Why does some mountains have tree lines and what the trees at the tree line look like?

Tree lines occur at high elevations where environmental conditions become too harsh for tree growth, primarily due to factors like temperature, wind, and soil quality. Above this elevation, trees typically become stunted and sparse, often appearing as small, gnarled, and twisted forms known as krummholz. These adaptations help them withstand extreme weather conditions and limited nutrients. The transition from dense forests to these resilient forms marks the ecological boundary between forested areas and alpine or tundra regions.