Background/Question/Methods I tested whether elevated CO₂ differentially affected which genotypes of the apomictic species Taraxacum officinale produced the largest number of viable seeds in two different field experiments, and identified morphological and physiological traits associated with fitness at elevated CO₂.  In one experiment, the T. officinale plants which persisted as weeds and were producing seeds three years after the establishment of alfalfa plots in open top chambers at ambient and elevated CO₂ were compared.  In a second experiment, T. officinale seeds collected from four diverse local habitats were mixed and scattered on bare soil in open top chambers at ambient and elevated CO₂, and plants producing viable seeds one and two years after seeding were compared.  In this second experiment the chambers were maintained as monocultures of T. officinale.  In both experiments all of the seeds produced in each chamber during the flowering season were collected, and many plants from the seed lot from each chamber were grown in controlled environment chambers to test whether the CO₂ of the chamber of origin affected the mean value of various morphological and physiological parameters. 

Results/Conclusions

In both experiments, the CO₂ of the chamber significantly affected the mean characteristics of the plants grown from seeds produced in that chamber, indicating that elevated CO₂ altered the relative fitness of genotypes.  In both experiments elevated CO₂ favored genotypes which produced biomass more rapidly at elevated CO₂, primarily because of faster rates of leaf initiation rather than because of differences in seed size or leaf CO₂ assimilation rates.  When grown at elevated CO₂, mean shoot dry mass at 35 days after planting averaged 20 to 50% higher for seed lots from elevated than from ambient CO₂ chambers, depending on the experiment and the year. Assaying the seed lots at ambient rather than elevated CO₂ eliminated most effects of the CO₂ of the chamber of origin in both experiments. This indicates that traits adapting genotypes to elevated CO₂ may not be evident when plants are grown at lower CO₂. Overall, the results suggest that genotypes of this species vary widely in fitness at elevated CO₂ whether grown in monocultures or in mixed communities, and that this species could adapt rapidly to rising atmospheric CO₂.