Weed flower dilemma: competitiveness with few costly flowers or ruderality with numerous cheap flowers?

Weed species are ecological models that recently received considerable attention due to their particular strategies linked to their ruderal-competitive traits. They are known to have the potential to provide additional floral resources for insects in flower-poor agroecosystems. However, their floral traits are much more scarcely studied than those of plants found in other habitats, such as grasslands. The aim of this study was to describe the floral phenotype of weeds and to determine to what extent their floral traits match their ecological strategies as described on the basis of leaf traits. We therefore cultivated 19 forb weeds from perennial agroecosystems, previously identified in Mediterranean fields, in a greenhouse for seven months and collected data on 12 floral and 5 leaf traits. We tested whether these traits covaried and whether they exhibited an ecological strategy at the phenotype scale. We found that in matters of flower production, weed species face a trade-off: either numerous small, low-stature flowers with small quantities of pollen and nectar, or few, large, higher-held flowers with more pollen and nectar. The floral traits were found to reflect Grime’s CSR strategies: the weed species producing fewer but costlier flowers belonged to C-strategy species, whereas those producing more but less costly flowers belonged to species dominated by an R strategy These findings indicate that the potential of weeds as floral resources for insects is related to their ecological strategies, which are known to be affected by agricultural practices that filter species composition. This implies that, as for the provision of other ecosystem services, weed communities can be managed so as to select species with interesting floral traits for pollinators.