This paper describes the changes in floristic composition and structure that occurred following the degradation of the riparian forests in the Benfica Settlement Project, in the southeast of Pará, Brazil. The composition and structure of the vegetation was analyzed in 15 transects of 10 x 50m distributed in five vegetation types, representing an increasing gradient of anthropic impact: Riparian forest (FRF), Riparian forest in pasture (FRP), Riparian forest with açaí palm (FRA), Shrubby marsh (BAB) and Herbaceous marsh (BHE). Three strata were considered: Upper Stratum – plants with DBH > 10cm, Middle Stratum – plants with DBH < 10cm and height greater than 2m, and Lower Stratum – plants less than 2m in height. Species were classified in two functional groups: forest species and pioneer species. In the 15 transects a total of 79 families, 176 genera and 279 species were identified. In all strata, the number of forest species predominated over pioneer species. The majority of species showed a low frequency, with 55% occurring in only one area. Differences were found between the basal area and height of the upper strata between the different vegetation types. The number of woody species is significantly greater in the FRF (mean of 57.33 species) and less in the BHE (mean of 7 species). The richness of woody forest species, woody pioneers, forest herbs and pioneers herbs showed differences between the different vegetation types. The densities of individual forest herbs did not present statistical differences between the vegetation types; however, the great number of individual of pionners herbs found in the BHE (mean of 15.95 individuals/m2) led to a statistical difference. The mean similarity between the areas was very low (Jmean = 0.171 + 0.102). In the upper stratum, all the indices used (richness, Shannon diversity, equitability and density) showed significant diference within categories. In the middle stratum, only richness and Shannon diversity showed a significant difference between vegetation types. In the lower stratum, as in the general analysis, species richness was the only index to show a significant difference between vegetation types. A floristic gradient can be visualized in the grouping analysis, with five distinct groups. The group of FRF, whose indicators were woody (91%) and forest species (100%), were Castilla ulei, Inga auristellae, Mollinedia laurina, Ouratea paraensis, and Trichilia lecointei. In the group of FRA, that also had forest species (100%) as indicators, although there was no predominant biological form, the most important species were Calathea sp. and Monstera obliqua. In the group of FRP that also had forest species (100%) as indicators, although there was no predominant biological form, the most important species were Calathea elliptica, Humirianthera ampla, Lomariopsis sp. and Protium sp. The group of BAB had 33% pioneers species as indicators, the most important being Panicum pilosum, Piperomia sp., Sapium lanceolatum and Urera caracasana. In the group of BHE the herbaceous species (83%) predominated as indicators and the number of pionners species (50%) was much greater than in the other groups, with the most important species being Paspalum paniculatum and Nephrolepis biserrata. Euterpe oleraceae (açaí) is an indicator species of almost all the riparian forests of the Benfica Settlement Project.
