Effects of slash and burn practices on a soil seed bank of caatinga vegetation in Northeastern Brazil

The semiarid tropical zone covers 20% of Brazil and is dominated by caatinga, a thorny deciduous savanna well adapted to seasonal water shortage and periodic drought years. This study was focused on effects of slash and burn agriculture on the soil seed bank in a Caatinga area, in Sobral, CE, Brazil. Caatinga is rich in species, called therophytes, which remain as seeds in the soil during unfavorable seasons and rely on regeneration from the soil seed bank for persistence in the environment. Although slash and burn agriculture has been intensified in the region for the past three centuries, its effects on the soil seed bank are not well known. A seedling emergence greenhouse experiment was conducted to evaluate differences in seed bank density and diversity among soil samples collected before and after an experimental burning. Soil samples were previously submitted to sequential sieving to assess fire effects on different-sized seeds. Fire significantly reduced overall seed bank density, with smaller sieving fractions being most strongly affected. Shanon′s diversity index was also lowered by fire. Combined, these results show that agricultural practices represent a serious threat to plant biodiversity conservation in the Caatinga biome.