The tree line, or timberline, is the elevation limit above which trees cannot grow due to harsh environmental conditions.

In the Rocky Mountains, the tree line (also called the timberline) represents the highest elevation at which trees can survive and grow. Above this line, conditions become too harsh, with factors like low temperatures, strong winds, short growing seasons, and limited soil development preventing tree growth. The landscape above the tree line is typically characterized by alpine tundra vegetation, such as grasses, shrubs, and wildflowers.