1. Calls for the application of ecological principles in agriculture have gained momentum. Intercropping systems are designed by growing two, or more, annual crop species in the same field, aiming for a better resource use efficiency. However, assembly rules for their design are lacking. Notably, it is unknown whether species performances are maximized during both the vegetative and reproductive phases given the sensitivity of reproductive allocation rules to resource limitation. Interestingly, ecological theory provides expectations regarding putative invariance of plant reproductive allometry (PRA) under non-limiting conditions for plant growth. Here we examined whether and how PRA changes in response to plant-plant interactions in intercropping systems, which can inform both ecological theory and the understanding of the functioning of intercropping systems.

2. We analyzed a dataset of 28 field cereal-legume intercropping trials from various climatic and management conditions across Western Europe. PRA was quantified in both mixing and single-species situations.

3. PRA was positively impacted in specific management conditions, leading to a greater increase in yield for a given increase in plant size. Variations in PRA were more beneficial for legumes grown in unfertilized mixtures, which explains their use as a key component in actual intercropping systems. The response for cereals was similar but less pronounced in magnitude, and was greater under resource limiting conditions. Focusing on intercropping conditions, hierarchical competition (indicated by biomass difference between intercropped species) appears as a strong driver of the reproductive output of a given species.

4. Synthesis and applications. PRA behaves in crop species in the same way as it does in wild species. However, contrary to theoretical expectations about an overall invariance of PRA, we highlighted taxon-specific and context-dependent effects of plant-plant interactions on PRA. This systematic deviation to PRA expectations could be leveraged to cultivate each species up to its reproductive optimum while accounting for the performance of the other, whether farmer’s objective is to favor one species or to reach an equilibrium in seed production. Sowing density and cultivar choice could regulate the biomass of each component, with specific targets derived from allometric relationships, aiming for an optimal reproductive allocation in mixtures.21-Jul-2021

We collected a set of experiments that compared different species and cultivars under intercropping and sole-cropping conditions under a variety of management practices in 9 locations in five European countries (France, Denmark, Italy, Germany, and the United Kingdom). The experiments covered 28 environments (location × year), of which 15 were managed as organic farming and 13 as conventional farming, with a total of 34 intercropping situations (environment × species) and 62 sole-cropping situations.
Since the experiments were not completely factorial, i.e. not all factors (cultivars, N fertilization, sowing density) were combined, we analyzed a total of 159 and 219 experimental units under intercropping and sole-cropping situations, respectively. In the experiments, 53% and 47% of the intercropped species were winter and spring crops, respectively. The mean temperature over the crop cycle (from sowing to harvest) ranged from 6.8-11.3 °C for winter crops and 12.3-15.1 °C for spring crops. Cumulative rainfall ranged from 278-713 mm for winter crops and 60-366 mm for spring crops.

The set of experiments included annual cereal-grain legume intercrops and their corresponding sole crops, with i) barley (Hordeum vulgare L.), durum wheat (Triticum turgidum L.) and soft wheat (Triticum aestivum L.) as the cereals (only Poaceae), and ii) faba bean (Vicia faba L.) and pea (Pisum sativum L.) as the legumes (Fabaceae). The cross between crop species and cropping seasons resulted in five intercropping combinations: two spring intercrops (barley-faba bean and barley-pea) and three winter intercrops (durum wheat-faba bean, durum wheat-pea and soft wheat-pea). In all experiments, the two intercropped species were sown and harvested at the same time, with sowing dates ranging from March 11 to May 03 for spring crops and October 25 to December 15 for winter crops.

To assess reproductive effort and allometry, all experiments measured at least three variables: grain yield (t.ha -1 ) at harvest, total aboveground biomass (t.ha -1 , including grains, flowers, pods and ears) at maturity, and actual plant density (plant.m -2 ). Plant density was used to convert per-ha variables into per-capita variables (i.e. g.plant-1).