Tree line

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.

The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and in frigid environments. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures or lack of moisture).[1]:51 The tree line should not be confused with a lower timberline or forest line, which is the line where trees form a forest with a closed canopy.[2]:151[3]:18

At the tree line, tree growth is often sparse and stunted, with the last trees forming densely matted bushes, known as krummholz (German for "twisted wood.")[4]:58

The tree line, like many other natural lines (lake boundaries, for example), appears well-defined from a distance, but upon sufficiently close inspection, it is a gradual transition in most places. Trees grow shorter towards the inhospitable climate until they simply stop growing.[4]:55

Types

There are several types of tree lines defined in ecology and geology:

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,[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.[16]

Desert

In a desert, the tree line marks the driest places where trees can grow; drier desert areas having insufficient rainfall to sustain them. These tend to be called the "lower" tree line and occur below about 5,000 ft (1,500 m) elevation in the Desert Southwestern United States.[17] The desert treeline tends to be lower on pole-facing slopes than equator-facing slopes, because the increased shade on the former keeps those cooler and prevents moisture from evaporating as quickly, giving trees a longer growing season and more access to water.

Desert-alpine

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.

Double tree line

Different tree species have different tolerances to drought and cold. Mountain ranges isolated by oceans or deserts may have restricted repertoires of tree species with gaps that are above the alpine tree line for some species yet below the desert tree line for others. For example, several mountain ranges in the Great Basin of North America have lower belts of Pinyon Pines and Junipers separated by intermediate brushy but treeless zones from upper belts of Limber and Bristlecone Pines.[18]:37

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

A mountain rising from a river with brownish water at its base. Its lower slopes are green with scattered evergreen trees, giving way to much denser forest cover midway up. At the top it yields abruptly to bare rock with some green ground cover. Above it is a partly cloudy sky.
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.[19] 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 timberlines, the northern timberline 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,[20] in contrast with the telescoped alpine timberlines.[8] North of the arctic timberline lies the low-growing tundra, and southwards lies the boreal forest.

Two zones can be distinguished in the arctic timberline:[21][22] 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 carpet of Cladonia spp. lichens.[21] The proportion of trees to lichen mat increases southwards towards the “forest line”, where trees cover 50 per cent or more of the landscape.[8][23]

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) don't.[24] Another treeline exists in the southwestern most parts of the Magellanic subpolar forests ecoregion, where the forest merges into the subantarctic tundra (termed Magellanic moorland or Magellanic tundra).[25] For example, the northern halves of Hoste and Navarino Islands have Nothofagus antarctica forests but the southern parts consist of moorlands and tundra.

Other tree lines

Several other reasons may cause the environment to be too extreme for trees to grow. This can include geothermal exposure associated with hot springs or volcanoes, such as at Yellowstone; high soil acidity near bogs; high salinity associated with playas or salt lakes; or ground that is saturated with groundwater that excludes oxygen from the soil, which most tree roots need for growth. The margins of muskegs and bogs are common examples of these types of open area. However, no such line exists for swamps, where trees, such as Bald cypress and the many mangrove species, have adapted to growing in permanently waterlogged soil. In some colder parts of the world there are tree lines around swamps, where there are no local tree species that can develop. There are also man-made pollution tree lines in weather-exposed areas, where new tree lines have developed because of the increased stress of pollution. Examples are found around Nikel in Russia and previously in the Erzgebirge.

Typical vegetation

Severe winter climate conditions at alpine tree line causes stunted krummholz growth. Karkonosze, Poland.
Dahurian Larch growing close to the Arctic tree line in the Kolyma region, Arctic northeast Siberia.

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

Eurasia

North America

South America

View of a Magellanic Lenga forest close to the tree line in Torres del Paine National Park, Chile.

Australia

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 (246 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).[29]

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 [29]
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[30]:85
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 [31]
Mount Katahdin, Maine, United States 46°N 1,150 3,800
Eastern Alps, Austria, Italy 46°N 1,750 5,700 more exposure to Russian cold winds than Western Alps
Sikhote-Alin, Russia 46°N 1,600 5,200 [32]
Alps of Piedmont, Northwestern Italy 45°N 2,100 6,900
New Hampshire, United States 44°N 1,350 4,400 [33] 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
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 [29] On warm southwest slopes
3,250 10,700 On northeast slopes
Japanese Alps 36°N 2,900 9,500
Yosemite, CA, United States 38°N 3,200 10,500 [34] West side of Sierra Nevada
3,600 11,800 [34] East side of Sierra Nevada
Sierra Nevada, Spain 37°N 2,400 7,900 Precipitation low in summer
Khumbu, Himalaya 28°N 4,200 13,800 [29]
Yushan, Taiwan 23°N 3,600 11,800 [35] Strong winds and poor soil restrict further grow of trees.
Hawaii, United States 20°N 3,000 9,800 [29] Geographic isolation and no local tree species with high tolerance to cold temperatures.
Pico de Orizaba, Mexico 19°N 4,000 13,100 [31]
Costa Rica 9.5°N 3,400 11,200
Mount Kinabalu, Borneo 6.1°N 3,400 11,200 [36]
Mount Kilimanjaro, Tanzania 3°S 3,950 13,000 [29]
New Guinea 6°S 3,850 12,600 [29]
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[37]
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
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[38]
Navarino Island, Chile 55°S 600 2,000 Strong maritime influence serves to cool summer and restrict tree growth[38]

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 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.
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.

