Abstract Riverine forests have high floristic diversity and are heterogeneous, even when they are near each other. Therefore, we aimed to compare three riverine forest remnants of Southern Brazil located in the Pampa biome regarding floristic and structural biodiversity, to: i) check whether the studied forest fragments maintain heterogeneity and high floristic richness as seen in riverine forests, even when close to each other, and ii) indicate the predominant successional groups in each remnant. A total of 81 sampling units were established, and all trees with diameter at breast height ≥ 15 cm were measured. We sampled 1,659 individuals, 67 species, and 34 families. The richness and structure of the three communities differed, confirming the high heterogeneity of riverine forests. One of the remnants had a higher predominance of pioneer species and individuals, which differentiated it from the other two. The latter, in turn, showed higher similarity in floristic and structural composition, with a predominance of secondary and late successional individuals and species. Differences in soil moisture, in the size and shape of fragments may be factors that contributed to the floristic and structural diversity observed. The remnants, although small and surrounded by monocultures, are heterogeneous and important for biodiversity conservation. Therefore i ii remnant 8 established 1 measured 1659 659 1,65 6 3 families differed two latter turn composition moisture observed monocultures conservation 165 65 1,6 16 1,

RESUMO Com o objetivo de caracterizar a estrutura da comunidade arbórea de um remanescente de floresta ribeirinha do rio Taquari, foram estabelecidas 42 unidades amostrais de 100 m2 (10 × 10 m) e calculados os parâmetros fitossociológicos e índices de diversidade de Shannon (H') e Equabilidade de Pielou (J). Foram amostradas 39 espécies, 21 famílias, 2,83 para H' e 0,77 para J. Dentre as espécies Callisthene inundata O.L. Bueno, A.D. Nilson & R.G. Magalh. é endêmica, estando, junto com Picrasma crenata (Vell.) Engl., na lista das espécies ameaçadas de extinção. A densidade encontrada foi de 1.557,14 ind. ha-1. Luehea divaricata Mart. e Lonchocarpus nitidus Benth. apresentaram os maiores índices de valores de importância. Além de contribuir para o conhecimento da distribuição das espécies e estrutura da comunidade, este estudo alerta para a necessidade de conservação dos remanescentes florestais nativos existentes.

ABSTRACT Aiming to characterize the structure of the arboreal community in a riverine forest remnant of the Taquari river, State of Rio Grande do Sul, 42 sampling units of 100 m2 (10 × 10 m) were located. Phytosociological parameters were also assessed and the indexes of Shannon diversity (H') and Pielou evenness (J) were evaluated. A total of 39 species, 21 families, 2.83 nats ind-1 for H' and 0.77 for J were recorded. Among the species found, the endemic Callisthene inundata O.L. Bueno, A.D. Nilson & R.G. Magalh. and Picrasma crenata (Vell.) Engl. are included in the list of endangered species. The density found was of 1,557.14 ind ha-1. Luehea divaricata Mart. and Lonchocarpus nitidus Benth. showed the highest indexes of importance values. Besides contributing to the knowledge of species distribution and community structure, this study points out the need for conservation of existing native forest remnants.