Forest researchers from Michigan (working in Wisconsin) published on the results of a 12 year study evaluating the effects of elevated carbon dioxide and ozone on tree growth (story here). Previous thinking assumed that althought he elevated CO2 would increase tree growth, that soil nitrogen would become limiting and the growth enhancement would be truncated. The MI study found elevated growth even in the final years of the 12 year study. Although soil nitrogen is thought to eventually become limiting, the onset of the constraints isn't known. Presumably, the onset would depend on the tree species, tree age, and soil.
For those interested in this subject, the link above has other related stories.