AbstractOld, climatically buffered and infertile landscapes (OCBILs) are remarkably species-diverse, but the assembly of their ecological communities is largely unknown. Our goal was to understand how edaphic filters drive the functional structure of plant communities in an ancient and nutrient-impoverished ecosystem. We carried out a functional screening across four types of campo rupestre habitats with different edaphic conditions. We investigated trait–soil relationships for ironstone and quartzitic substrate to obtain the optimal trait values of each condition. In addition, we built unipartite networks to explore trait–trait relationships to assess functional coordination among organs. Lower soil pH and less soil water retention potential increased the proportion of smaller leaves, tissue dry mass and tissue density, resulting in more resource-conservative plant species and communities. Trait functions were tightly correlated among organs at both the plot and the individual level, indicating a trend of phenotypic integration. Architectural traits were central to coordination, suggesting their key role in integrating sap transport, mechanical support and leaf display. We conclude that the trait adjustments in response to soil parameters are important for the coexistence of a large number of species in the campo rupestre ecosystem.