Cattle grazing and herbaceous competition shape woody seedling establishment across environmental gradients in Neotropical savannas

Abstract

Abstract Woody encroachment in savannas is a complex and global phenomenon that has negative impacts on the forage availability and livestock yield. We examined the impact of cattle grazing and herbaceous vegetation on Vachellia caven and Vachellia aroma, the main encroacher tree species in Neotropical savannas, where livestock production is the principal activity. Our experiments were conducted across a gradient of aridity and productivity (low-, medium-, and high-productivity sites), covering the central distribution of savannas in Argentina. We carried out manipulative experiments with cattle grazing (grazed–ungrazed) and herbaceous vegetation (with–without) to examine Vachellia seed loss (e.g. granivory), seedling emergence, survival, growth, and establishment. In the medium-productivity site, seed loss was higher under ungrazed conditions with herbaceous vegetation. Across all sites, cattle grazing decreased the final seedling survival regardless of the presence of the herbaceous vegetation. Herbaceous vegetation increased tree seedling survival in the medium-productivity site but decreased it in the low- and the high-productivity sites. Overall, the effect of grazing on tree establishment was neutral in the medium-productivity site as a consequence of increasing seed availability and decreasing seedling survival. However, seedling establishment was higher under ungrazed conditions and without herbaceous vegetation in the high- and low-productivity sites, because of the negative effects of grazing and herbaceous vegetation on seedling survival. From this demographic approach, we identified an ecological window during which grazing could be effectively managed to control the early stages of woody encroachment if stocking is adjusted and synchronized with tree species life history and site productivity.