Why do mountains have tree lines?

Answer: Temperature too cold for trees

Animals prevent tree growth — Wrong. While grazing animals can affect vegetation, tree lines are determined by climate (temperature), not animal activity.

Temperature too cold for trees ✓ — Correct! Tree lines mark the elevation where it's too cold for trees to grow. Above this altitude (~11,000 ft in temperate zones), low temperatures, short growing seasons, and frost prevent tree survival. Only alpine plants (grasses, shrubs) survive. The exact elevation varies by latitude—closer to sea level near poles, higher at equator!

Wind blows away all seeds — Wrong. High winds do stress trees at altitude (causing krummholz—twisted dwarf trees), but temperature is the main limiting factor for tree lines.