Reference
Calfapietra, C., Scarascia-Mugnozza, G., Karnosky, D.F., Loreto, F. and Sharkey, T.D. 2008. Isoprene emission rates under elevated CO2 and O3 in two field-grown aspen clones differing in their sensitivity to O3. New Phytologist 179: 55-61.
What was done
The authors measured emissions of isoprene -- C5H8 or 2-methyl-1,3-butadiene, which is a highly reactive non-methane hydrocarbon emitted by vegetation that is responsible for the production of vast amounts of harmful tropospheric O3 or ozone -- from sun-exposed upper-canopy leaves of an O3-tolerant clone and an O3-sensitive clone of trembling aspen (Populus tremuloides Michx.) trees at the Aspen FACE facility near Rhinelander, Wisconsin, USA, during the midday hours (10:00-16:00) of every sunny day of the period 26 June to 12 July 2006, where the trees were growing in either normal ambient air, air enriched with an extra 190-200 ppm CO2, air with 1.5 times the normal ozone concentration, or air simultaneously enriched with the identical concentrations of both of these atmospheric trace gases.
What was learned
For the trees growing in air of ambient ozone concentration, the extra 190 ppm of CO2 decreased the mean isoprene emission rate by 11.7% in the O3-tolerant aspen clone and by 22.7% in the O3-sensitive clone, while for the trees growing in air with 1.5 times the ambient ozone concentration, the extra CO2 also decreased the mean isoprene emission rate by 10.4% in the O3-tolerant clone and by 32.7% in the O3-sensitive clone. At the same time, and in the same order, net photosynthesis rates were increased by 34.9%, 47.4%, 31.6% and 18.9%.
What it means
Since it has been calculated by Poisson et al. (2000) that current levels of non-methane hydrocarbon emissions -- the vast majority of which are isoprene -- may increase surface ozone concentrations by up to 40% in the marine boundary-layer and 50-60% over land, and that the current tropospheric ozone concentration extends the atmospheric lifetime of methane (one of the world's most powerful greenhouse gases) by approximately 14%, it can be appreciated that the CO2-induced reduction of isoprene emissions from aspen trees is something to be highly desired, as is the CO2-induced increase in photosynthesis.
Reference
Poisson, N., Kanakidou, M. and Crutzen, P.J. 2000. Impact of non-methane hydrocarbons on tropospheric chemistry and the oxidizing power of the global troposphere: 3-dimensional modeling results. Journal of Atmospheric Chemistry 36: 157-230.