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The tree line is the edge of the habitat at which trees are capable of growing. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures or lack of moisture).:51 The tree line should not be confused with a lower timberline or forest line, which is often defined as the line where trees form a forest with a closed canopy.:151:18
At the tree line, tree growth is often very stunted, with the last trees forming low, densely matted bushes. If it is caused by wind, it is known as krummholz formation, from the German for 'twisted wood'.: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.:55
The highest elevation that sustains trees; higher up, it is too cold or snow cover persists for too much of the year to sustain trees.:151 Usually associated with mountains, the climate above the tree line is called an alpine climate,:21 and the terrain can be described as alpine tundra. In the northern hemisphere treelines on north-facing slopes are lower than on south-facing slopes because increased shade means the snowpack takes longer to melt which shortens the growing season for trees.:109 This is reversed in the southern hemisphere.
The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees. 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. The desert treeline tends to be higher on pole-facing slopes than equator-facing slopes, because the increased shade on a pole-facing slope keeps those slopes cooler and prevents moisture from evaporating as quickly, giving trees a longer growing season and more access to water.
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.
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.:37
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.
The northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold all year round to sustain trees. Extremely cold temperatures, especially when prolonged, can result in freezing of 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.
A southern treeline exists in the New Zealand Subantarctic Islands and the Australian Macquarie Island, with places where mean annual temperature above 5°C support trees and woody plants, and those below 5°C don't. 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). For example, the northern half of Hoste Island has a Nothofagus antarctica forest but the southern part is tundra.
The immediate environment is too extreme for trees to grow. This can be caused by 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 areas. 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.
Some typical Arctic and alpine tree line tree species (note the predominance of conifers):
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).
Here is a list of approximate tree lines from locations around the globe:
|Location||Approx. latitude||Approx. elevation of tree line||Notes|
|Finnmarksvidda, Norway||69°N||500||1,600||At 71°N, near the coast, the tree-line is below sea level (Arctic tree line).|
|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:85|
|Olympic Mountains WA, USA||47°N||1,500||4,900||Heavy winter snowpack buries young trees until late summer|
|Mount Katahdin, Maine, USA||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|
|Alps of Piedmont, Northwestern Italy||45°N||2,100||6,900|
|New Hampshire, USA||44°N||1,350||4,400|| Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua.|
|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, USA||40°N||2,900||9,500||Higher (nearly 11,000 feet or 3,400 metres in the Uintas)|
|Rocky Mountain NP, CO, USA||40°N||3,550||11,600|| On warm southwest slopes|
|3,250||10,700||On northeast slopes|
|Yosemite, CA, USA||38°N||3,200||10,500|| West side of Sierra Nevada|
|3,600||11,800|| East side of Sierra Nevada|
|Sierra Nevada, Spain||37°N||2,400||7,900||Precipitation low in summer|
|Hawaii, USA||20°N||3,000||9,800|| Geographic isolation and no local tree species with high tolerance to cold temperatures.|
|Pico de Orizaba, Mexico||19°N||4,000||13,100|||
|Mount Kilimanjaro, Tanzania||3°S||3,950||13,000|||
|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|
|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|
|Navarino Island, Chile||55°S||600||2,000||Strong maritime influence serves to cool summer and restrict tree growth|
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. 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 mild 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 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.|
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), Île Saint-Paul (38°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.
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.
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