In 1997, a research group in Wisconsin, USA, began growing five different aspen clones possessing different degrees of tolerance to O3 in 30-m diameter "Aspen FACE" plots that were maintained at atmospheric CO2 concentrations of 360 and 560 ppm with and without exposure to elevated O3 (1.5 times the ambient concentration) to study the effects of these environmental changes on growth in this important deciduous tree species.

During the first three months of the study, elevated O3 exposure caused visible foliar injuries in seedlings growing in ambient air (Wustman et al., 2001).  Seedlings exposed to elevated O3 and CO2, however, exhibited 40% fewer such injuries than those exposed to elevated O3 alone.  In addition, elevated O3 tended to increase the activities of several antioxidant enzymes, including catalase, glutathione reductase and ascorbate peroxidase; while elevated CO2 alone, and in combination with elevated O3, generally decreased the activities of these enzymes.

In a complementary study, Oksanen et al. (2001) found that O3 exposure caused significant structural injuries to thylakoid membranes and the stromal compartment within chloroplasts.  However, 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 chloroplasts were all decreased in the high O3 treatment; but in the case of these latter leaf characteristic changes, the simultaneous exposure of the O3-stressed trees to elevated CO2 more than compensated for the O3-induced reductions.

With respect to the effects of elevated CO2 alone, Takeuchi et al. (2001) report that photosynthetic rates were always greater in the CO2-enriched seedlings during the second complete growing season of the study.  In the upper parts of the trees' canopies, the stimulation was approximately 26%, while in their lower canopies it was about 3%.  Although this CO2-induced canopy-depth-dependent photosynthetic stimulation had no significant effect on tree volume in the first year of the study, it led to the CO2-enriched seedlings growing 18% taller than the ambient-treatment seedlings during the second full season of growth.

In the second and third years of the study, Isebrands et al. (2001) report that elevated CO2 increased apparent tree volume by 22 and 28%, respectively, while elevated O3 reduced it by 26% in both years.  Although these competing influences would appear to essentially cancel each other, in the treatment where both CO2 and O3 were increased together, apparent tree volumes were still 6 and 19% greater than they were in the treatment where only O3 was increased.

Last of all, after developing a competitive stress index based on the heights of the four nearest neighbors of each tree, McDonald et al. (2002) found that the relative CO2-induced growth stimulation of the young trees was greater for competitively advantaged as opposed to competitively disadvantaged trees.  In contrast, elevated O3 typically reduced aspen growth independent of competitive status.  Finally, in the combined treatment of elevated CO2 and O3 (+CO2 +O3), the competitive performance between clones and their nearest neighbors was reduced.  Commenting on this observation, the researchers say "the apparent convergence of competitive performance responses in +CO2 +O3 conditions does suggest that stand diversity may be maintained at a higher level."

In conclusion, the results of these several studies would appear to indicate that aspen trees of the future will likely be able to significantly increase their biomass while still maintaining clonal diversity in regenerating stands, even in the face of increasingly detrimental tropospheric O3 concentrations, but only, of course, if the air's ameliorative CO2 content continues to climb.

References
Isebrands, J.G., McDonald, E.P., Kruger, E., Hendrey, G., Percy, K., Pregitzer, K., Sober, J. and Karnosky, D.F.  2001.  Growth responses of Populus tremuloides clones to interacting elevated carbon dioxide and tropospheric ozone.  Environmental Pollution 115: 359-371.

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.

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.

Takeuchi, Y., Kubiske, M.E., Isebrands, J.G., Pregitzer, K.S., Hendrey, G. and Karnosky, D.F.  2001.  Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment.  Plant, Cell and Environment 24: 1257-1268.

Wustman, B.A., Oksanen, E., Karnosky, D.F., Noormets, A., Isebrands, J.G., Pregitzer, K.S., Hendrey, G.R., Sober, J. and Podila, G.K.  2001.  Effects of elevated CO2 and O3 on aspen clones varying in O3 sensitivity: Can CO2 ameliorate the harmful effects of O3?  Environmental Pollution 115: 473-481.