Evaluation of the growth response of six invasive species to past, present and future atmospheric carbon dioxide.

The response of plant species to future atmospheric carbon dioxide concentrations [CO(2)] has been determined for hundreds of crop and tree species. However, no data are currently available regarding the response of invasive weedy species to past or future atmospheric [CO(2)]. In the current study, the growth of six species which are widely recognized as among the most invasive weeds in the continental United States, Canada thistle (Cirsium arvense (L.) Scop.), field bindweed (Convolvulus arvensis L.), leafy spurge (Euphorbia esula L.), perennial sowthistle (Sonchus arvensis L.), spotted knapweed (Centaurea maculosa Lam.), and yellow star thistle (Centaurea solstitialis L.) were grown from seed at either 284, 380 or 719 micromol mol(-1) [CO(2)] until the onset of sexual reproduction (i.e. the vegetative period). The CO(2) concentrations corresponded roughly to the CO(2) concentrations which existed at the beginning of the 20th century, the current [CO(2)], and the future [CO(2)] projected for the end of the 21st century, respectively. The average stimulation of plant biomass among invasive species from current to future [CO(2)] averaged 46%, with the largest response (+72%) observed for Canada thistle. However, the growth response among these species to the recent [CO(2)] increase during the 20th century was significantly higher, averaging 110%, with Canada thistle again (+180%) showing the largest response. Overall, the CO(2)-induced stimulation of growth for these species during the 20th century (285-382 micromol mol(-1)) was about 3x greater than for any species examined previously. Although additional data are needed, the current study suggests the possibility that recent increases in atmospheric CO(2) during the 20th century may have been a factor in the selection of these species.