Crop type and within-field location as sources of intraspecific variations in the phenology and the production of floral and fruit resources by weeds

Abstract

In arable farming, weeds provide important floral and seed resources that have the potential to support the provision of ecosystem services such as pollination or pest control. Estimating the production of these weed resources in the landscape is however not trivial as large-scale surveys of weed communities are usually conducted once in the season with a timing that may not coincide with the flowering and fruiting stages of all weed species. More, intraspecific variation in the mortality and phenology of individual weed species may arise from differences in the quality of the growing environment of each plant. In this study, we monitored the phenology of 30 common weed species in the field core and the field edge of 64 commercial fields grown with 6 crop types. Our hypothesis was that the production of resources by an individual plant would be modulated by its within-field location and by the crop type where it grows. We quantified floral (proportion, starting date and duration of flowering, dry biomass at flowering as a proxy for the amount of flowers) and seed resource production (proportion and starting date of fruiting). For most species, flowering and fruiting success were higher in field edges than in field cores and were lower in cereal crops than in other crops. Weeds flowered and fruited earlier and the flowering period was longer in field edges, except those of cereal crops. Dry biomass at flowering varied with field location either way, depending on the weed species, but tended to be lower in cereal crops than in other crops. This important intraspecific phenological variability in the production of seed and/or flower or resources should be considered when evaluating the contribution of weed communities to ecosystem services. It also suggests that within an agricultural landscape, the amount, timing and duration of provision of services by weeds could be enhanced by maintaining sufficient lengths of field edges and by growing a diversity of crop types.