Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2002
Publication Date: N/A
Interpretive Summary: Invasive plants are generally recognized as those species whose introduction by human transport results in economic or environmental harm. In agriculture, invasive plants out compete crops for soil and water resources, reduce crop quality and interfere with harvesting operations and reduce land values. The USDA estimates the annual productivity loss due to invasive species at 7.4 billion. At present the number one invasive species in the US is Canada thistle. Because the increase in carbon dioxide associated with global warming is also known to stimulate the growth of plants, the growth and photosynthetic response of Canada thistle was determined at carbon dioxide concentrations which existed at the beginning of the 20th century, those which exist today, and those which are anticipated to occur by the end of the 21st century. This was done in order to determine the role increasing carbon dioxide has played and will play in the growth of this species. Data from this study indicate a strong, significant response of this species not only to future carbon dioxide (+72 percent) but also to the recent increase in carbon dioxide which has already occurred during the 20th century (+125 percent). In addition to the observed stimulation in growth, carbon dioxide also increased the number and length of leafy spines, potentially altering the rate of herbivory. These data indicate that the increase in carbon dioxide associated with global warming may be an important factor in the growth and spread of this invasive weed.
Using climate controlled growth chambers, the growth, morphology and photosynthetic response of Canada thistle, a recognized invasive weed, was determined at carbon dioxide (CO2) concentrations of 285, 382 and 721 parts per million (ppm). These concentrations correspond roughly to ambient CO2 levels from 1900, 2001 and those projected for 2100. Rates of single leaf photosynthesis increased significantly with increasing CO2 with no evidence of down-regulation (acclimation) through 35 days after sowing (DAS). At 54 DAS, leaf photosynthetic response was diminished for plants grown and measured at 720 ppm relative to the 380 ppm treatment, but only during the afternoon. No evidence of acclimation was observed between the 285 and 380 ppm CO2 treatments. Significant differences in leaf morphology, particularly the number and length of leaf spines were also observed by 54 DAS, with spine number and length increasing as a function of CO2. By 54 DAS, the start of floral initiation, whole plant biomass had significantly increased by 69 percent for the elevated (720 ppm) CO2 treatment relative to current ambient CO2. However, biomass increased by 126% for the current ambient (380 ppm) relative to the concentration which existed in 1900 (285 ppm). Data from this study indicate that rising CO2 levels associated with global warming may have already been a substantial factor in the early development of Canada thistle by significantly stimulating growth and photosynthesis, as well as altering leaf defenses to reduce potential herbivory. Overall, these changes could have important implications for future competitive abilities of this species as atmospheric CO2 continues to increase.