Resource allocation to reproduction is a critical trait for plant fitness1,2. This trait, called harvest index in the agricultural context3,4,5, 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 biomass6,7,8, plant diversity effects on seed yield of crops are ambiguous9 and dependent on the production syndrome10. 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 conditions11,12,13,14. 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 intensification15, 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 complementarity16 among crop species.
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The data that support the findings of this study and the corresponding R-code are available at the public repository Zenodo (https://doi.org/10.5281/zenodo.4750856).
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This work was financially supported by the Swiss National Science Foundation (no. PPOOP3_170645 to C.S.). J.C. was supported by the China Scholarship Council. We thank C. Barriga Cabanillas, E. P. Carbonell, H. Ramos Marcos, E. Hidalgo Froilán, A. García-Astillero Honrado, R. Hüppi and S. Baumgartner for field assistance.
The authors declare no competing interests.
Peer review information Nature Plants thanks Wopke van der Werf and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Average vegetative biomass (in g m-2) of eight monocultures, 24 different 2- and 16 different 4-species mixtures planted with eight different annual crop species in 0.25 m2 plots in Switzerland and Spain. Data are mean and 95% CI. n = 762 plots. The statistical analyses (type-I analysis of variance of a linear mixed model and significance tested with the Satterthwaite approximation method) show significant effects of country [F(1,20.7) = 195.8, P = 5.23×10-12], country × diversity [F(1,677.5) = 8.1, P = 0.004], country × species number [F(1,679.2) = 5.9, P = 0.015], and marginally significant effects of diversity [F(1,45.1) = 3.7, P = 0.062] on vegetative biomass. See Extended Data Fig. 2 for seed yield and vegetative biomass data of each species in monocultures and 2- and 4-species mixtures in Switzerland and Spain.
Extended Data Fig. 2 Crop plant diversity effects on seed yield and vegetative biomass of each species.
Average seed yield (a) and vegetative biomass (b) per species (in g m-2) of eight monocultures, 24 different 2- and 16 different 4-species mixtures planted with eight different annual crop species in 0.25 m2 plots in Switzerland and Spain. Data are mean and 1 SE based on raw data. To facilitate interpretation, data were extrapolated so that the values reflect the area of 1 m2 covered by the species in the community of interest. n = 2284 species in plots for seed yield and n = 2275 species in plots for vegetative biomass.
Seed yield and vegetative biomass increases (in g m-2) compared with monocultures averaged over 24 different 2- and 16 different 4-species mixtures, respectively. For the complementarity effect (a) n = 1274 biomass partitions (seed vs vegetative) in plots, for the sampling effect (b) n = 1181 biomass partitions (seed vs vegetative) in plots. Data are mean and 95% CI. See Supporting Information Table 6 for the complete statistical analyses. In Switzerland, complementarity effects contributed 25% more than sampling effects to the net biodiversity effect on seed yield, while in Spain only sampling effects could be detected. Complementarity effects in Switzerland were 59% lower for seed yield than for vegetative biomass. Sampling effects were 70% and 83% lower for seed yield than for vegetative biomass in Spain and Switzerland, respectively.
Extended Data Fig. 4 Harvest index of crops in response to the Home vs Away, Fertilization, Country and Species number (2- vs 4-species mixtures) treatments.
Data are mean and 95% CI. n = 4751 individuals.
Extended Data Fig. 5 Harvest index of the eight crop species planted in communities of different species composition.
Species were abbreviated as: Avena sativa = Av, Triticum aestivum = Tr, Camelina sativa = Ca, Coriandrum sativum = Co, Lens culinaris = Le, Lupinus angustifolius = Lu, Linum usitatissimum = Li and Chenopodium quinoa = Qu. Data are mean and 1 SE. n = 4751 individuals.
The harvest index quantifies the proportion of reproductive biomass, that is seed yield, from total aboveground biomass produced by the Spanish cultivars in Spain and the Swiss cultivars in Switzerland (Home) and vice versa (Away). Data are mean and 95% CI. n = 4751 individuals.
Extended Data Fig. 7 The harvest index at the community level in response to plant diversity and country.
Harvest index calculated at the community-level as the ratio between plot-level seed yield and plot-level aboveground biomass (that is the sum of aboveground vegetative biomass and seed yield), irrespective of species. Data shown are mean and 95% CI. n = 762 plots. The contrast of diversity vs mixture is marginally significant as a main effect [F(1,44.9) = 3.05, P = 0.088], and significant in interaction with country [F(1,675.1) = 4.96, P = 0.026], while the contrast between 2- and 4-species mixtures is not significant. Statistical tests were done on a linear mixed effects model with type-I analysis of variance and Satterthwaite approximation.
Monoculture and mixture communities are composed of four planting rows (divided by dashed lines and coloured strips), while plots with single plants are indicated in plain colour corresponding to the crop species. Plots (0.5 × 0.5 m) are delineated with a solid black frame. Beds (20 beds of 7 × 1 m each) are delineated with either a red (= fertilised) or green (= non-fertilised control) frame. Row numbers and column letters between the beds are used to identify each plot. Letters within the plots indicate the ecotype (S = Switzerland, E = Spain) of each crop species.
Monoculture and mixture communities are composed of four planting rows (divided by dashed lines and coloured strips), while plots with single plants are indicated in plain colour corresponding to the crop species. Plots (0.5 × 0.5 m) are delineated with a solid black frame. Beds (16 beds of 10 × 1 m each) are delineated with either a red (= fertilised) or green (= non-fertilised control) frame. Row numbers and column letters between the beds are used to identify each plot. Letters within the plots indicate the ecotype (S = Switzerland, E = Spain) of each crop species.
Extended Data Fig. 10 Phenological plant development for eight crop species as single plants, in monocultures and in mixtures in Switzerland and Spain.
Boxplots show median (black dot), upper and lower quartile (box), maximum and minimum values (whiskers) and outliers (open circles). Three plant development stages for each of the eight crop species averaged over all treatments per country, per diversity level. The specific development stages are for oat: beginning of tillering, flowering (tip of inflorescence emerged from sheath), seeds with milk texture; for wheat: beginning of tillering, flowering, seeds milk ripe; for lentil: first true leaf unfolded, flowering, full seeds (seeds fill the pod cavities); lupin: stem elongation and bases of several leaves clearly separated from each other, flowering, green pods (septa split and filled with seeds); camelina: first true leaf unfolded, flowering, start of grain formation; linseed: first basal branches expanded, flowering, capsules expanded and seeds formed; coriander: first true coriander leaf unfolded, flowering, formation of green capsules; quinoa: first true leaves unfolded, flower buds in pyramids, start of grain formation.
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Chen, J., Engbersen, N., Stefan, L. et al. Diversity increases yield but reduces harvest index in crop mixtures. Nat. Plants 7, 893–898 (2021). https://doi.org/10.1038/s41477-021-00948-4