The tree line is the edge of a

habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...

at which

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...

s are capable of growing and beyond which they are not. It is found at high

elevation The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...

s and high

latitude In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

s. 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 (German for "crooked wood"). 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 The climatic snow line is the boundary between a snow-covered and snow-free surface. The actual snow line may adjust seasonally, and be either significantly higher in elevation, or lower. The permanent snow line is the level above which snow wil ...

and roughly parallel to it.

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

conifers Conifers () are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All e ...

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 Fairbanks is a Municipal home rule, home rule city and the county seat, borough seat of the Fairbanks North Star Borough, Alaska, United States. Fairbanks is the largest city in the Interior Alaska, interior region of Alaska and the second la ...

. 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 A leaf (: leaves) is a principal appendage of the stem of a vascular plant, usually borne laterally above ground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, stem, f ...

, especially of shoots that project above the 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 Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucks 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 Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

, which leads to earlier snow melt and favorable conditions for tree establishment, the tree line in

North Cascades National Park North Cascades National Park is a List of national parks of the United States, national park of the United States in Washington (state), Washington. At more than , it is the largest of the three National Park Service units that comprise the No ...

has risen more than in 50 years.

Types

Several types of tree lines are defined in

ecology Ecology () is the natural science of the relationships among living organisms and their Natural environment, environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosystem, and biosphere lev ...

and

geography Geography (from Ancient Greek ; combining 'Earth' and 'write', literally 'Earth writing') is the study of the lands, features, inhabitants, and phenomena of Earth. Geography is an all-encompassing discipline that seeks an understanding o ...

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

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher t ...

s is called an

alpine climate Alpine climate is the typical climate for elevations above the tree line, where trees fail to grow due to cold. This climate is also referred to as a mountain climate or highland climate. Definition There are multiple definitions of alpine cli ...

, and the habitat can be described as the

alpine zone Alpine tundra is a type of natural region or biome that does not contain trees because it is at high elevation, with an associated alpine climate, harsh climate. As the latitude of a location approaches the poles, the threshold elevation for alp ...

. 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 Snowpack is an accumulation of snow that compresses with time and melts seasonally, often at high elevation or high latitude. Snowpacks are an important water resource that feed streams and rivers as they melt, sometimes leading to flooding. Snow ...

takes longer to melt. This shortens the growing season for trees. 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

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

es in central Mexico, and on mountains in each of the 11 western states and throughout much of Canada and Alaska". Environmentally dwarfed shrubs (''

krummholz ''Krummholz'' (, "crooked, bent, twisted" and ''Holz'', "wood") — also called ''knieholz'' ("knee timber") — is a type of stunted, deformed vegetation encountered in the subarctic and subalpine tree line landscapes, shaped by continual e ...

'') 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 Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

can create

microclimate A microclimate (or micro-climate) is a local set of atmosphere of Earth, atmospheric conditions that differ from those in the surrounding areas, often slightly but sometimes substantially. The term may refer to areas as small as a few square m ...

s 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 The Selkirk Mountains are a mountain range spanning the northern portion of the Idaho Panhandle, eastern Washington, and southeastern British Columbia which are part of a larger grouping of mountains, the Columbia Mountains. They begin at Mic ...

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 Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

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.

Exposure

coast A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake. Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, su ...

s and isolated mountains, the tree line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because strong

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

s 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

The Arctic tree line is the northernmost

in the

Northern Hemisphere The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined by humans as being in the same celestial sphere, celestial hemisphere relative to the invariable plane of the Solar ...

where trees can grow; farther north, it is too cold all year round to sustain trees. Extremely low temperatures, especially when prolonged, can freeze the internal sap of trees, killing them. In addition,

permafrost Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...

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 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 In physical geography, a tundra () is a type of biome where tree growth is hindered by frigid temperatures and short growing seasons. There are three regions and associated types of tundra: #Arctic, Arctic, Alpine tundra, Alpine, and #Antarctic ...

