Abstract
Biodiversity–ecosystem functioning (BEF) theory predicts that the diversity of consumer traits regulates ecosystem processes such as organic-matter decomposition. Variation in vegetation heterogeneity provides a direct test of this prediction, since structurally simpler vegetation can alter both the decomposer communities and the physical conditions under which they work. We examined how vegetation heterogeneity, spanning three forest sites that differed in land-use history (native forest, abandoned pine plantation, and actively managed eucalyptus plantation), affects microclimate, invertebrate functional diversity, and leaf-litter breakdown in subtropical phytotelmata. Artificial microcosms were installed across sites differing in vegetation heterogeneity, incubated with four litter species, and analysed with structural equation models fitted separately to each litter species. Vegetation heterogeneity reduced thermal variability inside phytotelmata consistently across all litter species. Contrary to BEF predictions, the functional diversity of aquatic invertebrates did not predict litter breakdown for any litter species, and thermal buffering translated into faster decomposition for only one substrate (Pinus elliottii). Vegetation heterogeneity thus buffers the microclimate of phytotelmata predictably, while its consequences for organic-matter processing remain contingent on the litter being processed. In small, resource-limited habitats, BEF relationships appear to be constrained by the abiotic context that vegetation heterogeneity imposes rather than expressed through trait complementarity among consumers.