Many weedy and invasive species are renowned for having substantial genetic and/or functional trait diversity, but how that variation is partitioned within and among populations is often unknown. We assessed trait variation in agricultural and natural riparian populations of the economically important agricultural weed giant ragweed (Ambrosia trifida). Although native to North America, giant ragweed has been expanding its weedy range north and west into the Great Plains from the Eastern Corn Belt. Previous studies have found substantial variation in seed traits within and among giant ragweed populations that may be adaptive, including earlier emergence in plants from successional habitats relative to agricultural sites. Thus we hypothesized that seeds from riparian populations would emerge faster than seeds from agricultural populations and that the resulting riparian plants would be larger and have higher relative growth rates. Using seeds from fourteen paired riparian and agricultural populations collected from throughout the Midwest, we quantified multiple traits that have implications for population spread and performance. We are also assessing seed morphology and germination traits from these greenhouse-grown plants to minimize the influence of maternal effects on our responses of interest.
Results/Conclusions
Our giant ragweed populations were highly variable, with often idiosyncratic habitat differences in functional traits across our study regions. Seedlings from riparian habitats emerged earlier than seedlings from agricultural sites on average, leading to plants that were slightly larger at maturity. Plants from sites farthest west produced more and smaller achenes, consistent with the hypothesis that individuals closer to the leading edge of the invasion front may experience selection for producing large numbers of readily dispersed seeds. Smaller achenes also tended to have less ornamentation, which may limit burial in the soil relative to more highly ornamented achenes. Previous work has identified a positive correlation between seed size and the degree of dormancy, thus we hypothesize that these smaller, less ornamented achenes produced by plants near the invasion front are also less likely to lay dormant in the soil relative to populations near the core of giant ragweed’s distribution.