Soil seed bank of floodable native and cultivated grassland in the Pantanal wetland: effects of flood gradient, season and species invasion

In the Pantanal, exotic grasses are being introduced into native grasslands. Information on impact of these plants and of flood on the soil seed bank dynamics of grassland vegetation regarding distribution and diversity are yet scarce. The aim of this work was to evaluate species similarity of soil seed bank of native and cultivated grassland in the Pantanal, under effects of flood levels, season and species invasion, evaluating effects of ecological filter upon distribution and diversity of flora. The soil seed bank was sampled in 1 km 2 in native and 1 km 2 in cultivated grassland. To test the hypothesis regarding flood gradient, we collected soil samples of 20 x 20 cm, 3 cm deep, at three ground levels (bottom, intermediate and top). To analyze species invasion, soil samples were collected in native and in cultivated grassland (Urochloa humidicola), a total of 120 in the dry season and 120 in the flood season. The soil seed bank was evaluated through emergence in trays in a greenhouse. From the soil seed bank emerged 91 species, of 22 families. The most representative species in density were Richardia grandiflora (< 7,558 seedling m(-2)) and Hyptis brevipes (< 6,725 seedlings m(-2) ). We conclude that flood influences distribution and diversity of species in native and cultivated grassland, the vegetation varies according to the gradient, similarity being higher among species in the rainy season. We also conclude that the soil seed bank is not dominated by exotic weedy species, what is attributed to flood action as an ecological filter hindering their establishment.