, and southwards lies the

boreal forest Taiga or tayga ( ; , ), also known as boreal forest or snow forest, is a biome characterized by pinophyta, coniferous forests consisting mostly of pines, spruces, and larches. The taiga, or boreal forest, is the world's largest land biome. I ...

. 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.

lichen A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...

s. The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.

Antarctic

A southern treeline exists in the

New Zealand Subantarctic Islands The New Zealand Subantarctic Islands comprise the five southernmost groups of the New Zealand outlying islands. They are collectively designated as a UNESCO World Heritage Site. Most of the islands lie near the southeast edge of the large ...

and the Australian

Macquarie Island Macquarie Island is a subantarctic island in the south-western Pacific Ocean, about halfway between New Zealand and Antarctica. It has been governed as a part of Tasmania, Australia, since 1880. It became a Protected areas of Tasmania, Tasmania ...

, 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 The Magellanic subpolar forests () are a terrestrial ecoregion of southernmost South America, covering parts of southern Chile and Argentina, and are part of the Neotropical realm. It is a temperate broadleaf and mixed forests ecoregion, and ...

ecoregion, where the forest merges into the subantarctic tundra (termed Magellanic moorland or Magellanic tundra). For example, the northern halves of Hoste and

Navarino Navarino or Navarin may refer to: Battle * Battle of Navarino, 1827 naval battle off Navarino, Greece, now known as Pylos Geography * Navarino is the former name of Pylos, a Greek town on the Ionian Sea, where the 1827 battle took place ** Old Na ...

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

Australia

* Snow gum (''Eucalyptus pauciflora'')

Eurasia

* Dahurian larch (''Larix gmelinii'') * Macedonian pine (''Pinus peuce'') * Swiss pine (''Pinus cembra'') *

Mountain pine ''Pinus mugo'', known as dwarf mountain pine, mountain pine, scrub mountain pine, Swiss mountain pine, bog pine, creeping pine, or mugo pine, is a species of conifer, native to high elevation habitats from southwestern to Central Europe and So ...

(''Pinus mugo'') * Arctic white birch (''Betula pubescens'' subsp. ''tortuosa'') *

Rowan The rowans ( or ) or mountain-ashes are shrubs or trees in the genus ''Sorbus'' of the rose family, Rosaceae. They are native throughout the cool temperate regions of the Northern Hemisphere, with the highest species diversity in the Himalaya ...

(''Sorbus aucuparia'')

North America

* Subalpine fir (''Abies lasiocarpa'') * Subalpine larch (''Larix lyallii'') * Mountain hemlock (''Tsuga mertensiana'') * Alaska yellow cedar (''Chaemaecyparis nootkatensis'') *

Engelmann spruce ''Picea engelmannii'', with the common names Engelmann spruce, white spruce, mountain spruce, and silver spruce, is a species of spruce native to western North America. It is highly prized for producing distinctive tone wood for acoustic guitars ...

(''Picea engelmannii'') * Whitebark pine (''Pinus albicaulis'') * Great Basin bristlecone pine (''Pinus longaeva'') * Rocky Mountains bristlecone pine (''Pinus aristata'') *

Foxtail pine ''Pinus balfouriana'', the foxtail pine, is a rare high-elevation pine that is endemic to California, United States. It is closely related to the Great Basin and Rocky Mountain bristlecone pines, in the subsection ''Balfourianae''. Descriptio ...

(''Pinus balfouriana'') * Limber pine (''Pinus flexilis'') *

Potosi pinyon ''Pinus culminicola'', commonly known as Potosí pinyon or Potosí Piñón, is a pine in the pinyon pine group, native and endemic to northeast Mexico. The range is highly localised, confined to a small area of high summits in the northern Sier ...

(''Pinus culminicola'') * Black spruce (''Picea mariana'') *

White spruce White spruce is a common name for several species of spruce (''Picea'') and may refer to: * '' Picea engelmannii'', native to the Rocky Mountains and Cascade Mountains of the United States and Canada * ''Picea glauca ''Picea glauca'', the whi ...

