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Tree line
Edge of the habitat at which trees are capable of growing

The tree line marks the edge of a habitat where trees can grow, typically found at high elevations and latitudes. Beyond this line, harsh conditions such as low temperatures and extreme snowpack prevent tree survival. Unlike the tree line, the timberline is the lower boundary where trees form a forest with a closed canopy. Near the tree line, trees often grow sparsely and are stunted by wind and cold, a form called krummholz. The transition can be gradual, with trees becoming shorter and less dense until they disappear. The tree line generally lies 300 to 1000 meters below and roughly parallel to the permanent snow line.

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Causes

Due to their vertical structure, trees are more susceptible to cold than more ground-hugging forms of plants.7 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.89 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.1011 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.

The actual tree line is set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging,12 or grazing13 Most human activities cannot change the actual tree line, unless they affect the climate.14 The tree line follows the line where the seasonal mean temperature is approximately 6 °C or 43 °F.1516 The seasonal mean temperature is taken over all days whose mean temperature is above 0.9 °C (33.6 °F). A growing season of 94 days above that temperature is required for tree growth.17

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 400 feet (120 m) in 50 years.18

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.19: 151  The climate above the tree line of mountains is called an alpine climate,20: 21  and the habitat can be described as the alpine zone.21 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.22: 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 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".23 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 3.5 °F (1.9 °C) per 1,000 feet (300 m) of elevation gain in the dry mountains of the western United States,24 to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the eastern United States.25 Skin effects and topography can create microclimates that alter the general cooling trend.26

Compared with arctic tree lines, alpine tree lines may receive fewer than half of the number of degree days (above 10 °C (50 °F)) 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.27

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,28 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 400 metres (1,300 ft) lower than on exposed intervening shoulders.29

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 8,000 ft (2,400 m) is also above the condensation zone and results due to a lack of moisture.

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

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.30 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.

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 145 kilometres (90 mi) and widening markedly from west to east,31 in contrast with the telescoped alpine timberlines.32 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:3334 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.35 The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.3637

Antarctic

Further information: Antipodes Subantarctic Islands tundra and Tierra del Fuego

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) do not.38 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).39 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

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

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 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 (245 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).48

Here is a list of approximate tree lines from locations around the globe:

