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Both plant species diversity and genetic diversity within a plant species can affect community properties and ecosystem processes. However, the relative contribution of species diversity and genetic diversity to ecosystem functioning is poorly known. Furthermore, ecosystem processes may respond non-additively to interactions between species diversity and genetic diversity. If interactive effects exist, the impact of biodiversity loss may not be predictable from simple assessments of either species diversity or genetic diversity alone. * Here, we addressed how plant species diversity and genetic diversity within a dominant species independently and interactively influenced plant community biomass in a Great Lakes sand dune ecosystem. To test the independent effects of diversity, we established two experiments. In one, we manipulated genetic diversity within the dominant dune species, Ammophila breviligulata. In the other, we manipulated the number of plant species, excluding A.breviligulata. Then, to test for interactive effects, we constructed communities that varied the number of species and levels of genetic diversity within A.breviligulata. Although there were no independent effects of either species diversity or genetic diversity within A.breviligulata on biomass production in this system, interactive effects of species diversity and genetic diversity significantly influenced overall above-ground biomass production of the plant communities. Specifically, as genetic diversity within the dominant species increased, the relationship between species diversity and community-level biomass shifted from negative to positive. Negative non-additive effects of diversity drove this pattern. Synthesis.These results show, for the first time, that interactions between plant species diversity and genetic diversity within a dominant species can alter biomass production, highlighting the importance of incorporating interactions between levels of biodiversity into our understanding of how biodiversity influences ecosystem function.