Started talking about the tree line in the Alps and looked up this page and this page.
Factors:
Depends on aspect of slope, rain, and distance to the poles (or how close it is to the equator), and in some cases vegetation…
Swiss Alps: 2100 m
Mount Kilmanjaro, Tanzania: 3000 m
Andes, South America: 3900 to 5200 m
I remember that back in school we learned that the tree line in the swiss alps is at 1800m. Klimaerwärmung?