity. For structural analyses, all individuals with circumference at soil level (CSL ≥ 3 cm) and DBH < 15 cm were sampled, from which density, frequency and dominance, species richness and Simpson diversity were obtained. On the edge and interior of each fragment, we calculated the percentage of dead standing individuals and of rare, common and abundant species. A CCA was applied in order to check if there was a relationship between microclimatic variables and understory structure among environments. The results showed that: a high richness in the understory and a great similarity with the canopy stratum indicate that forest understory is predominantly composed by canopy regenerants. In general, these canopy species had high regeneration indices, greater densities and/ or frequencies and were evenly distributed amongst size classes, which demonstrate the regeneration potential of canopy species. In larger fragments, edges showed higher temperatures and lower humidity compared to forest interior, whereas in smaller fragments, these differences did not exist. In smaller fragments, there was a greater proportion of dead individuals and a higher diversity. According to CCA, microclimatic variables explained a small part of the total variance on forest structure (10.7% in large and 12.9% in small fragments). It was verified that area and edge effects had a greater influence on smaller fragments, considering that there were no differences on microclimate between edges and interiors. Although the understory component did not validate the species-area relationship, on large fragments the composition of understory assemblages was indeed related to edge-interior microclimatic differences.