Who is who in the understory: the contribution of resident and transitory groups of species to plant richness in forest assemblages.

The forest understory is made up of resident and transitory species and can be much richer than the canopy. With the purpose to describe the contribution of these groups to the woody understory, five Atlantic Forest fragments were selected and studied in Northeastern Brazil. In order to analyze the understory's structure, the sample included woody individuals with circumference at breast height (CBH) smaller than 15cm and circumference at ground level (CGL) greater than 3cm, regardless of height. The recorded species were quantified and classified into functional stratification categories (resident and transitory), and the floristic similarity between the understory and the tree stratum was calculated. Species' importance in the understory was analyzed by height and total natural regeneration classes based on a regeneration index. The understory was richer in species (median=63.8, SD=21.72, n=5 fragments) than the tree stratum (43.8, 18.14, 5), and the similarity between these components was relatively high (median=0.54, SD=0.09). The results also showed that the studied understory in the forest fragments was mainly composed by transitory species (median=67.01%, SD=3.76), that were well distributed among height classes and had the highest densities, which may favor their future presence in the canopy's structure and composition. The typical understory species were grouped into two strata: the lower understory, made up of species that generally do not reach more than 4m in height (mostly species from families Piperaceae, Rubiaceae and Melastomataceae); and the upper understory, with intermediate heights between the lower understory and the canopy, but with average heights that were not higher than 10m (mainly of species from families Anonnaceae, Clusiaceae and Myrtaceae). These families' richness was commonly used as an indicator of the vegetation's successional stage; however, such results must be seen with caution as they show that these families co-occurred and were highly important in different strata. Studying the understory is fundamental because it represents a floristically rich stratum with a unique structure, which promotes the natural regeneration of the tree stratum.