A tree line or timberline is the line at which trees stop growing. The best-known tree lines include the alpine tree line, polar tree line, desert tree line, and exposure tree line, though there are others. On mountains, the tree line begins anywhere between 2,600 ft (800 m) and 17,000 ft (5,200 m) above sea level, though between 7,000 ft (2,100 m) and 10,000 ft (3,000 m) is most typical. The lowest alpine tree lines are in places where it is already very cold and challenging for trees to grow, such as northern Sweden and Norway, and the highest alpine tree line is in the Bolivian Andes, where hardy trees grow up to 17,000 ft (5,200 m) above sea level.

A suite of complex factors determine the altitude of the alpine tree line. These include ambient temperature, local species, and degree of exposure. Many mountains have lower tree lines on south-facing slopes, because these receive less sun and are therefore colder and less hospitable. Usually, the trees that make it to the highest altitudes are conifers, especially various species of pine, as these are best adapted to cold conditions. While alpine forests may host a variety of animal species, the biodiversity tends to drop above the tree line, due to the lack of food and places to hide from predators. Some animals do indeed live above the tree line, however, eating small shrubs. These include the Mountain Goat, Alpine Ibex, Bighorn Sheep, and various rodents and birds including the Golden Eagle.

The Arctic and Antarctic tree lines occur in areas that are too far north or south to have ground suitable for tree growth. Generally this is around 70 degrees from the poles, but it can be as close as 52 degrees from the poles depending on climate. In Eurasia, the tree line varies between 66 and 72 degrees north, meaning that only the northern tips of Norway, Sweden, Finland, and Russia have a tree line. The most northerly Arctic tree line is in the Central Siberian Plateau, where the influence of an extreme continental climate heats up the ground, and the most southerly Arctic tree line is in Quebec, where the extremely cold Hudson Bay discourages tree growth. Few continental masses reach very far south, making it difficult to delineate the Antarctic tree line. Most of the southernmost areas of Tierra del Fuego in South America are barren, except for Hoste Island, located at 55 degrees south, the home of the world's southernmost trees.