I’m fairly certain “tree lines” are due to average temperature, not CO2.
It is true that the thinner atmosphere is also colder at elevations, it heres an example of what you can easily find if you search:
At high altitude trees dont grow (tree line), but grass and heather does. That is not because of the lower level of Oxygen (H2O), or lower temperature level, as we probably all learn in school, its because of the low level of Carbon Dioxide (CO2).