Karnosky et al. (1999) grew O3-sensitive and O3-tolerant aspen clones in 30-m diameter plots at the Aspen FACE site near Rhinelander, Wisconsin, USA, which were maintained at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times ambient ozone concentration).  After one year of growth at ambient CO2, elevated O3 had caused visible injury to leaves of both types of aspen, with the average percent damage in O3-sensitive clones being more than three times as great as that observed in O3-tolerant clones (55% vs. 17%, respectively).  In combination with elevated CO2, however, O3-induced damage to leaves of these same clones was only 38% and 3%, respectively.  Thus, elevated CO2 ameliorated much of the foliar damage induced by high O3 concentrations.

King et al. (2001) studied the same plants for a period of two years, concentrating on belowground growth, where elevated O3 alone had no effect on fine-root biomass.  When the two aspen clones were simultaneously exposed to elevated CO2 and O3, however, there was an approximate 66% increase in the fine-root biomass of both of them.

Also in the same experiment, Noormets et al. (2001) studied the interactive effects of O3 and CO2 on photosynthesis, finding that elevated CO2 increased rates of photosynthesis in both clones at all leaf positions.  Maximum rates of photosynthesis were increased in the O3-tolerant clone by averages of 33 and 49% due to elevated CO2 alone and in combination with elevated O3, respectively, while in the O3-sensitive clone they were increased by 38% in both situations.  Hence, CO2-induced increases in maximal rates of net photosynthesis were typically maintained, and sometimes even increased, during simultaneous exposure to elevated O3.

Yet again in the same experiment, Oksanen et al. (2001) reported that after three years of treatment, ozone exposure caused significant structural injuries to thylakoid membranes and the stromal compartment within chloroplasts, but that these injuries were largely ameliorated by atmospheric CO2 enrichment.  Likewise, leaf thickness, mesophyll tissue thickness, the amount of chloroplasts per unit cell area, and the amount of starch in leaf chloroplasts were all decreased in the high ozone treatment; but in the case of these leaf properties, simultaneous exposure of the ozone-stressed trees to elevated CO2 more than compensated for the ozone-induced reductions.

After four years of growing five aspen clones with varying degrees of tolerance to ozone under the same experimental conditions, McDonald et al. (2002) developed what they termed a "competitive stress index," based on the heights of the four nearest neighbors of each tree, to study the influence of competition on the CO2 growth response of the various clones as modified by ozone.  In general, elevated O3 reduced aspen growth independent of competitive status, while the authors noted an "apparent convergence of competitive performance responses in +CO2 +O3 conditions," which they say suggests that "stand diversity may be maintained at a higher level" in such circumstances.

Percy et al. (2002) utilized the same experimental setting to assess a number of the trees' growth characteristics, as well as the responses of one plant pathogen and two insects with different feeding strategies that typically attack the trees.  Of the plant pathogen studied, they say that "the poplar leaf rust, Melampsora medusae, is common on aspen and belongs to the most widely occurring group of foliage diseases."  As for the two insects, they report that "the forest tent caterpillar, Malacosoma disstria, is a common leaf-chewing lepidopteran in North American hardwood forests" and that "the sap-feeding aphid, Chaitophorus stevensis, infests aspen throughout its range."  Hence, the rust and the two insect pests the scientists studied are widespread and have significant deleterious impacts on trembling aspen and other tree species.  As but one example of this fact, the authors note that, "historically, the forest tent caterpillar has defoliated more deciduous forest than any other insect in North America" and that "outbreaks can reduce timber yield up to 90% in one year, and increase tree vulnerability to disease and environmental stress."

By itself, Percy et al. found that elevated O3 decreased tree height and trunk diameter, increased rust occurrence by nearly fourfold, improved tent caterpillar performance by increasing female pupal mass by 31%, and had a strong negative effect on the natural enemies of aphids.  The addition of the extra CO2, however, completely ameliorated the negative effects of elevated O3 on tree height and trunk diameter, reduced the O3-induced enhancement of rust development from nearly fourfold to just over twofold, completely ameliorated the enhancement of female tent caterpillar pupal mass caused by elevated O3, and completely ameliorated the reduction in the abundance of natural enemies of aphids caused by elevated O3.

In a final study from the Aspen FACE site, Holton et al. (2003) raised parasitized and non-parasitized forest tent caterpillars on two quaking aspen genotypes (O3-sensitive and O3-tolerant) alone and in combination for one full growing season; and they too found that elevated O3 improved tent caterpillar performance under ambient CO2 conditions, but not in CO2-enriched air.

In summary, it is clear that elevated ozone concentrations have a number of significant negative impacts on the well-being of North America's most widely distributed tree species, while elevated carbon dioxide concentrations have a number of significant positive impacts.  In addition, elevated CO2 often completely eliminates the negative impacts of elevated O3.  Hence, if the tropospheric O3 concentration continues to rise as expected (Percy et al. note that "damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100"), we had better hope that the air's CO2 content continues to rise as well.  If it doesn't, aspen trees will be in a world of hurt; for Percy et al. additionally report that "since the mid-1990s, aspen use has increased almost exponentially in the US Great Lakes Region and in the boreal mixed wood region of Canada, where the resource is now almost fully utilized."

References
Holton, M.K., Lindroth, R.L. and Nordheim, E.V.  2003.  Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO2, O3, and plant genotype.  Oecologia 137: 233-244.

Karnosky, D.F., Mankovska, B., Percy, K., Dickson, R.E., Podila, G.K., Sober, J., Noormets, A., Hendrey, G., Coleman, M.D., Kubiske, M., Pregitzer, K.S. and Isebrands, J.G.  1999.  Effects of tropospheric O3 on trembling aspen and interaction with CO2: results from an O3-gradient and a FACE experiment.  Water, Air, and Soil Pollution 116: 311-322.

King, J.S., Pregitzer, K.S., Zak, D.R., Sober, J., Isebrands, J.G., Dickson, R.E., Hendrey, G.R. and Karnosky, D.F.  2001.  Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric CO2 and tropospheric O3.  Oecologia 128: 237-250.

McDonald, E.P., Kruger, E.L., Riemenschneider, D.E. and Isebrands, J.G.  2002.  Competitive status influences tree-growth responses to elevated CO2 and O3 in aggrading aspen stands.  Functional Ecology 16: 792-801.

Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F.  2001.  Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3.  Plant, Cell and Environment 24: 327-336.

Oksanen, E., Sober, J. and Karnosky, D.F.  2001.  Impacts of elevated CO2 and/or O3 on leaf ultrastructure of aspen (Populus tremuloides) and birch (Betula papyrifera) in the Aspen FACE experiment.  Environmental Pollution 115: 437-446.

Percy, K.E., Awmack, C.S., Lindroth, R.L., Kubiske, M.E., Kopper, B.J., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Dickson, R.E., Zak, D.R., Oksanen, E., Sober, J., Harrington, R. and Karnosky, D.F.  2002.  Altered performance of forest pests under atmospheres enriched by CO2 and O3.  Nature 420: 403-407.