Diversity increases yield but reduces harvest index in crop mixtures.

Resource allocation to reproduction is a critical trait for plant fitness. This trait, called harvest index in the agricultural context, determines how plant biomass is converted to seed yield and consequently financial revenue from numerous major staple crops. While plant diversity has been demonstrated to increase plant biomass, plant diversity effects on seed yield of crops are ambiguous and dependent on the production syndrome. This discrepancy might be explained through changes in the proportion of resources invested in reproduction in response to changes in plant diversity, namely through changes in species interactions and microenvironmental conditions. Here, we show that increasing crop plant diversity from monocultures over two- to four-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 harvest index 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, the use of currently available cultivars may compromise larger gains in seed yield. We therefore advocate regional breeding programmes for crop varieties to be used in mixtures that should exploit complementarity among crop species.