干旱区生态环境知识资源中心

(Floristics and phytosociology of grassland vegetation subject to sandy desertification in southwestern Rio Grande do Sul State, Brazil). Extensive areas of native grassland in southwestern Rio Grande do Sul are susceptible to sandy desertification, which occurs in pedologically fragile areas influenced by anthropic interference and abiotic factors. A floristic and phytosociological study was carried out in grazed native grasslands to test vegetation cover variation and sandy desertification dynamics over time and according to the influence of slope distance. Two sub-areas that differed as to the sandy desertification process were selected in Sao Francisco de Assis municipality, Rio Grande do Sul. The floristic survey revealed the presence of 102 species, distributed in 25 families. For the phytosociological study, 35 permanent plots (0.25 m(2) each) in each sub-area were arranged at different distances from the hillside slope. Cover of plant species and litter, and bare soil were recorded for three periods. Richness, species diversity, and life forms were evaluated by analysis of variance using randomization tests, considering temporal and spatial factors (slope distance). Higher reduction of species diversity and vegetation cover in sub-area I (with less vegetation cover) occurred due to temporal variation. In sub-area 2 (with more vegetation cover), the increase in bare soil and a subsequent reduction of vegetation cover were highly influenced by slope distance. Vegetation cover change and sandy desertification expansion observed throughout this period resulted from the joint influence of rainfall dynamics, soil type, presence of tableland slopes with little vegetation, and constant grazing pressure.