Changes in functional diversity and intraspecific trait variability of weeds in response to crop sequences and climate

Summary How weed communities assemble represents one the key issues of weed science. For a decade, functional approaches have been applied to investigate the processes that govern weed community assembly. In most previous studies, trait values have been generally averaged over multiple populations and habitats. Consequently, conspecifics display similar trait values while neglecting the considerable influence of intraspecific variability to detect changes in functional diversity in response to environmental drivers. However, this influence has been shown to be critical, especially, at local scales. Here, we studied changes in weed functional diversity at the field scale in four crop sequences. We focused on intra- and interspecific variability of four key functional traits involved in response to resource acquisition processes, the latter being modified by climate, management and competition. The relative influence of intra- and interspecific variability among the crop sequence types was highlighted using a diversity partitioning approach. It provides evidence for substantial amount of intraspecific variability in the weed community and underlines its essential role in response to fine-scale environmental drivers. In addition, we investigated the response of the three most abundant species to competition with the crop, the growing season and the crop sequence type. We highlighted that these species showed a wide range of combinations of trait values, suggesting the co-existence of several successful strategies. Based on these results, we emphasise that neglecting intraspecific variability can lead to substantial underestimations of the functional weed response to management and crop-weed competition at the field scale.