Ever wonder why there is a tree line on mountains, above which there are no trees? Its not temperature related.
As Blueflag points out, CO2 mixes with other gases in the and does not settle out at lower elevations. If the latter were true, how would you explain the lack of trees at near sea level in the arctic? Temperature is the controlling variable there though length of day, wind conditions and lack of moisture also impact wood growth. In mountain areas in addition to temperature, snow pack duration and wind also minimize tree growth.