What was done
Open-top chambers were constructed around two clones of silver birch (Betula pendula Roth) saplings that were rooted in the ground and growing in that manner for the past seven years at a Finnish research center.  Chambers were fumigated with air containing 360 and 720 ppm CO2 in combination with 30 and 50 ppb ozone for two growing seasons to study the competing effects of these trace atmospheric components on crown structure and growth in these two birch clones.

What was learned
Fumigation with ozone significantly decreased branching in crowns of both birch clones.  However, elevated CO2 exposure almost completely counteracted this negative response to elevated ozone.  In addition, the positive influence of elevated CO2 on shoot growth was dependent upon branch position, having a greater relative effect on shoot initiation and growth in the lower portions of sapling crowns than in higher crown positions.

What it means
As the atmospheric CO2 concentration increases, it is likely that saplings of silver birch will respond positively by increasing shoot initiation and growth in their canopies.  Moreover, crown development likely will be preferentially enhanced in the lower canopy, where incident light levels are generally lower than in the upper canopy.  Thus, overall carbon uptake by sapling canopies should be greatly enhanced as the air's CO2 content continues to rise.  In fact, this positive response will likely occur even if tropospheric ozone concentrations concurrently double with the atmospheric CO2 concentration.  Indeed, as stated by the authors, "in spite of slight differences in the reactions of the two clones, the conclusion is that O3 decreased overall ramification, whereas CO2 increased leaf initiation and shoot growth," leading them to conclude that, "in that sense, CO2 ameliorated the negative effect of ozone."