Biodiversity effects on seedling biomass growth are modulated by light environment across functional groups

1. Tree biodiversity has the potential to ensure consistency in the functioning of forest ecosystems, not just over space, but over long-timescales by maintaining composition through recruitment. However, for continued buffering in the face of global environmental change, the sensitivity of biodiversity-ecosystem functioning relationships to heterogeneous environments needs to be understood. 2. Seedling recruitment in carbon-rich tropical forests is a result of biotic and abiotic drivers but their combined outcomes at the community-level remain poorly understood. Although biodiversity in seedling communities can potentially increase their growth and biomass accumulation, abiotic drivers like light can alter this effect through divergent effects on constituent species and functional groups. In forests with high baseline heterogeneity in microclimates, these processes can enhance or constrain regeneration. 3. We tested the effects and interactions between species richness and canopy cover on the growth of seedling communities consisting of tropical broad-leaved evergreen and deciduous forest species using a fully crossed manipulated experiment in the Andaman Islands, India and compared these with field observations from a long-term forest plot in the same landscape. 4. We show that in the critical seedling establishment phase, species richness and light increase community biomass independently. Accounting for variation across species, individual species on average accumulated more biomass in communities with both higher light and higher diversity. 5. We also show that overyielding in species rich communities fits expectations from a model of complementarity with non-random overyielding than selection or spatial insurance effects. 6. Synthesis Taken together, our results show that the potential for biodiversity to increase ecosystem functioning in seedling communities is modulated by light. Further understanding on the interaction of biodiversity with multiple abiotic drivers and their effect on regeneration dynamics is crucial for predicting future ecosystem functioning. ### Competing Interest Statement The authors have declared no competing interest.