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. But no trees live in Antarctica. In fact, only a few species of grass, mosses, and lichens survive on the peninsula. 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 600 – 800 hours of sun annually. Campbell Island (52°S) further south is treeless, except for one stunted pine, planted by scientists. 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.

Long term monitoring of alpine treelines

There are several monitoring protocols developed for long term monitoring of alpine biodiversity. One such network which is developed on the line of GLORIA, in India HIMADRI.

See also

Look up tree line in Wiktionary, the free dictionary.

References

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  2. 1 2 Jørgensen, S.E. (2009). Ecosystem Ecology. Academic Press. ISBN 978-0-444-53466-8.
  3. ↑ Körner, C.; Riedl, S. (2012). Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Springer. ISBN 9783034803960.
  4. 1 2 Zwinger, A.; Willard, B. E. (1996). Land Above the Trees: A Guide to American Alpine Tundra. Big Earth Publishing. ISBN 1-55566-171-8.
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  6. ↑ "Alpine Tundra Ecosystem". Rocky Mountain National Park. National Park Service. Retrieved 2011-05-13.
  7. 1 2 3 Peet, R.K. (2000). "Forests and Meadows of the Rocky Mountains". In Barbour, M.G.; Billings, M.D. North American Terrestrial Vegetation (2nd ed.). Cambridge University Press. ISBN 978-0-521-55986-7.
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  14. ↑ Viereck, L.A.; Van Cleve, K.; Dyrness, C.T (1986). "Forest ecosystem distribution in the taiga environment". In Van Cleve, K.; Chapin, F.S.; Flanagan, P.W.; Viereck, L.A.; Dyrness, C.T. Forest Ecosystems in the Alaskan Taiga. New York NY: Springer-Verlag. pp. 22–43. doi:10.1007/978-1-4612-4902-3_3. ISBN 1461249023.
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  16. ↑ Shaw, C.H. (1909). "The causes of timberline on mountains: the role of snow". Plant World. 12: 169–181.
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  20. ↑ Timoney, K.P.; La Roi, G.H.; Zoltai, S.C.; Robinson, A.L. (1992). "The high subarctic forest–tundra of northwestern Canada: position, width, and vegetation gradients in relation to climate". Arctic. 45: 1–9. doi:10.14430/arctic1367. JSTOR 40511186.
  21. 1 2 Löve, Dd (1970). "Subarctic and subalpine: where and what?". Arctic and Alpine Res. 2: 63–73. doi:10.2307/1550141. JSTOR 1550141.
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  23. ↑ R.A., Black; Bliss, L.C. (1978). "Recovery sequence of Picea mariana–Vaccinium uliginosum forests after burning near Inuvik, Northwest Territories, Canada". Can. J. Bot. 56 (6): 2020–2030. doi:10.1139/b78-243.
  24. ↑ "Antipodes Subantarctic Islands tundra". Terrestrial Ecoregions. World Wildlife Fund.
  25. ↑ "Magellanic subpolar Nothofagus forests". Terrestrial Ecoregions. World Wildlife Fund.
  26. ↑ Chalupa, V. (1992). "Micropropagation of European Mountain Ash (Sorbus aucuparia L.) and Wild Service Tree [Sorbus torminalis (L.) Cr.]". In Bajaj, Y.P.S. High-Tech and Micropropagation II. Biotechnology in Agriculture and Forestry. 18. Springer Berlin Heidelberg. pp. 211–226. doi:10.1007/978-3-642-76422-6_11. ISBN 978-3-642-76424-0.
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  30. ↑ "Action For Scotland's Biodiversity" (PDF).
  31. 1 2 Körner, Ch. "High Elevation Treeline Research". Archived from the original on 2011-05-14. Retrieved 2010-06-14.
  32. ↑ "Physiogeography of the Russian Far East".
  33. ↑ "Mount Washington State Park". New Hampshire State Parks. Archived from the original on 2013-04-03. Retrieved 2013-08-22. Tree line, the elevation above which trees do not grow, is about 4,400 feet in the White Mountains, nearly 2,000 feet below the summit of Mt. Washington.
  34. 1 2 Schoenherr, Allan A. (1995). A Natural History of California. UC Press. ISBN 0-520-06922-6.
  35. ↑ "台灣地帶性植被之區劃與植物區系之分區" (PDF).
  36. ↑ "Mount Kinabalu National Park". www.ecologyasia.com. Ecology Asia. 4 September 2016. Retrieved 6 September 2016.}
  37. ↑ Lara, Antonio; Villalba, Ricardo; Wolodarsky-Franke, Alexia; Aravena, Juan Carlos; Luckman, Brian H.; Cuq, Emilio (2005). "Spatial and temporal variation in Nothofagus pumilio growth at tree line along its latitudinal range (35°40′–55° S) in the Chilean Andes" (PDF). Journal of Biogeography. 32 (5): 879–893. doi:10.1111/j.1365-2699.2005.01191.x.
  38. 1 2 Aravena, Juan C.; Lara, Antonio; Wolodarsky-Franke, Alexia; Villalba, Ricardo; Cuq, Emilio (2002). "Tree-ring growth patterns and temperature reconstruction from Nothofagus pumilio (Fagaceae) forests at the upper tree line of southern, Chilean Patagonia". Revista Chilena de Historia Natural. Santiago. 75 (2). doi:10.4067/S0716-078X2002000200008.

Further reading

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