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The tree line is the edge of a habitat at which are capable of growing and beyond which they are not. It is found at high and high . Beyond the tree line, trees cannot tolerate the environmental conditions (usually low temperatures, extreme snowpack, or associated lack of available moisture). The tree line is sometimes distinguished from a lower timberline, which is the line below which trees form a forest with a closed canopy.
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"). (1996). 9781555661717, Big Earth Publishing. ISBN 9781555661717
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. Given a certain latitude, the tree line is approximately 300 to 1000 meters below the permanent snow line and roughly parallel to it.
The actual tree line is 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 . The seasonal mean temperature is taken over all days whose mean temperature is above . A growing season of 94 days above that temperature is required for tree growth.
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 in 50 years.
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 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 per of elevation gain in the dry mountains of the western United States, to per in the moister mountains of the eastern United States. Skin effects and topography can create 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 ) 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 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 treeline than the northern faces because of increased sun exposure and aridity. Hawaii's treeline of about is also above the condensation zone and results due to a lack of moisture.
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 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 Taiga.
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. . The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.
Averaging over many locations and local , the treeline rises when moving 1 degree south from 70 to 50°N, and per degree from 50 to 30°N. Between 30°N and 20°S, the treeline is roughly constant, between .
Here is a list of approximate tree lines from locations around the globe:
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 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.
Southern Metrosideros forests exist on Enderby Island and Auckland Islands (both 50°S) and these grow up to an elevation of in sheltered valleys. These trees seldom grow above in height and they get smaller as one gains altitude, so that by 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. Summers are very cold with an average January temperature of . Winters are mild but wet. Macquarie Island (Australia) is located at 54°S and has no vegetation beyond snow grass and alpine grasses and mosses.
Causes
Due to their vertical structure, trees are more susceptible to cold than more ground-hugging forms of plants. Summer warmth generally sets the limit to which tree growth can occur: while tree line Pinophyta are very frost-hardy during most of the year, they become sensitive to just 1 or 2 degrees of frost in mid-summer. 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. (1986). 9781461249023, Springer-Verlag. ISBN 9781461249023 Survival depends on a sufficiency of new growth to support the tree. Wind can flag tree, including blasting with windborne particles, and may also contribute to the desiccation of leaf, especially of shoots that project above the snow cover.
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 is called an alpine climate, and the habitat can be described as the alpine zone. 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. In the southern hemisphere, the south-facing slopes have the shorter growing season.
Exposure
On and isolated mountains, the tree line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because strong reduce tree growth. In addition, the lack of suitable soil, such as along scree or exposed rock formations, prevents trees from gaining an adequate foothold and exposes them to drought and sun.
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. (2025). 9781402021268, Springer. ISBN 9781402021268 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.
Antarctic
A southern treeline exists in the New Zealand Subantarctic Islands and the Australian Macquarie Island, with places where mean annual temperatures above support trees and woody plants, and those below do not.
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). For example, the northern halves of Hoste Island and Navarino Island Islands have Nothofagus antarctica forests but the southern parts consist of moorlands and tundra.
Tree species near tree line
Some typical Arctic and alpine tree line tree species (note the predominance of Pinophyta):
Australia
Eurasia
North America
South America
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 zone environment can result in lower tree lines than one might expect by climate alone.
Finnmarksvidda, Norway 69°N At 71°N, near the coast, the tree-line is below sea level (Arctic tree line). Abisko, Sweden 68°N Chugach Mountains, Alaska 61°N Tree line around or lower in coastal areas Southern Norway 61°N Much lower near the coast, down to . Scotland, United Kingdom 57°N Strong maritime influence serves to cool summer and restrict tree growth Northern Quebec 56°N 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 Ural Mountains 55°N Canadian Rockies 51°N Tatra Mountains 49°N Olympic Mountains, Washington, United States 47°N Heavy winter snowpack buries young trees until late summer Swiss Alps 47°N Mount Katahdin, Maine, United States 46°N Eastern Alps, Austria, Italy 46°N More exposure to Siberian High than Western Alps Sikhote-Alin, Russia 46°N Alps of Piedmont, Northwestern Italy 45°N New Hampshire, United States 44°N 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 Caucasus Mountains 42°N Rila and Pirin Mountains, Bulgaria 42°N Up to on favorable locations. Mountain Pine is the most common tree line species. Pyrenees Spain, France, Andorra 42°N Mountain Pine is the tree line species Steens Mountain, Oregon, US 42°N Wasatch Mountains, Utah, United States 40°N Higher (nearly in the Uintas) Rocky Mountain NP, Colorado, United States 40°N On warm southwest slopes On northeast slopes Yosemite, California, United States 38°N West side of Sierra Nevada East side of Sierra Nevada Sierra Nevada, Spain 37°N Precipitation low in summer Japanese Alps 36°N Khumbu, Himalaya 28°N Yushan, Taiwan 23°N Strong winds and poor soil restrict further grow of trees. Hawaii, United States 20°N Geographic isolation and no local tree species with high tolerance to cold temperatures. Pico de Orizaba, Mexico 19°N Costa Rica 9.5°N Mount Kinabalu, Borneo 6.1°N Mount Kilimanjaro, Tanzania 3°S Upper limit of forest trees; woody ericaeous scrub grows up to 3900m New Guinea 6°S Andes, Peru 11°S East side; on west side tree growth is restricted by dryness Andes, Bolivia 18°S Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana) Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate) Sierra de Córdoba, Argentina 31°S Precipitation low above trade winds, also high exposure Australian Alps, New South Wales, Australia 36°S Despite the far inland location, summers are cool relative to the latitude, with occasional summer snow; and heavy springtime snowfalls are common Andes, Laja Lake, Chile 37°S Temperature rather than precipitation restricts tree growth Mount Taranaki, North Island, New Zealand 39°S Strong maritime influence serves to cool summer and restrict tree growth Northeast Tasmania, Australia 41°S Although sheltered on the leeward side of the island, summers are still cool for the latitude. Southwest Tasmania, Australia 43°S Exposed to the Roaring Forties, 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 Very snowy springs, strong cold winds and cool summers with frequent summer snow restrict tree growth Lago Argentino, Argentina 50°S Nothofagus pumilio Torres del Paine, Chile 51°S Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth Navarino Island, Chile 55°S Strong maritime influence serves to cool summer and restrict tree growth
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:
Norway 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 66°N Central Siberian Plateau 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) 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 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 69°N Reaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures. Nunavut 61°N Influence of the very cold Hudson Bay moves the treeline southwards. Labrador Peninsula 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 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.
See also
Further reading
(1984). 9780898860856, The Mountaineers. ISBN 9780898860856
[[Category:Trees|Line]]
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