Tree line

The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures, extreme snowpack, or associated lack of available moisture).[1]: 51  The tree line is sometimes distinguished from a lower timberline, which is the line below which trees form a forest with a closed canopy.[2]: 151 [3]: 18 

Tree line above St. Moritz, Switzerland. May 2009
In this view of an alpine tree line, the distant line looks particularly sharp. The foreground shows the transition from trees to no trees. These trees are stunted in growth and one-sided because of cold and constant wind.

At the tree line, tree growth is often sparse, stunted, and deformed by wind and cold. This is sometimes known as krummholz (German for "crooked wood").[4]: 58 

The tree line often appears well-defined, but it can be a more gradual transition. Trees grow shorter and often at lower densities as they approach the tree line, above which they are unable to grow at all.[4]: 55  Given a certain latitude, the tree line is approximately 300 to 1000 meters below the snow line and roughly parallel to it; however, the snow line, by definition, exhibits seasonal variations, whereas the tree line does not.[5]

Causes

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, for 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 treeline 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 snow cover.

The actual tree line is set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging.[6] Most human activities cannot change the actual tree line, unless they affect the climate.[6] The tree line roughly follows the line where the highest monthly mean temperature is at 10 °C (50 °F).[11] A more proximate cause of the tree line is the ground temperature: the tree line occurs when the seasonal mean ground temperature is approximately 6–7 °C or 43–45 °F.[12] The ground temperature threshold depends on climate type: it is slightly lower (5–6 °C or 41–43 °F) near the equator, and slightly higher (7–8 °C or 45–46 °F) in Mediterranean and temperate climates.

Types
This map of the "Distribution of Plants in a Perpendicular Direction in the Torrid, the Temperate, and the Frigid Zones" was first published 1848 in "The Physical Atlas". It shows tree lines of the Andes, Himalaya, Alps and Pyrenees.
Alpine tree line of mountain pine and European spruce below the subalpine zone of Bistrishko Branishte, with the surmounting Golyam Rezen Peak, Vitosha Mountain, Sofia, Bulgaria

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

Alpine
An alpine tree line in the Tararua Range

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,[13]: 21  and the habitat can be described as the alpine zone.[14] 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. This shortens the growing season for trees.[15]: 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".[16] 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,[16] to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the eastern United States.[17] Skin effects and topography can create microclimates that alter the general cooling trend.[18]

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.[16]

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,[19] 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.[20]

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 treeline than the northern faces because of increased sun exposure and aridity. Hawaii's treeline of about 8,000 ft (2,400 m) feet is also above the condensation zone and results due to a lack of moisture.

Exposure

On coasts and isolated mountains the tree line is often much lower than in 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.

Arctic
Treeline on a mountain in the Canadian 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.[21] Extremely cold 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,[22] in contrast with the telescoped alpine timberlines.[16] 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:[23][24] 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.[23] The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.[16][25]

Antarctic

A southern treeline 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.[26] Another treeline 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).[27] For example, the northern halves of Hoste and Navarino Islands have Nothofagus antarctica forests but the southern parts consist of moorlands and tundra.

Tree species near tree line
Coniferous species tree line below Vihren Peak, Pirin Mountain, Bulgaria
Dahurian larch growing close to the Arctic tree line in the Kolyma region, Arctic northeast Siberia
View of a Magellanic lenga forest close to the tree line in Torres del Paine National Park, Chile

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[28] (Sorbus aucuparia)

North America
  • Subalpine fir (Abies lasiocarpa)[15]: 106 
  • Subalpine larch (Larix lyallii)[29]
  • Mountain hemlock (Tsuga mertensiana)
  • Alaska yellow cedar (Chaemaecyparis nootkatensis)
  • Engelmann spruce (Picea engelmannii)[15]: 106 
  • Whitebark pine (Pinus albicaulis)[29]
  • 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)[1]: 53 
  • White spruce (Picea glauca)
  • Tamarack (Larix laricina)
  • Hartweg's pine (Pinus hartwegii)

South America
  • Antarctic beech (Nothofagus antarctica)
  • Lenga beech (Nothofagus pumilio)[30]
  • 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).[31]

Worldwide distribution

Alpine tree lines

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.

Averaging over many locations and local microclimates, the treeline 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 treeline is roughly constant, between 3,500 and 4,000 metres (11,500 and 13,100 ft).[32]

