Reference
Kostiainen, K., Kaakinen, S., Warsta, E., Kubiske, M.E., Nelson, N.D., Sober, J., Karnosky, D.F., Saranpaa, P. and Vapaavuori, E. 2008. Wood properties of trembling aspen and paper birch after 5 years of exposure to elevated concentrations of CO2 and O3. Tree Physiology 28: 805-813.
What was done
The authors investigated the interactive effects of elevated concentrations of carbon dioxide (CO2) and ozone (O3) on radial growth, wood chemistry and structure of five 5-year-old trembling aspen (Populus tremuloides Michx.) clones and the wood chemistry of paper birch (Betula papyrifera Marsh.) saplings at the Aspen free-air CO2 enrichment (FACE) facility in Rhinelander, Wisconsin (USA), where the saplings had been exposed to four treatments -- control, elevated CO2 (560 ppm), elevated O3 (1.5 x ambient) and their combination -- for five growing seasons.
What was learned
Kostiainen et al. report that "elevated CO2 in the presence of ambient O3 tended to increase, and elevated O3 in the presence of ambient CO2 tended to decrease, stem radial growth in aspen after a 5-year exposure, whereas stem radial growth of trees in the combined elevated CO2 + O3 treatment did not differ from controls." In addition, they say that none of the structural variables of the aspen wood were affected by the elevated CO2 treatment, but that elevated O3 tended to decrease vessel lumen diameter.
With respect to the paper birch saplings, which also exhibited "a tendency for increased stem diameter in elevated CO2," they write that "elevated CO2 caused ... an increase in extractives" -- such as fats, waxes, triterpenoids and steroids -- "which have important roles in defense against pathogens and other biotic attacks." And here, too, they found that elevated O3 tended to decrease vessel lumen diameter.
What it means
The nine researchers conclude that the increased growth they observed in response to elevated CO2 "can be foreseen to shorten rotation lengths, with only moderate changes in wood properties," which is good. On the other hand, they say that "in response to elevated O3, stem wood production decreased and was accompanied by changes in vessel properties, which may indicate decreasing efficiency of water and nutrient transport," which is not good. Hence, it is fortunate indeed that the major negative effects of the elevated O3 concentration were negated by the positive effects of the elevated CO2 concentration.