What was done
Seedlings of one-year-old maple (Acer saccharum) and two-year old aspen (Populus tremuloides) were grown in glasshouses with atmospheric CO2 concentrations of 356 and 645 ppm for 70 days to determine the effects of elevated CO2 on photosynthesis and growth.  In addition, after 49 days of differential CO2 exposure, 50% of the leaf area was removed from half of the trees of each species in order to study the impact of simulated herbivory on the CO2 responses of these species.

What was learned
The modest 70-day CO2 enrichment treatment increased total sapling dry weight of non-defoliated maple and aspen by about 10% relative to their non-defoliated controls grown at ambient CO2.  Defoliation, however, resulted in differential CO2 responses among species and CO2 level.  Defoliation, for example, decreased final dry weights of both species at ambient CO2 concentration.  However, at elevated CO2, defoliated maples exhibited final dry weights that were equivalent to those of CO2-enriched non-defoliated controls, while defoliated aspen attained dry weights that were less than, but not significantly different from, their non-defoliated counterparts.

In expressing the data another way, it is important to note that elevated CO2 had its greatest positive impact on plant dry weight when species were stressed by simulated herbivory.  CO2-enriched defoliated maples exhibited 28% more dry weight than their defoliated counterparts grown at ambient CO2, while similarly manipulated CO2-enriched aspen displayed a smaller 11% increase in dry weight relative to their defoliated controls.

Elevated CO2 and defoliation did not affect photosynthetic rates in first-flush foliage of maple.  However, elevated CO2 caused photosynthetic down regulation in second-flush foliage of non-defoliated plants.  Interestingly, this acclimation was not apparent in defoliated plants, which actually exhibited rates of photosynthesis in second-flush foliage that were about two-fold and three-fold higher than rates observed in non-defoliated plants grown at ambient and elevated CO2 levels, respectively.  Photosynthesis was not affected by defoliation in aspen, but was enhanced by approximately 20 to 30% by atmospheric CO2 enrichment depending on leaf age.

What it means
As the CO2 content of the air rises, saplings of maple and aspen will likely respond positively by increasing their overall biomass.  Aspen, which has a relatively rapid growth rate, will likely display enhanced rates of photosynthesis, regardless of herbivory and leaf age.  Moreover, in the short-term, photosynthetic acclimation should not occur to any significant degree in this species.  Maple, on the other hand, which has a much slower relative growth rate, is more susceptible to photosynthetic acclimation.  It can, however, physiologically overcome photosynthetic acclimation when stressed by herbivory.  Such manipulations effectively increase the sink strength within plants and allow excess carbohydrate to be used in tissue construction rather than feedback to induce photosynthetic down regulation.  The observed response of maple dry weight to elevated CO2 and simulated herbivory was so dramatic the authors stated that "in future atmospheric conditions [of elevated CO2], sugar maple might be more capable of tolerating severe defoliation events which in the past have been implicated in widespread maple declines."  In addition, the phenomenal recovery of maple to herbivory under CO2-enriched conditions lead the authors to conclude that species with inherently rapid growth rates (like aspen) do not necessarily respond more favorably to CO2 enrichment than slower growing species (like maple).  Thus, species diversity may not be at as great a risk as the CO2 content of the air continues to rise.