An 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.^[2]:151 The climate above the tree line of mountains is called an alpine climate,^[5]:21 and the terrain can be described as alpine tundra.^[6] In the northern hemisphere treelines on north-facing slopes are lower than on south-facing slopes because the increased shade on north-facing slopes means the snowpack takes longer to melt. This shortens the growing season for trees.^[7]:109 In the southern hemisphere, the south-facing slopes have the shorter growing season.

The alpine tree line boundary is seldom abrupt: it usually forms a transition zone between closed forest below and treeless alpine tundra above. 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”.^[8] Environmentally dwarfed shrubs (krummholz) commonly forms the upper limit.

The decrease in air temperature due to increasing elevation causes the alpine climate. The rate of decrease can vary in different mountain chains, from 3.5 °F (1.9 °C) per 1,000 feet (300 m) of elevation gain in the dry mountains of the Western United States,^[8] to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the Eastern United States.^[9] Skin effects and topography can create microclimates that alter the general cooling trend.^[10]

Compared with arctic timberlines, alpine timberlines may receive fewer than half of the number of degree days (>10 °C) based on air temperature because solar radiation intensities are greater at alpine than at arctic timberlines. However, the number of degree days calculated from leaf temperatures may be very similar in the two kinds of timberlines.^[8]

Summer warmth generally sets the limit to which tree growth can occur, for while timberline conifers are very frost-hardy during most of the year, they become sensitive to just 1 or 2 degrees of frost in mid-summer.^[11][12] A series of warm summers in the 1940s seems to have permitted the establishment of “significant numbers” of spruce seedlings above the previous treeline in the hills near Fairbanks, Alaska.^[13][14] Survival depends on a sufficiency of new growth to support the tree. The windiness of high-elevation sites is also a potent determinant of the distribution of tree growth. Wind can mechanically damage tree tissues directly, including blasting with wind-borne particles, and may also contribute to the desiccation of foliage, especially of shoots that project above snow cover.

At the alpine timberline, tree growth is inhibited when excessive snow lingers and shortens the growing season to the point where new growth would not have time to harden before the onset of fall frost. Moderate snowpack, however, may promote tree growth by insulating the trees from extreme cold during the winter, curtailing water loss,^[15] and prolonging a supply of moisture through the early part of the growing season. However, snow accumulation in sheltered gullies in the Selkirk Mountains of southeastern British Columbia causes timberline to be 400 metres (1,300 ft) lower than on exposed intervening shoulders