Response of Quaking Aspen Genotypes to Enriched Co 2 : Foliar Chemistry and Tussock Moth Performance

1 Genetic variation in the phytochemical responses of plants to CO 2 enrichment is likely to alter trophic dynamics, and to shift intraspecific selection pressures on plant populations. We evaluated the independent and interactive effects of atmospheric CO 2 and quaking aspen (Populus tremuloides Michx.) genotype on chemical composition of foliage and performance of the whitemarked tus-sock moth (Orgyia leucostigma J. E. Sm.). 2 This research was conducted at the Aspen FACE (Free Air CO 2 Enrichment) site in northern Wisconsin, U.S.A. Leaf samples were collected periodically from each of three genetically variable aspen genotypes growing under ambient and elevated CO 2 , and analysed for levels of primary and secondary meta-bolites. Tussock moth larvae were reared in situ on experimental trees, and development times and pupal masses were recorded. 3 Foliar chemical composition varied among aspen genotypes and in response to CO 2 enrichment. However, chemical responses of trees to elevated CO 2 were generally consistent across genotypes. 4 Larval development times varied among host genotypes and increased slightly for insects on high-CO 2 plants. Enriched CO 2 tended to reduce insect pupal masses, particularly for females on one of the three aspen genotypes. 5 CO 2 Â genotype interactions observed for plant chemistry and insect performance in this study with a small number of genotypes are probably too few, and too weak, to shift selection pressures in aspen populations. These results differ, however, from earlier work in which more substantial CO 2 Â genotype interactions were observed for plant chemistry.