Skin effects and topography can create microclimates that alter the general cooling trend. The rate of decrease can vary in different mountain chains, from 3.5 ☏ (1.9 ☌) per 1,000 feet (300 m) of elevation gain in the dry mountains of the western United States, to 1.4 ☏ (0.78 ☌) per 1,000 feet (300 m) in the moister mountains of the eastern United States. The decrease in air temperature with increasing elevation creates the alpine climate. Environmentally dwarfed shrubs ( krummholz) commonly form the upper limit. This zone of transition occurs "near the top of the tallest peaks in the northeastern United States, high up on the giant volcanoes in central Mexico, and on mountains in each of the 11 western states and throughout much of Canada and Alaska". The alpine tree line boundary is seldom abrupt: it usually forms a transition zone between closed forest below and treeless alpine tundra above. : 109 In the southern hemisphere, the south-facing slopes have the shorter growing season. This shortens the growing season for trees.
Treelines on north-facing slopes in the northern hemisphere are lower than on south-facing slopes, because the increased shade on north-facing slopes means the snowpack takes longer to melt. : 151 The climate above the tree line of mountains is called an alpine climate, : 21 and the terrain can be described as alpine tundra. : 55Īn alpine tree line is the highest elevation that sustains trees higher up it is too cold, or the snow cover lasts for too much of the year, to sustain trees. Trees grow shorter and often at lower densities as they approach the tree line, above which they cease to exist. The tree line often appears well-defined, but it can be a more gradual transition. This is sometimes known as krummholz (German for "crooked wood"). : 151 : 18Īt the tree line, tree growth is often sparse, stunted, and deformed by wind and cold. : 51 The tree line is sometimes distinguished from a lower timberline or forest line, which is the line below which trees form a forest with a closed canopy. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures, extreme snowpack, or associated lack of available moisture). It is found at high elevations and high latitudes. The tree line is the edge of the habitat at which trees are capable of growing. These trees are stunted in growth and one-sided because of cold and constant wind. The foreground shows the transition from trees to no trees. In this view of an alpine tree line, the distant line looks particularly sharp.