More diverse plant communities generally produce more biomass than monocultures. This benefit of plant diversity is supposed to stem from resource complementarity of species in mixtures. Different plant species might use the resources spatially, temporally, or chemically in different ways. Along the same lines, for agricultural production crop mixtures outperform monocultures. Complementarity in water use of crop species in mixtures could result in higher yield. Therefore, complementarity in water use could be relevant for crop production in the future when water for agriculture becomes increasingly limited.Here we used the stable isotopes of water and a Bayesian model to investigate the spatial water uptake patterns of six different crops species and how these patterns change upon increasing crop diversity. In addition, we calculated niche overlaps of water uptake and compared them among the different diversity levels.The spatial water uptake pattern differed among crop species. The effect of crop diversity had a minor effect on water uptake but varied strongly depending on the crop species. Niche overlap in water uptake was highest in monocultures and decreased strongly in mixtures. Additionally, we show that increased competition intensity caused stronger changes in water uptake patterns.Synthesis. We found evidence for niche partitioning of water uptake in crop mixtures which might lead to a more efficient use of the limited resource water in intercropping systems. Intercropping can therefore increase and stabilise crop yields under forthcoming drier climatic conditions.

More diverse plant communities are generally more productive than monocultures. This benefit of species diversity is supposed to stem from resource partitioning of species in mixtures where different species use the resources spatially, temporally, or chemically in distinct ways. With respect to water, the simultaneous cultivation of crops with distinct water uptake patterns might reduce niche overlaps and thus result in higher productivity. However, little is known about whether and how spatial water uptake patterns of crop species differ among different planting arrangements and whether these changes result in increased niche partitioning and explain overyielding in mixtures. Stable isotopes of water and a Bayesian model were used to investigate the spatial water uptake patterns of six different crop species and how these patterns change depending on the planting arrangement (monocultures vs mixtures). Niche overlaps and niche widths in spatial water uptake were compared among the different crop diversity levels and linked to productivity. Furthermore, spatial water uptake was related to competition intensity and overyielding in mixtures. We found evidence for increased niche partitioning in spatial water uptake, and therefore complementary spatial root distributions of crop species, and higher expected productivity in mixtures compared to expected productivity in monocultures both due to inherent species‐level differences in water uptake and plasticity in the water uptake pattern of species. We also found a significant relationship of competition and overyielding with observed patterns in spatial water uptake. These results suggest that competition was most intense in shallow soil layers and enhanced overyielding was related to a gradual increase of water uptake in deeper soil layers. Thus, overyielding might be related to a more complete spatial exploitation of available water sources. Synthesis. Differences in spatial water uptake and niche partitioning of intercropped species, driven most likely by a complementary spatial root distribution, might explain why mixtures outperform monocultures. These findings underpin the potential of intercropping systems for a more sustainable agriculture with a more efficient use of soil resources and hence reduced input demands.

Growing crops in more diverse crop systems (i.e. intercropping) is one way to produce food more sustainably. Even though intercropping, compared to average monocultures, is generally more productive, the full yield potential of intercropping might not yet have been achieved as modern crop cultivars are bred to be grown in monoculture. Breeding plants for more familiarity in mixtures, i.e. plants that are adapted to more diverse communities (i.e. adaptation) or even to coexist with each other (i.e. coadaptation) might have the potential to sustainably enhance productivity. In this study, the productivity benefits of familiarity through evolutionary adaptation, where one species adapts to its neighbourhood, and coevolutionary coadaptation, where two or more species adapt to each other, were disentangled in a crop system through an extensive common garden experiment. Furthermore, evolutionary and coevolutionary effects on species-level and community-level productivity were linked to corresponding changes in functional traits. We found evidence for higher productivity and trait convergence with increasing familiarity of the plants composing the community. Furthermore, our results provide evidence for coevolution of plants in mixtures leading to higher productivity of coadapted species. However, with the functional traits measured in our study we could not fully explain the productivity benefits found upon coevolution. Our study is, to our knowledge, the first study that investigated coevolution among randomly interacting plants and was able to demonstrate that coadaptation through coevolution of coexisting species in mixtures promote ecosystem functioning (i.e. higher productivity). This result is particularly relevant for the diversification of agricultural and forest ecosystems, demonstrating the added value of artificially selecting plants for the communities they are familiar with.

Background. Interactions among species are a fundamental aspect of biodiversity and driving ecosystem functioning and services. Species interactions include direct (pairwise) interactions among two species and indirect interactions that occur when a third species interacts with the two others and changes the direct interactions between the two. In a three-species interaction network, these interactions can be transitive (where one species outperforms all others) or intransitive (where each species outperforms another). How direct and indirect interactions influence ecosystem functions in crop systems, and how diversification and evolutionary adaptation can influence those interactions and therefore ecosystem functions has not been studied. Methods. A common garden experiment was conducted with crop communities in monocultures, 2- and 3-species mixtures that had either a common or no coexistence history (i.e. community adaptation) for three years. Net, direct and indirect interaction intensities were estimated and compared between the diversity levels and coexistence histories. Furthermore, species interaction networks were inspected for transitive/intransitive interactions. Results. We found evidence for lower competition in mixtures and for reduced negative direct interaction intensity and enhance facilitative effects upon community adaptation. We could further show that indirect interactions were generally less important for community adaptation than direct interactions. Additionally, we showed that community adaptation has the potential to shift interactions in the species interaction networks from competitive intransitive into pairwise competitive interactions where interactions occurred mainly between two species. Synthesis. Co-adapted crop species with reduced negative interactions might have the potential to enhance productivity especially in more diverse cropping systems. This supports the notion that intercropping is a vital part towards a more sustainable agriculture and one with further yield potential when developing cultivars adapted to grow in mixtures.

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