The relationship between plant diversity and total plant biomass is of great current interest in ecology. Niche differences among plants are widely thought to promote both plant coexistence, and higher biomass in more diverse polycultures. Using simple mechanistic models, I demonstrate that not all niche differences among plants are equally likely to promote high total biomass in polyculture. In particular, transgressive overyielding (when a polyculture of plants outperforms any monoculture) occurs for a wide range of parameter values when plants coexist in polyculture due to differences in resource use. Transgressive overyielding occurs only for a limited set of parameter values when specialist herbivores mediate plant coexistence, and is impossible when generalist herbivores mediate coexistence. Niche differences among plants promote high biomass in polyculture only when plants coexist in polyculture at the expense of other trophic levels – that is, by converting into biomass resources that would otherwise be bound in herbivores, or exist in a free state. A major challenge for future work will be to identify the joint consequences of different coexistence mechanisms for plant diversity and ecosystem performance.