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

Location Approx. latitude Approx. elevation of tree line Notes
(m) (ft)
Finnmarksvidda, Norway 69°N 500 1,600 At 71°N, near the coast, the tree-line is below sea level (Arctic tree line).
Abisko, Sweden 68°N 650 2,100 [32]
Chugach Mountains, Alaska 61°N 700 2,300 Tree line around 1,500 feet (460 m) or lower in coastal areas
Southern Norway 61°N 1,100 3,600 Much lower near the coast, down to 500–600 metres (1,600–2,000 ft).
Scotland 57°N 500 1,600 Strong maritime influence serves to cool summer and restrict tree growth[33]: 79 
Northern Quebec 56°N 0 0 The cold Labrador Current originating in the arctic makes eastern Canada the sea-level region with the most southern tree-line in the northern hemisphere.
Southern Urals 55°N 1,100 3,600
Canadian Rockies 51°N 2,400 7,900
Tatra Mountains 49°N 1,600 5,200
Olympic Mountains WA, United States 47°N 1,500 4,900 Heavy winter snowpack buries young trees until late summer
Swiss Alps 47°N 2,200 7,200 [34]
Mount Katahdin, Maine, United States 46°N 1,150 3,800
Eastern Alps, Austria, Italy 46°N 1,750 5,700 More exposure to cold Russian winds than Western Alps
Sikhote-Alin, Russia 46°N 1,600 5,200 [35]
Alps of Piedmont, Northwestern Italy 45°N 2,100 6,900
New Hampshire, United States 44°N 1,350 4,400 [36] Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua.
Wyoming, United States 43°N 3,000 9,800
Rila and Pirin Mountains, Bulgaria 42°N 2,300 7,500 Up to 2,600 m (8,500 ft) on favorable locations. Mountain Pine is the most common tree line species.
Pyrenees Spain, France, Andorra 42°N 2,300 7,500 Mountain Pine is the tree line species
Steens Mountain, Oregon, USA 42°N 2,500 8,200
Wasatch Mountains, Utah, United States 40°N 2,900 9,500 Higher (nearly 11,000 feet or 3,400 metres in the Uintas)
Rocky Mountain NP, CO, United States 40°N 3,550 11,600 [32] On warm southwest slopes
3,250 10,700 On northeast slopes
Yosemite, CA, United States 38°N 3,200 10,500 [37] West side of Sierra Nevada
3,600 11,800 [37] East side of Sierra Nevada
Sierra Nevada, Spain 37°N 2,400 7,900 Precipitation low in summer
Japanese Alps 36°N 2,900 9,500
Khumbu, Himalaya 28°N 4,200 13,800 [32]
Yushan, Taiwan 23°N 3,600 11,800 [38] Strong winds and poor soil restrict further grow of trees.
Hawaii, United States 20°N 3,000 9,800 [32] Geographic isolation and no local tree species with high tolerance to cold temperatures.
Pico de Orizaba, Mexico 19°N 4,000 13,100 [34]
Costa Rica 9.5°N 3,400 11,200
Mount Kinabalu, Borneo 6.1°N 3,400 11,200 [39]
Mount Kilimanjaro, Tanzania 3°S 3,100 10,200 [32] Upper limit of forest trees; woody ericaeous scrub grows up to 3900m
New Guinea 6°S 3,850 12,600 [32]
Andes, Peru 11°S 3,900 12,800 East side; on west side tree growth is restricted by dryness
Andes, Bolivia 18°S 5,200 17,100 Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana)
4,100 13,500 Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate)
Sierra de Córdoba, Argentina 31°S 2,000 6,600 Precipitation low above trade winds, also high exposure
Australian Alps, Australia 36°S 2,000 6,600 West side of Australian Alps
1,700 5,600 East side of Australian Alps
Andes, Laguna del Laja, Chile 37°S 1,600 5,200 Temperature rather than precipitation restricts tree growth[40]
Mount Taranaki, North Island, New Zealand 39°S 1,500 4,900 Strong maritime influence serves to cool summer and restrict tree growth
Tasmania, Australia 41°S 1,200 3,900 Cold winters, strong cold winds and cool summers with occasional summer snow restrict tree growth
Fiordland, South Island, New Zealand 45°S 950 3,100 Cold winters, strong cold winds and cool summers with occasional summer snow restrict tree growth
Lago Argentino, Argentina 50°S 1,000 3,300 Nothofagus pumilio[41]
Torres del Paine, Chile 51°S 950 3,100 Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth[42]
Navarino Island, Chile 55°S 600 2,000 Strong maritime influence serves to cool summer and restrict tree growth[42]

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 enough inland effect to have summers well above the threshold for the tree line. Here are some typical polar treelines:

Location Approx. longitude Approx. latitude of tree line Notes
Norway 24°E 70°N The North Atlantic current makes Arctic climates in this region warmer than other coastal locations at comparable latitude. In particular the mildness of winters prevents permafrost.
West Siberian Plain 75°E 66°N
Central Siberian Plateau 102°E 72°N Extreme continental climate means the summer is warm enough to allow tree growth at higher latitudes, extending to northernmost forests of the world at 72°28'N at Ary-Mas (102° 15' E) in the Novaya River valley, a tributary of the Khatanga River and the more northern Lukunsky grove at 72°31'N, 105° 03' E east from Khatanga River.
Russian Far East (Kamchatka and Chukotka) 160°E 60°N The Oyashio Current and strong winds affect summer temperatures to prevent tree growth. The Aleutian Islands are almost completely treeless.
Alaska, United States 152°W 68°N Trees grow north to the south-facing slopes of the Brooks Range. The mountains block cold air coming off of the Arctic Ocean.
Northwest Territories, Canada 132°W 69°N Reaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures.
Nunavut 95°W 61°N Influence of the very cold Hudson Bay moves the treeline southwards.
Labrador Peninsula 72°W 56°N Very strong influence of the Labrador Current on summer temperatures as well as altitude effects (much of Labrador is a plateau). In parts of Labrador, the treeline extends as far south as 53°N. Along the coast the northernmost trees are at 58°N in Napartok Bay.
Greenland 50°W 64°N Determined by experimental tree planting in the absence of native trees because of isolation from natural seed sources; a very few trees are surviving, but growing slowly, at Søndre Strømfjord, 67°N. There is one natural forest in the Qinngua Valley.

Antarctic 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.

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.

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.

Trees growing along the north shore of the Beagle Channel, 55°S.

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, planted by scientists. [43] The climate on these islands is not severe, but tree growth is limited by almost continual rain and wind. Summers are very cold with an average January temperature of 9 °C (48 °F). Winters are 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.

See also
Look up tree line in Wiktionary, the free dictionary.
  • Montane ecosystems
  • Ecotone: a transition between two adjacent ecological communities
  • Edge effects: the effect of contrasting environments on an ecosystem
  • Massenerhebung effect
  • Snow line

References
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Further reading
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