Negative impact of slash-and-burn agriculture on the seed rain in a tropical dry forest

Abstract

Forest’s recovery potential in human-modified landscapes is increasingly threatened by agricultural activities that disrupt critical sources of forest regeneration, such as the seed rain. Slash-and-burn agriculture is a good example. By slashing and burning the vegetation, this farming method can promote seed source and seed dispersal limitation, but this hypothesis remains poorly tested. Here, we sampled the seed rain during a complete year in nine plots exposed to slash-and-burn agriculture (i.e., burned plots) and in forest stands (control plots) in the Caatinga biome – a species-rich tropical dry forest endemic to Brazil that is increasingly threatened by slash-and-burn agriculture. We compared seed density, seed species diversity, and the taxonomic and functional composition of seed assemblages between burned and control plots. As expected, seed density was 15 times lower in burned plots than in control plots. Species diversity was also lower in burned plots, but only when considering the number of rare species. However, burned plots showed a higher β-diversity of rare species than forest plots, mainly caused by species replacement (i.e. species turnover) from plot to plot. Burned plots also showed 30% more species with dry fruits and abiotic dispersal than control plots, but this difference was not statistically significant. Taken together, our findings highlight the low tolerance of the seed rain to this dominant agricultural practice in tropical dry forests. This is likely related to the lack of seed sources and seed dispersers in burned plots. Therefore, increasing the availability of seed sources and ecological connectivity in the surrounding landscape are critical management strategies for enhancing seed dispersal and forest recovery in forest areas exposed to this farming method.