Due to their vertical structure, trees are more susceptible to cold than more ground-hugging forms of plants.^[6] Summer warmth generally sets the limit to which tree growth can occur: while tree line conifers are very frost-hardy during most of the year, they become sensitive to just 1 or 2 degrees of frost in mid-summer.^[7][8] A series of warm summers in the 1940s seems to have permitted the establishment of "significant numbers" of spruce seedlings above the previous tree line in the hills near Fairbanks, Alaska.^[9][10] Survival depends on a sufficiency of new growth to support the tree. Wind can mechanically damage tree tissues directly, including blasting with windborne particles, and may also contribute to the desiccation of foliage, especially of shoots that project above the snow cover.^{[citation needed]}

The actual tree line is set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging,^[6] or grazing^[11] Most human activities cannot change the actual tree line, unless they affect the climate.^[6] The tree line follows the line where the seasonal mean temperature is approximately 6 °C or 43 °F.^[12][6] The seasonal mean temperature is taken over all days whose mean temperature is above 0.9 °C (33.6 °F). A growing season of 94 days above that temperature is required for tree growth.^[13]

Because of climate change, which leads to earlier snowmelt and favorable conditions for tree establishment, the tree line in North Cascades National Park has risen more than 400 feet (120 m) in 50 years.^[14]

Several types of tree lines are defined in ecology and geography:

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,^[15]: 21 and the habitat can be described as the alpine zone.^[16] Tree lines 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. This shortens the growing season for trees.^[17]: 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 zone 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".^[18] Environmentally dwarfed shrubs (krummholz) commonly form the upper limit.

The decrease in air temperature with increasing elevation creates 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,^[18] to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the eastern United States.^[19] Skin effects and topography can create microclimates that alter the general cooling trend.^[20]

Compared with arctic tree lines, alpine tree lines may receive fewer than half of the number of degree days (above 10 °C (50 °F)) based on air temperature, but because solar radiation intensities are greater at alpine than at arctic tree lines the number of degree days calculated from leaf temperatures may be very similar.^[18]

At the alpine tree line, 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,^[21] 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 the tree line to be 400 metres (1,300 ft) lower than on exposed intervening shoulders.^[22]

In some mountainous areas, higher elevations above the condensation line, or on equator-facing and leeward slopes, can result in low rainfall and increased exposure to solar radiation. This dries out the soil, resulting in a localized arid environment unsuitable for trees. Many south-facing ridges of the mountains of the Western U.S. have a lower tree line than the northern faces because of increased sun exposure and aridity. Hawaii's tree line of about 8,000 ft (2,400 m) is also above the condensation zone and results due to a lack of moisture.^{[citation needed]}

On coasts and isolated mountains, the tree line is often much lower than corresponding altitudes inland and in larger, more complex mountain systems, because strong winds reduce tree growth. In addition, the lack of suitable soil, such as along talus slopes or exposed rock formations, prevents trees from gaining an adequate foothold and exposes them to drought and sun.^{[citation needed]}

The Arctic tree line is the northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold all year round to sustain trees.^[23] Extremely low temperatures, especially when prolonged, can freeze the internal sap of trees, killing them. In addition, permafrost in the soil can prevent trees from getting their roots deep enough for the necessary structural support.^{[citation needed]}

Unlike alpine tree lines, the northern tree line occurs at low elevations. The Arctic forest-tundra transition zone in northwestern Canada varies in width, perhaps averaging 145 kilometres (90 mi) and widening markedly from west to east,^[24] in contrast with the telescoped alpine timberlines.^[18] North of the arctic tree line lies the low-growing tundra, and southwards lies the boreal forest.

