An enemy's enemy is an ally: Competitive indirect interactions mediate coexistence of trees, grasses, and subshrubs in neotropical savanna

Abstract Savannas are complex ecosystems where multiple growth forms, including grasses, trees, and subshrubs, coexist through intricate ecological interactions. Understanding the mechanisms that promote this coexistence is key to conserving savanna biodiversity. This study investigated the effects of grass competition on the survival and growth of trees and subshrubs, as well as the interactions between these two growth forms. Using three species of trees and three species of subshrubs, we conducted a greenhouse experiment to assess how competition between trees and subshrubs is influenced by the presence of grass. We found that grass competition significantly reduced the survival of tree seedlings, while subshrubs were unaffected. When trees and subshrubs competed directly, subshrub biomass was reduced, but only in the absence of grass. Tree seedling biomass was not affected by subshrubs, but was negatively impacted by grass in the absence of subshrubs. Both root and stem biomass of trees and subshrubs were reduced by grass competition; however, this effect was mitigated when grasses, subshrubs, and trees competed simultaneously. These results indicate that when grasses, subshrubs, and trees compete together, the intensity of competition is reduced, promoting coexistence and contributing to the balance of growth forms in savanna ecosystems. While fire is an important factor in savanna dynamics, our study emphasizes the critical role of competition in maintaining this balance. Future studies should explore how fire and competition interact to further our understanding of biodiversity and ecosystem function in Neotropical savannas. Abstract in Portuguese is available with online material.