> Higher concentrations of CO2 enables a higher tree line.

While this may make some intuitive sense, [CO2] actually has very little bearing on tree line. The major factors are temperature and length of the growing season. Trees need liquid water to be able to transpire and grow. The growing season also needs to be long enough that trees can harvest enough energy (sugars) from sunlight to power respiration during cold nights and long winters. If you are aware of some articles showing that [CO2] is a limiting factor at high elevations, please share them :)

Tree line is tends to be higher at lower latitudes