Tree line

Variation

The tree line elevation at a location is generally set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging, or grazing. Most human activities cannot change the actual tree line, unless they affect the climate. The tree line follows the line where the seasonal mean temperature is approximately 6 °C or 43 °F. 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.

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.

Other local factors can locally change the elevation of tree line, such as aspect of slope, rain shadow. 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. In the southern hemisphere, the south-facing slopes have the shorter growing season. On coasts and isolated mountains, the tree line is often much lower than corresponding altitudes inland and in larger, more complex mountain systems. This is known as the Massenerhebung effect, and is caused by large mountain ranges retaining more heat and reducing wind velocity downwind, compared to isolated mountains. In addition, in some tropical or island localities, the lack of local drought- and cold-adapted species can result in lower tree lines than one might expect by climate alone.

Types

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

Alpine

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. The climate above the tree line of mountains is called an alpine climate, and the habitat can be described as the alpine zone.

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". 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, to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the eastern United States. Skin effects and topography can create microclimates that alter the general cooling trend.

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.

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, 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.

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 7,900–9,500 ft (2,400–2,900 m) is above a temperature inversion which blocks moisture from reaching the highest slopes.

Arctic

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. 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.

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, in contrast with the telescoped alpine timberlines. 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: 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. The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.

Tree species near tree line

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

Australia

• Snow gum (Eucalyptus pauciflora)

Eurasia

• Dahurian larch (Larix gmelinii)
• Macedonian pine (Pinus peuce)
• Swiss pine (Pinus cembra)
• Mountain pine (Pinus mugo)
• Arctic white birch (Betula pubescens subsp. tortuosa)
• Rowan (Sorbus aucuparia)

North America

• Subalpine fir (Abies lasiocarpa)
• Subalpine larch (Larix lyallii)
• Mountain hemlock (Tsuga mertensiana)
• Alaska yellow cedar (Chaemaecyparis nootkatensis)
• Engelmann spruce (Picea engelmannii)
• Whitebark pine (Pinus albicaulis)
• Great Basin bristlecone pine (Pinus longaeva)
• Rocky Mountains bristlecone pine (Pinus aristata)
• Foxtail pine (Pinus balfouriana)
• Limber pine (Pinus flexilis)
• Potosi pinyon (Pinus culminicola)
• Black spruce (Picea mariana)
• White spruce (Picea glauca)
• Tamarack (Larix laricina)
• Hartweg's pine (Pinus hartwegii)

South America

• Antarctic beech (Nothofagus antarctica)
• Lenga beech (Nothofagus pumilio)
• Alder (Alnus acuminata)
• Pino del cerro (Podocarpus parlatorei)
• Polylepis (Polylepis tarapacana)
• Eucalyptus (not native to South America but grown in large amounts in the high Andes).

Worldwide distribution

Alpine tree lines

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).

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

Arctic tree lines

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. 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:

Antarctic tree lines

The southernmost trees in the world are on Isla Hornos (56°S), at the southern tip of South America. Trees do not exist on subantarctic islands nor in Antarctica. Therefore, there is no continental Antarctic tree line: the Southern Ocean acts as a tree boundary.

The subantarctic islands (South Georgia, Prince Edward, Crozet, Kerguelen, Heard and McDonald, and Macquarie Islands) lie in the Antarctic Circumpolar Current between 46.4° and 54.6°S. While the climate of these islands is cold and wet with long growing seasons, none of these islands have trees, due to the strong winds of the Roaring Forties and Furious Fifties.

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. 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.