Impacts of defoliation on the growth and physiology of sugar maple (Acer saccharum Marsh.) and trembling aspen (Populus tremuloides Michx.) were examined in ambient and CO2-enriched atmospheres. Saplings were grown for 70 d in controlled environments, wherein CO2 mole fractions averaged either 356 μmol mol−1 or 645 μmol mol−1, under a PPF of 500 μmol m−2 s−1. On day 49 of the study, 50% of the leaf area was removed from a subset of each species in both CO2 environments. Relative growth rate (rgr) and its physiological and morphological determinants were monitored before and after defoliation. For non-defoliated saplings of both species, a slight stimulation of rgr (c. 5%) in elevated CO2 led to a modest increase (9–11%) in final sapling weight. In the case of maple, the minimal growth response corresponded with minor CO2 effects on specific leaf area (sla) and leaf weight ratio (lwr), and an apparent CO2-induced down-regulation of photosynthetic metabolism. For aspen, the CO2 stimulation of photosynthesis was largely offset by a decrease in sla. Responses to defoliation differed markedly between species and CO2 environments. Defoliation decreased maple rgr in ambient CO2, whereas the opposite occurred in elevated CO2. The latter led to complete recovery of plant weight (compensation), and was attributed to a defoliation-induced increase in carbon allocation to new leaves, along with a reversal of photosynthetic CO2 acclimation in that foliage. In both environments, aspen rgr increased after defoliation, facilitating almost full compensation. Defoliation increased light penetration into the aspen canopy, and it was estimated that the resultant stimulation of photosynthesis in lower leaves would have more than offset the concomitant decrease in lwr. CO2 enrichment might substantially enhance the ability of certain tree species to recover from herbivory. Moreover, responses to elevated CO2 might be largest in the presence of stresses, such as herbivory, that decrease plant source[ratio ]sink ratios.

Due to their different physiological effects, elevated carbon dioxide and elevated ozone might have interactive impacts on plants, and differentially so on plants differing in photosynthetic pathway and growth rate. To test several hypotheses related to these issues, we examined the physiological, morphological and growth responses of six perennial species grown at various atmospheric concentrations of carbon dioxide and ozone. The species involved (two C3 trees: Populus tremuloides Michx., Quercus rubra L.; two C3 grasses: Agropyron smithii Rybd., Koeleria cristata L.; two C4 grasses: Bouteloua curtipendula Michx., Schizachyrium scoparium Michx.) differed in growth form, stomatal conductance and photosynthetic pathway. In situ photosynthesis, relative growth rate (RGR) and its determinants (leaf area ratio, specific leaf area, leaf weight ratio and root weight ratio) were determined via sequential harvests of seedlings that were grown in all combinations of 366 or 672 μmol mol−1 CO2 and 3 or 95 nmol mol−1 O3 over a 101-d period. Elevated CO2 had minimal effect on either photosynthesis or RGR. By contrast, RGR for all six species was lower in high O3 concentrations at ambient CO2, significantly so in A. smithii and P. tremuloides. Five of the six species also exhibited reductions in in situ photosynthesis at ambient CO2 in high-O3-grown compared with low-O3-grown plants. For all species, these O3-induced reductions in RGR and photosynthesis were absent in the elevated CO2 environment. Root weight ratio was significantly reduced by elevated O3 in A. smithii and P. tremuloides in ambient but not elevated CO2. Species with high stomatal conductance were the most susceptible to oxidant injury, while those with low stomatal conductance, such as the C4 species and Q. rubra, were not as detrimentally affected by O3. Elevated levels of CO2 will reduce stomatal conductance and O3 uptake, and might therefore reduce the potential for oxidant damage. However, there was a stronger relationship of the percent reduction in whole-plant mass due to O3, related to the ratio of photosynthesis to stomatal conductance. In general, results of this study of six functionally diverse plant species suggest that O3 pollution effects on carbon balance and growth are likely to be ameliorated by elevated concentrations of atmospheric CO2.