Tree line

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Template:Short description

File:Tree line above St. Moritz.jpg
Tree line above St. Moritz, Switzerland. May 2009
File:Tree line.jpg
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 a habitat at which trees are capable of growing and beyond which they are not. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions (usually low temperatures, extreme snowpack, or associated lack of available moisture).[1]Template:Rp 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]Template:Rp[3]Template:Rp

At the tree line, tree growth is often sparse, stunted, and deformed by wind and cold. This is sometimes known as Script error: No such module "Lang". (German for "crooked wood").[4]Template:Rp

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]Template:Rp Given a certain latitude, the tree line is approximately 300 to 1000 meters below the permanent snow line and roughly parallel to it.[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: 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 the snow cover.Script error: No such module "Unsubst".

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 Template:Convert.[12][6] The seasonal mean temperature is taken over all days whose mean temperature is above Template:Convert. A growing season of 94 days above that temperature is required for tree growth.[13]

Because of climate change, which leads to earlier snow melt and favorable conditions for tree establishment, the tree line in North Cascades National Park has risen more than Template:Convert in 50 years.[14]

Types

File:Distribution of Plants in a Perpendicular Direction in the Torrid, the Temperate, and the Rigid Zones 1848 Alexander Keith Johnston.png
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, Tenerife, Himalaya, Alps, Pyrenees, and Lapland.
File:Bistrishko-Branishte.jpg
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

File:Treeline In The Tararuas.JPG
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]Template:Rp The climate above the tree line of mountains is called an alpine climate,[15]Template:Rp and the habitat can be described as the alpine zone.[16] 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.[17]Template:Rp 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 Template:Convert per Template:Convert of elevation gain in the dry mountains of the western United States,[18] to Template:Convert per Template:Convert 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 Template:Convert) 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 Template:Convert 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 treeline than the northern faces because of increased sun exposure and aridity. Hawaii's treeline of about Template:Convert is also above the condensation zone and results due to a lack of moisture.Script error: No such module "Unsubst".

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.Script error: No such module "Unsubst".

Arctic

An aerial photo viewing down to Earth with rivers visible. Ground is covered by snow, with trees in the lower left and in the valleys of the rivers.
Treeline visible in lower left, northern Quebec, Canada, while trees also grow in the sheltered river valleys

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.Script error: No such module "Unsubst".

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 Template:Convert 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]

Antarctic

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A southern treeline exists in the New Zealand Subantarctic Islands and the Australian Macquarie Island, with places where mean annual temperatures above Template:Convert support trees and woody plants, and those below Template:Convert do not.[28] 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).[29] 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

File:Vihren Peak.jpg
Coniferous species tree line below Vihren Peak, Pirin Mountains, Bulgaria
File:Larix gmelinii0.jpg
Dahurian larch growing close to the Arctic tree line in the Kolyma region, Arctic northeast Siberia
File:Valle del Frances.jpg
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

Eurasia

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North America

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South America

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Worldwide distribution

Alpine tree lines

File:Treeline elevation.gif
Treeline elevation by latitude[34]

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.Script error: No such module "Unsubst".

Averaging over many locations and local microclimates, the treeline rises Template:Convert when moving 1 degree south from 70 to 50°N, and Template:Convert per degree from 50 to 30°N. Between 30°N and 20°S, the treeline is roughly constant, between Template:Convert.[35]

