The tree line is the natural boundary beyond which trees cannot grow because conditions are too cold, dry, windy, or otherwise harsh.
Tree lines occur at high elevations on mountains and at high latitudes near the Arctic and Antarctic regions. The exact boundary varies with temperature, moisture, wind, slope, soil, and local geography. It is therefore not a perfectly sharp line everywhere; scattered, stunted trees may appear near the transition.
At high elevations, the tree line is often called the timberline or alpine tree line. Beyond it, vegetation may consist of shrubs, grasses, mosses, and other low-growing plants. Trees near the boundary are frequently short, twisted, or shaped by persistent wind and snow.
Tree line is not identical to the edge of a forest cleared by people. A cleared field may have no trees because of land use, while a natural tree line reflects environmental limits. Climate warming can shift some tree lines upward or poleward, although local factors affect the response.