What was done
Parasitized and non-parasitized forest tent caterpillars (Malacosoma disstria) were reared on two quaking aspen (Populus tremuloides) genotypes (216, which is O3-tolerant, and 259, which is O3-sensitive) alone and in combination at the Aspen FACE site in northern Wisconsin, USA, in plots exposed to ambient air, ambient air + 200 ppm extra CO2, ambient air + 50% extra ozone, and ambient air + 200 ppm extra CO2 and 50% extra O3 during the daylight hours of one full growing season.

What was learned
It was determined, in the words of the authors, that "elevated CO2 had little effect on both primary and secondary metabolites of aspen."  Hence, it was not surprising to learn, as they report, that "elevated CO2 had few biologically significant effects on forest tent caterpillar performance."  Elevated O3, on the other hand, altered foliar composition much more than did elevated CO2; and, as they discovered, it improved tent caterpillar performance under ambient CO2 conditions, but not in CO2-enriched air.  In addition, elevated O3 decreased the larval survivorship of the parasite of the caterpillar.

What it means
These findings suggest that the ongoing rise in the air's CO2 content will likely be very important to the future well-being of quaking aspen trees, especially when atmospheric O3 concentrations are increasing concurrently.  For one thing, elevated levels of atmospheric CO2 are known to significantly increase the productivity of aspen trees, making them more robust [see Percy et al. (2002) and Populus tremuloides in our Plant Growth Data section).  For another, the extra CO2 of this study totally thwarted the positive impact of the extra O3 on caterpillar performance, thus eliminating a major negative consequence for the trees.

As the authors describe the situation, "aspen growth is significantly increased under high CO2, but decreased under high O3," and "that difference is likely to be exacerbated by the impacts of CO2 and O3 on herbivorous insects and parasitoids as documented in this study."  Without an increase in the air's CO2 content that is commensurate with that projected for O3 over the coming half-century, therefore, quaking aspen -- the most widely distributed tree in North America -- would be in a world of hurt.

Reference
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.