Intraspecific Functional Trait Variation in Weeds: A Strategy in Response to Competition with Crop and Weed Plants

Weeds are a key component of agroecosystems and compete with crop plants. Crop-weed competition has been widely investigated over the past decades, although how functional traits mediate these competitive interactions has rarely been explored. In this study, we measured functional traits in 4400 individual plants belonging to 32 weed species, growing among four crops differing in functional traits. We assessed the intraspecific variation in weed functional traits in response to competition with crops but also with the other weeds. We found that the most abundant weeds have a lower intraspecific trait variability than other weed species for leaf dry matter content (LDMC), but this pattern was not observed for canopy height (CH) nor for specific leaf area (SLA). We found that most of the weed species tended to grow taller and have higher LDMC values when crop CH and LDMC increased. We propose that these species display a 'crop matching' strategy, tending to match crop functional trait values in response to competition. We also found that weed-weed competition affected weed functional traits, sometimes more strongly than crop-weed competition. This study highlights that intraspecific trait variation is important to consider when investigating the processes underlying weed species community assembly, especially resource competition. Further studies are required to identify the source of intraspecific trait variation in weeds and the consequences for weed fitness.