Effects of Elevated Atmospheric Carbon Dioxide and Tropospheric Ozone on Phytochemical Composition of Trembling Aspen ( Populus tremuloides ) and Paper Birch ( Betula papyrifera ).

Anthropogenic activities are altering levels of atmospheric carbon dioxide (CO ₂ ) and tropospheric ozone (O ₃ ). These changes can alter phytochemistry, and in turn, influence ecosystem processes. We assessed the individual and combined effects of elevated CO ₂ and O ₃ on the phytochemical composition of two tree species common to early successional, northern temperate forests. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown at the Aspen FACE (Free-Air Carbon dioxide and ozone Enrichment) facility under four combinations of ambient and elevated CO ₂ and O ₃ . We measured, over three years (2006-08), the effects of CO ₂ and O ₃ on a suite of foliar traits known to influence forest functioning. Elevated CO ₂ had minimal effect on foliar nitrogen and carbohydrate levels in either tree species, and increased synthesis of condensed tannins and fiber in aspen, but not birch. Elevated O ₃ decreased nitrogen levels in both tree species and increased production of sugar, condensed tannins, fiber, and lignin in aspen, but not birch. The magnitude of responses to elevated CO ₂ and O ₃ varied seasonally for both tree species. When co-occurring, CO ₂ offset most of the changes in foliar chemistry expressed under elevated O ₃ alone. Our results suggest that levels of CO ₂ and O ₃ predicted for the mid-twenty-first century will alter the foliar chemistry of northern temperate forests with likely consequences for forest community and ecosystem dynamics.