Abstract Resource allocation to reproduction is a critical trait for plant fitness 1,2 . This trait, called harvest index in the agricultural context 3–5 , determines how plant biomass is converted to seed yield and consequently financial revenue of numerous major staple crops. While plant diversity has been demonstrated to increase plant biomass 6–8 , plant diversity effects on seed yield of crops are ambiguous 9 . This discrepancy could be explained through changes in the proportion of resources invested into reproduction in response to changes in plant diversity, namely through changes of species interactions and microenvironmental conditions 10–13 . Here we show that increasing crop plant diversity from monoculture over 2- to 4-species mixtures increased annual primary productivity, resulting in overall higher plant biomass and, to a lesser extent, higher seed yield in mixtures compared with monocultures. The difference between the two responses to diversity was due to a reduced reproductive effort of the eight tested crop species in mixtures, possibly because their common cultivars have been bred for maximum performance in monoculture. While crop diversification provides a sustainable measure of agricultural intensification 14 , the use of currently available cultivars may compromise larger gains in seed yield. We therefore advocate regional breeding programs for crop varieties to be used in mixtures that should exploit facilitative interactions 15 among crop species.

Diversity increases yield but reduces reproductive effort in crop mixtures