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 CO # -enriched atmospheres. Saplings were grown for 70 d in controlled environments, wherein CO # mole fractions averaged either 356 µmol mol −" or 645 µmol mol −" , under a PPF of 500 µmol m −# s −" . On day 49 of the study, 50 % of the leaf area was removed from a subset of each species in both CO # environments. Relative growth rate () and its physiological and morphological determinants were monitored before and after defoliation. For non-defoliated saplings of both species, a slight stimulation of  (c. 5 %) in elevated CO # led to a modest increase (9-11 %) in final sapling weight. In the case of maple, the minimal growth response corresponded with minor CO # effects on specific leaf area () and leaf weight ratio (), and an apparent CO # -induced down-regulation of photosynthetic metabolism. For aspen, the CO # stimulation of photosynthesis was largely offset by a decrease in . Responses to defoliation differed markedly between species and CO # environments. Defoliation decreased maple  in ambient CO # , whereas the opposite occurred in elevated CO # . 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 CO # acclimation in that foliage. In both environments, aspen  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 . CO # enrichment might substantially enhance the ability of certain tree species to recover from herbivory. Moreover, responses to elevated CO # might be largest in the presence of stresses, such as herbivory, that decrease plant source : sink ratios.

Elevated CO 2 enhances resprouting of a tropical savanna tree

“…Other studies have that found CO 2 stimulates resprouting following defoliation (Fajer et al 1991;Wilsey et al 1994Wilsey et al , 1997Pearson and Brooks 1996) and coppicing (Will and Ceulemans 1997), but in these studies, the effect of CO 2 was not greater for clipped than unclipped individuals. In agreement with the current study, Kruger et al (1998) found the CO 2 effect to be greater for defoliated Acer saccharum. Similarly, Hättenschwiler et al (1997) found significant CO2 enhancement in coppiced Quercus ilex stands but no enhancement after the stands had matured.…”

Effects of an invasive herbivore at the single plant scale do not extend to population-scale seedling dynamics

“…We found that high levels of defoliation decreased individual seedling survival, but low levels had negligible effects. This is consistent with previous reports of sugar maple seedling growth (Ellsworth et al 1994;Kruger et al 1998) and survival (Gardescu 2003). However, we did not find evidence that mortality associated with high defoliation scales up to population-level impacts on total sugar maple seedling biomass or growth.…”

CO2 and light effects on deciduous trees: growth, foliar chemistry, and insect performance

“…We observed no response of R:S ratio to elevated CO 2 . Other studies with aspen and maple, however, have yielded dierent and sometimes con¯icting results for CO 2 eects on R:S ratios (Lindroth et al 1993;Volin and Reich 1996;Kruger et al 1998). With regard to a possible limitation of root sink strength, we observed no evidence of root restriction when roots were harvested.…”