Two zones can be distinguished in the Arctic tree line:^[25][26] a forest–tundra zone of scattered patches of krummholz or stunted trees, with larger trees along rivers and on sheltered sites set in a matrix of tundra; and "open boreal forest" or "lichen woodland", consisting of open groves of erect trees underlain by a carpet of Cladonia spp. lichens.^[25] The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.^[18][27]

A southern tree line exists in the New Zealand Subantarctic Islands and the Australian Macquarie Island, with places where mean annual temperatures above 5 °C (41 °F) support trees and woody plants, and those below 5 °C (41 °F) do not.^[28] Another tree line exists in the southwesternmost parts of the Magellanic subpolar forests ecoregion, where the forest merges into the subantarctic tundra (termed Magellanic moorland or Magellanic tundra).^[29] For example, the northern halves of Hoste and Navarino Islands have Nothofagus antarctica forests, but the southern parts consist of moorlands and tundra.

Some typical Arctic and alpine tree line tree species (note the predominance of conifers):

• Snow gum (Eucalyptus pauciflora)

The alpine tree line at a location is dependent on local variables, such as aspect of slope, rain shadow and proximity to either geographical pole. In addition, in some tropical or island localities, the lack of biogeographical access to species that have evolved in a subalpine environment can result in lower tree lines than one might expect by climate alone.^{[citation needed]}

Averaging over many locations and local microclimates, the tree line rises 75 metres (245 ft) when moving 1 degree south from 70 to 50°N, and 130 metres (430 ft) per degree from 50 to 30°N. Between 30°N and 20°S, the tree line is roughly constant, between 3,500 and 4,000 metres (11,500 and 13,100 ft).^[35]

Here is a list of approximate tree lines from locations around the globe:

Like the alpine tree lines shown above, polar tree lines are heavily influenced by local variables such as aspect of slope and degree of shelter. In addition, permafrost has a major impact on the ability of trees to place roots into the ground. When roots are too shallow, trees are susceptible to windthrow and erosion. Trees can often grow in river valleys at latitudes where they could not grow on a more exposed site. Maritime influences such as ocean currents also play a major role in determining how far from the equator trees can grow as well as the warm summers experienced in extreme continental climates.^{[citation needed]} In northern inland Scandinavia, there is substantial maritime influence on high parallels that keep winters relatively mild, but with enough inland effect to have summers well above the threshold for the tree line. Here are some typical polar tree lines:

Trees exist on Tierra del Fuego (55°S) at the southern end of South America, but generally not on subantarctic islands and not in Antarctica. Therefore, there is no explicit Antarctic tree line.^{[citation needed]}

Kerguelen Island (49°S), South Georgia (54°S), and other subantarctic islands are all so heavily wind-exposed and with a too-cold summer climate (tundra) that none have any indigenous tree species. The Falkland Islands (51°S) summer temperature is near the limit, but the islands are also treeless, although some planted trees exist.^{[citation needed]}

Antarctic Peninsula is the northernmost point in Antarctica (63°S) and has the mildest weather—it is located 1,080 kilometres (670 mi) from Cape Horn on Tierra del Fuego—yet no trees survive there; only a few mosses, lichens, and species of grass do so. In addition, no trees survive on any of the subantarctic islands near the peninsula.^{[citation needed]}

Southern Rata forests exist on Enderby Island and Auckland Islands (both 50°S) and these grow up to an elevation of 370 metres (1,200 ft) in sheltered valleys. These trees seldom grow above 3 m (9.8 ft) in height and they get smaller as one gains altitude, so that by 180 m (600 ft) they are waist-high. These islands have only between 600 and 800 hours of sun annually. Campbell Island (52°S) further south is treeless, except for one stunted spruce, probably planted in 1907.^[48] The climate on these islands is not severe, but tree growth is limited by almost continual rain and wind. The summers are very cold, with an average January temperature of 9 °C (48 °F), while winters are a mild 5 °C (41 °F) but wet. Macquarie Island (Australia) is located at 54°S and has no vegetation beyond snow grass and alpine grasses and mosses.^{[citation needed]}

• Trees portal
• Montane ecosystems
• Ecotone: a transition between two adjacent ecological communities
• Edge effects: the effect of contrasting environments on an ecosystem
• Massenerhebung effect
• Snow line