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Relatively little is known about the implications of atmospheric CO 2  enrichment for tree responses to biotic disturbances such as folivory. We examined the combined effects of elevated CO 2  concentration ([CO 2 ]) and defoliation on growth and physiology of sugar maple ( Acer saccharum  Marsh.) and trembling aspen ( Populus tremuloides  Michx.). Seedlings were planted in the ground in eight open-top chambers. Four chambers were ventilated with CO 2 -enriched air (ambient + 283 μmol mol −1 ) and four chambers were supplied with ambient air. After 6 weeks of growth, half of the leaf area was removed on a subset of seedlings of each species in each CO 2  treatment. We monitored subsequent biomass gain and allocation, along with leaf gas exchange and chemistry. Defoliation did not significantly affect final seedling biomass in either species or CO 2  treatment. Growth recovery following defoliation was associated with increased allocation to leaf mass in maple and a slight enhancement of mean photosynthesis in aspen. Elevated [CO 2 ] did not significantly affect aspen growth, and the observed stimulation of maple growth was significant only in mid-season. Correspondingly, simulated responses of whole-tree photosynthesis to elevated [CO 2 ] were constrained by a decrease in photosynthetic capacity in maple, and were partially offset by reductions in specific leaf area and biomass allocation to foliage in aspen. There was a significant interaction between [CO 2 ] and defoliation on only a few of the measured traits. Thus, the data do not support the hypothesis that atmospheric CO 2  enrichment will substantially alter tree responses to folivory. However, our findings do provide further indication that regeneration-stage growth rates of certain temperate tree species may respond only moderately to a near doubling of atmospheric [CO 2 ].

Responses of deciduous broadleaf trees to defoliation in a CO 2 enriched atmosphere