The interspecific and intraspecific variation of functional traits in weeds: diversified ecological strategies within arable fields

Arable weeds are a key component of the biodiversity of agroecosystems, but have faced a marked decline due to agricultural intensification. Recently, the crop edge has been considered as a potential refugia for many species. Indeed, weed species richness and abundance are higher in the crop edge than in the field margin and the field core. In this study we question whether weed functional diversity also varies among field elements and whether it is higher in the crop edge. We studied the interspecific and intraspecific variation of three functional traits (specific leaf area, canopy height and above-ground biomass) related to the response of weeds to competition and to agricultural practices, for seven weed species sampled in the crop edge, the field margin and the field core area in four winter-wheat fields. We show that trait values varied significantly with the species, the field element and their interaction. Within the field, all species had high specific leaf area, low canopy height and biomass, suggesting a shade-tolerance syndrome that could be a strategy in response to both competition with the crop and the disturbances induced by agricultural practices. In the crop edge, where the functional variation was the highest, two distinct functional strategies were observed, suggesting a resource partitioning under the predominance of weed-weed competition. In conclusion, the crop edge plays a key role in sustaining weed diversity, mostly because of its intermediate environmental properties that allow the coexistence of weeds with different ecological strategies.