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 Template:Convert At 71°N, near the coast, the tree-line is below sea level (Arctic tree line).
Abisko, Sweden 68°N Template:Convert [35]
Chugach Mountains, Alaska 61°N Template:Convert Tree line around Template:Convert or lower in coastal areas
Southern Norway 61°N Template:Convert Much lower near the coast, down to Template:Convert.
Scotland, United Kingdom 57°N Template:Convert Strong maritime influence serves to cool summer and restrict tree growth[36]Template:Rp
Northern Quebec 56°N Template:Convert 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 Template:Convert
Canadian Rockies 51°N Template:Convert
Tatra Mountains 49°N Template:Convert
Olympic Mountains, Washington, United States 47°N Template:Convert Heavy winter snowpack buries young trees until late summer
Swiss Alps 47°N Template:Convert [37]
Mount Katahdin, Maine, United States 46°N Template:Convert
Eastern Alps, Austria, Italy 46°N Template:Convert More exposure to cold Russian winds than Western Alps
Sikhote-Alin, Russia 46°N Template:Convert [38]
Alps of Piedmont, Northwestern Italy 45°N Template:Convert
New Hampshire, United States 44°N Template:Convert [39] 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 Template:Convert
Caucasus Mountains 42°N Template:Convert [40]
Rila and Pirin Mountains, Bulgaria 42°N Template:Convert Up to Template:Convert on favorable locations. Mountain Pine is the most common tree line species.
Pyrenees Spain, France, Andorra 42°N Template:Convert Mountain Pine is the tree line species
Steens Mountain, Oregon, US 42°N Template:Convert
Wasatch Mountains, Utah, United States 40°N Template:Convert Higher (nearly Template:Convert in the Uintas)
Rocky Mountain NP, CO, United States 40°N Template:Convert [35] On warm southwest slopes
Template:Convert On northeast slopes
Yosemite, CA, United States 38°N Template:Convert [41] West side of Sierra Nevada
Template:Convert [41] East side of Sierra Nevada
Sierra Nevada, Spain 37°N Template:Convert Precipitation low in summer
Japanese Alps 36°N Template:Convert
Khumbu, Himalaya 28°N Template:Convert [35]
Yushan, Taiwan 23°N Template:Convert [42] Strong winds and poor soil restrict further grow of trees.
Hawaii, United States 20°N Template:Convert [35] Geographic isolation and no local tree species with high tolerance to cold temperatures.
Pico de Orizaba, Mexico 19°N Template:Convert [37]
Costa Rica 9.5°N Template:Convert
Mount Kinabalu, Borneo 6.1°N Template:Convert [43]
Mount Kilimanjaro, Tanzania 3°S Template:Convert [35] Upper limit of forest trees; woody ericaeous scrub grows up to 3900m
New Guinea 6°S Template:Convert [35]
Andes, Peru 11°S Template:Convert East side; on west side tree growth is restricted by dryness
Andes, Bolivia 18°S Template:Convert Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana)
Template:Convert Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate)
Sierra de Córdoba, Argentina 31°S Template:Convert Precipitation low above trade winds, also high exposure
Australian Alps, New South Wales, Australia 36°S
Template:Convert Despite the far inland location, summers are cool relative to the latitude, with occasional summer snow; and heavy springtime snowfalls are common[44]
Andes, Laguna del Laja, Chile 37°S Template:Convert Temperature rather than precipitation restricts tree growth[45]
Mount Taranaki, North Island, New Zealand 39°S Template:Convert Strong maritime influence serves to cool summer and restrict tree growth
Northeast Tasmania, Australia 41°S Template:Convert Although sheltered on the leeward side of the island, summers are still cool for the latitude.
Southwest Tasmania, Australia 43°S Template:Convert Exposed to the westerly storm track, summer is extraordinarily cool for the latitude, with frequent summer snow. Springtime receives an extreme amount of cold, heavy precipitation; winds are likewise extreme.
Fiordland, South Island, New Zealand 45°S Template:Convert Very snowy springs, strong cold winds and cool summers with frequent summer snow restrict tree growthScript error: No such module "Unsubst".
Lago Argentino, Argentina 50°S Template:Convert Nothofagus pumilio[46]
Torres del Paine, Chile 51°S Template:Convert Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth[47]
Navarino Island, Chile 55°S Template:Convert Strong maritime influence serves to cool summer and restrict tree growth[47]

Arctic tree lines

File:Canada tree line map.png
Map of tree line in Canada

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.Script error: No such module "Unsubst". 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 Template:Sort 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 Template:Sort 66°N
Central Siberian Plateau Template:Sort 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) Template:Sort 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 Template:Sort 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 Template:Sort 69°N Reaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures.
Nunavut Template:Sort 61°N Influence of the very cold Hudson Bay moves the treeline southwards.
Labrador Peninsula Template:Sort 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 Template:Sort 69°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.Script error: No such module "Unsubst".

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.Script error: No such module "Unsubst".

Antarctic Peninsula is the northernmost point in Antarctica (63°S) and has the mildest weather—it is located Template:Convert 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.Script error: No such module "Unsubst".

File:BeagleChannelGlacier.jpg
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 Template:Convert in sheltered valleys. These trees seldom grow above Template:Convert in height and they get smaller as one gains altitude, so that by Template:Convert 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. Summers are very cold with an average January temperature of Template:Convert. Winters are mild Template:Convert but wet. Macquarie Island (Australia) is located at 54°S and has no vegetation beyond snow grass and alpine grasses and mosses.Script error: No such module "Unsubst".

See also

References

Template:Reflist

Further reading

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