Can spatial variation and inter-annual variation in precipitation explain the seed density and species richness of the germinable soil seed bank in a tropical dry forest in north-eastern Brazil?

The interaction between microhabitat and inter-annual variation in precipitation has an important role on the dynamics of the seed bank and can play a crucial role in survival and maintenance of plant populations in semi-arid environments. We hypothesized that the type of microhabitat and the inter-annual variability in precipitation can explain the richness and density of the seed bank in a semi-arid region in Brazil. The study was conducted in an area of tropical dry forest with shrub-tree physiognomy, locally called caatinga. We collected 35 soil samples in three distinct microhabitats, at the end of rainy and dry seasons, respectively, over three years, totalling 630 samples. The seed bank (richness and seed density) were determined by seedling emergence method. Over the three years, 79 species emerged from the seed bank, 64, 45 and 42 in riparian, non-riparian and rocky microhabitats, respectively. We recorded differences in species richness and average density between microhabitats and between years, with significant statistical interaction between them. Inter-annual precipitation explained 48% and 5% of the variation in richness and seed density, respectively. Spatial variation explained 7% of the species richness and 31% of the density. Our results show that the interaction between spatial variation and precipitation has an important role on the spatial and temporal heterogeneity of the richness and density of seed banks in dry environments.