This study presents the structure of forest stands of an Evergreen Seasonal Forest along the Pacas River and evaluates the floristic similarity of ten areas located in the Pacuneiro and Pacas river basins in Mato Grosso state, Brazil. Each section (source, middle, and mouth) was divided into 14 plots (25x10 m). All individuals with diameter at breast height (DBH) > 15 cm were sampled. Phytosociological parameters and floristic similarity (by Jaccard index) were analyzed. Total density was 1,606 ind./ha. The most commonly found families in number of species were Anonnaceae (9), Fabaceae (9), Melastomataceae (6), and Burseraceae (5). The species Jacaranda copaia (Aubl.) D. Don, Ocotea caudata (Nees) Mez, Ocotea guianensis Aubl., and Zigia cataractae (Kunth) L. Rico presented the highest importance values. The Shannon (3.67) and equability (0.86) indexes found were similar to those of other riparian forests. The areas of the source and middle of the river showed the highest floristic similarity (J = 0.71).
phytosociology; Xingu; Amazon Forest