What was done
The authors studied the effects of elevated carbon dioxide (CO2) and ozone (O3) on various wood properties of two initially seven-year-old fast-growing silver birch (Betula pendula Roth) clones that were grown out-of-doors at Suonenjoki, Finland, for three additional years in open-top chambers maintained at ambient and 1.9x ambient CO2 concentrations in combination with ambient and 1.5x ambient O3 concentrations.

What was learned
The elevated CO2 treatment had no effects on wood structure; but it increased annual ring width by 21%, woody biomass by 23% and trunk starch concentration by 7%. Elevated O3, on the other hand, decreased stem vessel percentage in one of the clones by 10%; but it had no effect on vessel percentage in the presence of elevated CO2.

What it means
Kostiainen et al. note that "in the xylem of angiosperms, water movement occurs principally in vessels (Kozlowski and Pallardy, 1997)," and that "the observed decrease in vessel percentage by elevated O3 may affect water transport," obviously lowering it. "However," as they continue, "elevated CO2 ameliorated the O3-induced decrease in vessel percentage."

The Finnish researchers also note that "the concentration of nonstructural carbohydrates (starch and soluble sugars) in tree tissues is considered a measure of carbon shortage or surplus for growth (Korner, 2003)." Hence, they say that "starch accumulation observed under elevated CO2 in this study indicates a surplus of carbohydrates produced by enhanced photosynthesis of the same trees (Riikonen et al., 2004)." In addition, they report that "during winter, starch reserves in the stem are gradually transformed to soluble carbohydrates involved in freezing tolerance (Bertrand et al., 1999; Piispanen and Saranpaa, 2001), so the increase in starch concentration may improve acclimation in winter."

Considering these several responses, it can be appreciated that the ongoing rise in the air's CO2 content should be a boon to silver birch (and likely many other trees) in both summer and winter in both pristine and ozone-polluted air.

References
Bertrand, A., Robitaille, G., Nadeau, P. and Castonguay, Y. 1999. Influence of ozone on cold acclimation in sugar maple seedlings. Tree Physiology 19: 527-534.

Korner, C. 2003. Carbon limitation in trees. Journal of Ecology 91: 4-17.

Kozlowski, T.T. and Pallardy, S.G. 1997. Physiology of Woody Plants. Academic Press, San Diego, CA, USA.

Piispanen, R. and Saranpaa, P. 2001. Variation of non-structural carbohydrates in silver birch (Betula pendula Roth) wood. Trees 15: 444-451.

Riikonen, J., Lindsberg, M.-M., Holopainen, T., Oksanen, E., Lappi, J., Peltonen, P. and Vapaavuori, E. 2004. Silver birch and climate change: variable growth and carbon allocation responses to elevated concentrations of carbon dioxide and ozone. Tree Physiology 24: 1227-1237.