INFLUENCES OF ATMOSPHERIC CO2 ENRICHMENT ON THE RESPONSES OF SUGAR MAPLE AND TREMBLING ASPEN TO DEFOLIATION

Impacts of defoliation on the growth and physiology of sugar maple (Ac ev saccharum Marsh.) and trembling aspen (Populus tremuloides Michx.) were examined in ambient and CO2-enriched atmospheres. Saplings were g rown for 70 d in controlled environments, wherein CO2 mole fractions a veraged either 356 mu mol mol(-1) or 645 mu mol mol(-1), under a PPF o f 500 mu mol m(-2) s(-1). On day 49 of the study, 50 % of the leaf are a was removed from a subset of each species in both CO2 environments. Relative growth rate (RGR) and its physiological and morphological det erminants were monitored before and after defoliation. For non-defolia ted 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 (L WR), and an apparent CO2-induced down-regulation of photosynthetic met abolism. For aspen, the CO2 stimulation of photosynthesis was largely offset by a decrease in SLA. Responses to defoliation differed markedl y between species and CO2 environments. Defoliation decreased maple RG R in ambient CO2, whereas the opposite occurred in elevated CO2. The l atter 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 defolia tion, facilitating almost full compensation. Defoliation increased lig ht penetration into the aspen canopy, and it was estimated that the re sultant stimulation of photosynthesis in lower leaves would have more than offset the concomitant decrease in LWR. CO2 enrichment might subs tantially 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:s ink ratios.