The real question is whether there will be more biomass and more photosynthesis in response to more CO2, not whether individual trees grow more. The climate skeptics don't say tree trunks will get wider, they say there will be more plant biomass.
And also this is adding in confounding variables and saying more CO2 could change precipitation in certain places, and thus cause lower growth. Other studies that show more growth with more CO2 are using greenhouses to control for confounding variables.
And even if you are trying to answer the question in terms of a holistic analysis, you can't just study one forest in Canada and then make any kind of conclusion.
You'd need to study trees from most of the major forests, and look at the density of the foliage in each.