Growth and reproductive output of Xanthium strumarium grown at four sites varying in the extent of urbanization

Environmental changes associated with urbanization may alter growth and reproductive allocation of plants, which may in turn alter the invasive potential of invasive plants. We examined growth and reproductive allocation of an annual, invasive plant, Xanthium strumarium (common cocklebur), at four sites that differed in the extent of urbanization. Plants were grown in pots with homologous soil in un-shaded exclosures, and received both natural and supplemental water. The number of main stem leaves, flowers, fruit and branches, and height, were recorded for each plant once each week. Seed mass and number, allocation to reproduction, and total plant mass were measured after complete senescence. All measured variables were significantly lower for plants grown in the most rural site relative to plants in the more urban sites. Further, with the exception of number of branches, measured variables for the plants in the two suburban sites had significantly lower values relative to those in the most urban site. In general, differences in total plant mass accounted for differences among sites in other growth and reproductive parameters. Phenologic differences among the sites, based on the number of main stem leaves, did not account for differences among sites in growth and reproductive output. Further, all plants flowered at similar ages. The increased reproductive output in the more urbanized sites is probably the result of increased dry biomass production rather than due to accelerated development, suggesting that site differences in reproductive output were size driven rather than stage or age driven.