Influences of atmospheric CO₂ enrichment on the responses of sugar maple and trembling aspen to defoliation

Abstract

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.