Cattle grazing and herbaceous vegetation reduce Vachellia seedling establishment in Neotropical savannas

Woody encroachment threatens the biodiversity, functioning, and ecosystem services of savannas worldwide. Few systematic—experimental—manipulative studies have evaluated the mechanisms that determine demographic transitions of encroaching species across broad environmental gradients. We evaluated cattle and herbaceous vegetation impacts on two widespread tree-encroaching species in Neotropical savannas, Vachellia caven and Vachellia aroma . At two savanna sites with contrasting aridity and productivity, a humid, high-productivity site and a semiarid, low-productivity site, we manipulated cattle grazing (grazed vs. ungrazed) and herbaceous vegetation (present vs. removed) to evaluate their effects on Vachellia seed loss, seedling emergence, survival, growth, and establishment. In the low-productivity site, seed loss ( e.g., by granivory) was higher in ungrazed plots with herbaceous vegetation than in all other treatments. Vachellia seedling emergence rate did not vary significantly among treatments or sites. At both sites, cattle grazing decreased final survival (0.33 in ungrazed vs 0.01 grazed plots) and seedling growth in height. Herbaceous vegetation also decreased Vachellia seedling survival (0.004 vs. 0.197). Finally, regardless of the site, Vachellia seedling establishment was higher in ungrazed plots without herbaceous vegetation. Our findings suggest that cattle effects on Vachellia seedlings are consistent among sites with contrasting environmental conditions and could therefore be effectively managed to address woody encroachment in savannas. We developed a conceptual model suggesting that grazing management should consider that increasing stocking rates after seedling emergence may enhance the likelihood of encounters between cattle and tree seedlings. But grazing intensification should not compromise grass regrowth capacity, to maximize tree-grass competitive interactions. • The impact of cattle on Vachellia varies across the tree life stages. • Cattle consistently reduce Vachellia seedling survival, regardless of site conditions. • Herbaceous layer reduces Vachellia seedling survival, regardless of site conditions. • Rotational cattle grazing could limit early woody encroachment.