Drastic impoverishment of the soil seed bank in a tropical dry forest exposed to slash-and-burn agriculture

Forest ecosystems are increasingly threatened by unsustainable agricultural practices, especially by those that damage their regenerative potential. This can be the case of slash-and-burn agriculture – a farming method that can negatively impact the soil seed bank, potentially limiting the resilience of forest ecosystems. To test this hypothesis, and thus look for management practices aimed at enhancing forest recovery, we examined the impact of fire throughout an experiment of slash-and-burn agriculture on the soil seed bank of woody plants in the Caatinga tropical dry forest, northeast Brazil. We compared seed damage and viability, and the structure (seed density and diversity) and composition (taxonomic and functional) of seed bank assemblages before and after fire. We found a significant decrease in the frequency and proportion of intact (undamaged) seeds after fire, and a 3.6-fold decrease in the proportion of viable seeds. While seed density remained constant, species diversity drastically decreased after fire, especially the number of rare species. The compositional dissimilarity (β-diversity) between plots also dropped after fire, particularly its turnover component, thus causing the homogenization of seed assemblages across space. The functional composition of seed assemblages was also altered, with the relative frequency of shrub species increasing after fire, especially species with fleshy fruits and biotic dispersal. Taken together, our findings highlight the low resistance of the soil seed bank to this common farming method in tropical dry forests. Therefore, the recovery of this and potentially of other species-rich tropical forests exposed to slash-and-burn agriculture cannot rest on the soil seed bank, but on other processes such as seed dispersal and resprouting – an interesting avenue for future research.