(''Picea glauca'') * Tamarack (''Larix laricina'') * Hartweg's pine (''Pinus hartwegii'')

South America

* Antarctic beech (''Nothofagus antarctica'') * Lenga beech (''Nothofagus pumilio'') *

Alder Alders are trees of the genus ''Alnus'' in the birch family Betulaceae. The genus includes about 35 species of monoecious trees and shrubs, a few reaching a large size, distributed throughout the north temperate zone with a few species ex ...

(''Alnus acuminata'') * Pino del cerro (''Podocarpus parlatorei'') * Polylepis (''Polylepis tarapacana'') *

Eucalyptus ''Eucalyptus'' () is a genus of more than 700 species of flowering plants in the family Myrtaceae. Most species of ''Eucalyptus'' are trees, often Mallee (habit), mallees, and a few are shrubs. Along with several other genera in the tribe Eucalyp ...

(not native to South America but grown in large amounts in the high Andes).

Worldwide distribution

Alpine tree lines

The alpine tree line at a location is dependent on local variables, such as aspect of slope,

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from body of water, bodies of water (such as oceans and larg ...

and proximity to either

geographical pole A geographical pole or geographic pole is either of the two points on Earth where its axis of rotation intersects its surface. The North Pole lies in the Arctic Ocean while the South Pole is in Antarctica. North and South poles are also defined ...

. In addition, in some tropical or island localities, the lack of biogeographical access to species that have evolved in a

subalpine Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures fall as elevation increases, causing the ecosystem to stratify. This stratification is a crucial f ...

environment can result in lower tree lines than one might expect by climate alone. Averaging over many locations and local

s, 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:

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,

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 In forestry, windthrow refers to trees uprooted by wind. Breakage of the tree bole (botany), bole (trunk) instead of uprooting is called windsnap. Blowdown refers to both windthrow and windsnap. Causes Windthrow is common in all forested ...

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 An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours ...

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 Scandinavia is a subregion#Europe, subregion of northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. ''Scandinavia'' most commonly refers to Denmark, Norway, and Sweden. It can sometimes also ...

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:

Antarctic tree lines

Trees exist on

Tierra del Fuego Tierra del Fuego (, ; Spanish for "Land of Fire", rarely also Fireland in English) is an archipelago off the southernmost tip of the South America, South American mainland, across the Strait of Magellan. The archipelago consists of the main is ...

(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 South Georgia is an island in the Atlantic Ocean, South Atlantic Ocean that is part of the British Overseas Territories, British Overseas Territory of South Georgia and the South Sandwich Islands. It lies around east of the Falkland Islands. ...

(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 The Falkland Islands (; ), commonly referred to as The Falklands, is an archipelago in the South Atlantic Ocean on the Patagonian Shelf. The principal islands are about east of South America's southern Patagonian coast and from Cape Dub ...

(51°S) summer temperature is near the limit, but the islands are also treeless, although some planted trees exist.

Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martin in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctica. ...

is the northernmost point in Antarctica (63°S) and has the mildest weather—it is located from

Cape Horn Cape Horn (, ) is the southernmost headland of the Tierra del Fuego archipelago of southern Chile, and is located on the small Hornos Island. Although not the most southerly point of South America (which is Águila Islet), Cape Horn marks the nor ...

—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. BeagleChannelGlacier

Southern Rata forests exist on

Enderby Island Enderby Island is part of New Zealand's uninhabited Auckland Islands archipelago, south of mainland New Zealand. It is situated just off the northern tip of Auckland Island, the largest island in the archipelago. Geography and geology Enderby ...

and

Auckland Islands The Auckland Islands ( Māori: ''Motu Maha'' "Many islands" or ''Maungahuka'' "Snowy mountains") are an archipelago of New Zealand, lying south of the South Island. The main Auckland Island, occupying , is surrounded by smaller Adams Island ...

(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.

(Australia) is located at 54°S and has no vegetation beyond snow grass and alpine grasses and mosses.