Why is there a tree line on mountains?

Question

Why is there a tree line on mountains? I have a feeling it's because of low oxygen but could somebody tell me exactly why?

Answer 1

Temperature ,shallow soils ,

Answer 2

Yep, trees and plants convert carbon dioxide into energy during the day via photsynthesis, whilst a byproduct of this is oxygen, at night plants are not converting energy and so take in oxygen to make use of the energy they have stored.

They use oxygen to burn energy, the same way we do.

Answer 3

Temperature. Above the tree line the temperatures are too low for the tree seeds to germinate.

Answer 4

that is correct.