Functional diversity drives leaf decomposition across rainfall–disturbance gradients, but phylogenetic effects vary

Plant functional traits driving ecosystem functioning can have either similar or distinct evolutionary histories, meaning that both functional and phylogenetic diversity can jointly influence ecosystem dynamics. However, the relative importance of this relationship depends on the environmental context. In this study, we combined species into assemblages by using a full-factorial experiment varying low and high levels of functional and phylogenetic diversity. We conducted a diversity-decomposition assessment along independent gradients of rainfall and landscape disturbance in the seasonally tropical dry forest of the Brazilian Caatinga. We demonstrated that functional diversity, even holding species richness constant at four, promoted decomposition and played a greater role in decomposition than phylogenetic diversity. Equally or more importantly, the extent to which differences in traits and evolutionary lineages affected leaf decomposition was contingent on rainfall and landscape disturbance. Functional diversity increased mass loss, particularly in dry conditions and disturbed landscapes. In contrast, increasing phylogenetic diversity was associated with lower leaf decomposition in dry regions, but this effect reversed in wetter regions. Our experimental findings suggest that ecological gradients likely mediate ecosystem functioning. Furthermore, we identified a potential role of functional diversity as a buffer against climate change and disturbance in dry ecosystems, reinforcing the benefit of biodiversity conservation efforts aimed at protecting as many species and traits as possible.