What was done
The authors grew pre-flushed hardwood stem cuttings of the interamerican (Populus trichocarpa Torr. & Gray ex Hook. x P. deltoides Bartr. ex Marsh) hybrid poplar clone 'Boelare' out-of-doors in eight open-top chambers for two growing seasons of 132 and 186 days (first and second years, respectively), during which time they measured a number of plant properties and physiological processes. Two of the eight chambers were maintained at ambient carbon dioxide (350 ppm) and ozone concentrations (A), two at ambient CO2 with daily O3 episodes rising to a mid-day peak of 100 ppb (AO), two at elevated CO2 (700 ppm) and ambient O3 (E), and two at elevated CO2 and O3 (EO) throughout the first year of the study, while only CO2 was elevated during the second year.

What was learned
With respect to the effect of CO2 alone, Gardner et al. report that mainstem dry weight "was increased by 38% in 700-ppm CO2 compared with that in 350-ppm CO2 at the end of the first growing season," and that "during year 2 mainstem dry weight increased by about 5-fold and the relative effect of elevated CO2 remained similar in magnitude (32%) to that seen in the first year." They also report that during the first season of exposure, mainstem dry mass was decreased by 45% in the O3-episode treatment in 350-ppm CO2, but by only 34% in the O3-episode treatment in 700-ppm CO2; and because of the strong growth-promoting effect of the extra CO2, the O3-induced change in growth when going from the ambient-CO2-ambient-O3 treatment to the elevated-CO2-elevated-O3 treatment was only a reduction of 10%, as compared to the O3-induced reduction of 45% when CO2 was not increased concurrently.

What it means
The British researchers conclude that "elevated levels of CO2 can play a key role in ameliorating the worst effects of severe ozone episodes on a relatively sensitive tree species," and that "O3 episodes are less likely to be detrimental to P. trichocarpa x P. deltoides in the CO2 concentrations of the future."