LocationApprox. latitudeApprox. elevation of tree lineNotes
(m)(ft)
Finnmarksvidda, Norway69°N5001,600At 71°N, near the coast, the tree-line is below sea level (Arctic tree line).
Abisko, Sweden68°N6502,10049
Chugach Mountains, Alaska61°N7002,300Tree line around 1,500 feet (460 m) or lower in coastal areas
Southern Norway61°N1,1003,600Much lower near the coast, down to 500–600 metres (1,600–2,000 ft).
Scotland, United Kingdom57°N5001,600Strong maritime influence serves to cool summer and restrict tree growth50: 79 
Northern Quebec56°N00The 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 Urals55°N1,1003,600
Canadian Rockies51°N2,4007,900
Tatra Mountains49°N1,6005,200
Olympic Mountains, Washington, United States47°N1,5004,900Heavy winter snowpack buries young trees until late summer
Swiss Alps47°N2,2007,20051
Mount Katahdin, Maine, United States46°N1,1503,800
Eastern Alps, Austria, Italy46°N1,7505,700More exposure to cold Russian winds than Western Alps
Sikhote-Alin, Russia46°N1,6005,20052
Alps of Piedmont, Northwestern Italy45°N2,1006,900
New Hampshire, United States44°N1,3504,40053 Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua.
Wyoming, United States43°N3,0009,800
Caucasus Mountains42°N2,4007,90054
Rila and Pirin Mountains, Bulgaria42°N2,3007,500Up to 2,600 m (8,500 ft) on favorable locations. Mountain Pine is the most common tree line species.
Pyrenees Spain, France, Andorra42°N2,3007,500Mountain Pine is the tree line species
Steens Mountain, Oregon, US42°N2,5008,200
Wasatch Mountains, Utah, United States40°N2,9009,500Higher (nearly 11,000 feet or 3,400 metres in the Uintas)
Rocky Mountain NP, CO, United States40°N3,55011,60055 On warm southwest slopes
3,25010,700On northeast slopes
Yosemite, CA, United States38°N3,20010,50056 West side of Sierra Nevada
3,60011,80057 East side of Sierra Nevada
Sierra Nevada, Spain37°N2,4007,900Precipitation low in summer
Japanese Alps36°N2,9009,500
Khumbu, Himalaya28°N4,20013,80058
Yushan, Taiwan23°N3,60011,80059 Strong winds and poor soil restrict further grow of trees.
Hawaii, United States20°N3,0009,80060 Geographic isolation and no local tree species with high tolerance to cold temperatures.
Pico de Orizaba, Mexico19°N4,00013,10061
Costa Rica9.5°N3,40011,200
Mount Kinabalu, Borneo6.1°N3,40011,20062
Mount Kilimanjaro, Tanzania3°S3,10010,20063 Upper limit of forest trees; woody ericaeous scrub grows up to 3900m
New Guinea6°S3,85012,60064
Andes, Peru11°S3,90012,800East side; on west side tree growth is restricted by dryness
Andes, Bolivia18°S5,20017,100Western Cordillera; highest treeline in the world on the slopes of Sajama Volcano (Polylepis tarapacana)
4,10013,500Eastern Cordillera; treeline is lower because of lower solar radiation (more humid climate)
Sierra de Córdoba, Argentina31°S2,0006,600Precipitation low above trade winds, also high exposure
Australian Alps, New South Wales, Australia36°S
1,8005,900Despite the far inland location, summers are cool relative to the latitude, with occasional summer snow; and heavy springtime snowfalls are common65
Andes, Laguna del Laja, Chile37°S1,6005,200Temperature rather than precipitation restricts tree growth66
Mount Taranaki, North Island, New Zealand39°S1,5004,900Strong maritime influence serves to cool summer and restrict tree growth
Northeast Tasmania, Australia41°S1,2003,900Although sheltered on the leeward side of the island, summers are still cool for the latitude.
Southwest Tasmania, Australia43°S7502,500Exposed 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 Zealand45°S9503,100Very snowy springs, strong cold winds and cool summers with frequent summer snow restrict tree growth
Lago Argentino, Argentina50°S1,0003,300Nothofagus pumilio67
Torres del Paine, Chile51°S9503,100Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth68
Navarino Island, Chile55°S6002,000Strong maritime influence serves to cool summer and restrict tree growth69

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:

LocationApprox. longitudeApprox. latitude of tree lineNotes
Norway24°E70°NThe 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 Plain75°E66°N
Central Siberian Plateau102°E72°NExtreme 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°E60°NThe Oyashio Current and strong winds affect summer temperatures to prevent tree growth. The Aleutian Islands are almost completely treeless.
Alaska, United States152°W68°NTrees grow north to the south-facing slopes of the Brooks Range. The mountains block cold air coming off of the Arctic Ocean.
Northwest Territories, Canada132°W69°NReaches north of the Arctic Circle because of the continental nature of the climate and warmer summer temperatures.
Nunavut95°W61°NInfluence of the very cold Hudson Bay moves the treeline southwards.
Labrador Peninsula72°W56°NVery 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.
Greenland50°W69°NDetermined 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.

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—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 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 between 600 and 800 hours of sun annually. Campbell Island (52°S) further south is treeless, except for one stunted Spruce, probably planted in 1907.70 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.

See also

Further reading

References

  1. Elliott-Fisk, D.L. (2000). "The Taiga and Boreal Forest". In Barbour, M.G.; Billings, M.D. (eds.). North American Terrestrial Vegetation (2nd ed.). Cambridge University Press. ISBN 978-0-521-55986-7. 978-0-521-55986-7 ↩

  2. Jørgensen, S.E. (2009). Ecosystem Ecology. Academic Press. ISBN 978-0-444-53466-8. 978-0-444-53466-8 ↩

  3. Körner, C. (2012). Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Illustrated by S. Riedl. Springer. ISBN 978-3-0348-0396-0. 978-3-0348-0396-0 ↩

  4. Zwinger, A.; Willard, B.E. (1996). Land Above the Trees: A Guide to American Alpine Tundra. Big Earth Publishing. ISBN 978-1-55566-171-7. 978-1-55566-171-7 ↩

  5. Zwinger, A.; Willard, B.E. (1996). Land Above the Trees: A Guide to American Alpine Tundra. Big Earth Publishing. ISBN 978-1-55566-171-7. 978-1-55566-171-7 ↩

  6. "Why treelines?". https://www.geo.uzh.ch/microsite/alpecole/static/course/lessons/10/10c.htm ↩

  7. Körner, Christian (November 1, 2021). "The cold range limit of trees". Trends in Ecology & Evolution. 36 (11): 979–989. Bibcode:2021TEcoE..36..979K. doi:10.1016/j.tree.2021.06.011. PMID 34272073. S2CID 235999977. /wiki/Bibcode_(identifier) ↩

  8. Tranquillini, W. (1979). Physiological Ecology of the Alpine Timberline: tree existence at high altitudes with special reference to the European Alps. New York, NY: Springer-Verlag. ISBN 978-3-642-67107-4. 978-3-642-67107-4 ↩

  9. Coates, K.D.; Haeussler, S.; Lindeburgh, S; Pojar, R.; Stock, A.J. (1994). Ecology and silviculture of interior spruce in British Columbia. OCLC 66824523. /wiki/OCLC_(identifier) ↩

  10. Viereck, L.A. (1979). "Characteristics of treeline plant communities in Alaska". Holarctic Ecology. 2 (4): 228–238. Bibcode:1979Ecogr...2..228V. doi:10.1111/j.1600-0587.1979.tb01294.x. JSTOR 3682417. /wiki/Ecography ↩

  11. 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. (eds.). Forest Ecosystems in the Alaskan Taiga. New York, NY: Springer-Verlag. pp. 22–43. doi:10.1007/978-1-4612-4902-3_3. ISBN 978-1-4612-4902-3. 978-1-4612-4902-3 ↩

  12. Körner, Christian (November 1, 2021). "The cold range limit of trees". Trends in Ecology & Evolution. 36 (11): 979–989. Bibcode:2021TEcoE..36..979K. doi:10.1016/j.tree.2021.06.011. PMID 34272073. S2CID 235999977. /wiki/Bibcode_(identifier) ↩

  13. Wang, Xiaoyi; Wang, Tao (2022). "Enhanced habitat loss of the Himalayan endemic flora driven by warming-forced upslope tree expansion". Nature Ecology & Evolution. 6 (7): 890–899. Bibcode:2022NatEE...6..890W. doi:10.1038/s41559-022-01774-3. PMID 35654898. /wiki/Bibcode_(identifier) ↩

  14. Körner, Christian (November 1, 2021). "The cold range limit of trees". Trends in Ecology & Evolution. 36 (11): 979–989. Bibcode:2021TEcoE..36..979K. doi:10.1016/j.tree.2021.06.011. PMID 34272073. S2CID 235999977. /wiki/Bibcode_(identifier) ↩

  15. Körner, Christian; Paulsen, Jens (May 2004). "A World-Wide Study of High Altitude Treeline Temperatures". J. Biogeogr. 31 (5): 713–732. Bibcode:2004JBiog..31..713K. doi:10.1111/j.1365-2699.2003.01043.x. JSTOR 3554841. S2CID 59025355. /wiki/Bibcode_(identifier) ↩

  16. Körner, Christian (November 1, 2021). "The cold range limit of trees". Trends in Ecology & Evolution. 36 (11): 979–989. Bibcode:2021TEcoE..36..979K. doi:10.1016/j.tree.2021.06.011. PMID 34272073. S2CID 235999977. /wiki/Bibcode_(identifier) ↩

  17. Paulsen, Jens; Körner, Christian (2014). "A climate-based model to predict potential treeline position around the globe" (PDF). Alpine Botany. 124 (1): 1–12. Bibcode:2014AlBot.124....1P. doi:10.1007/s00035-014-0124-0. S2CID 8752987. http://doc.rero.ch/record/324784/files/35_2014_Article_124.pdf ↩

  18. "Climate Change Resource Brief - North Cascades National Park". U.S. National Park Service. January 30, 2018. Retrieved May 13, 2025. https://www.nps.gov/noca/learn/nature/climate-change-resource-brief.htm#:~:text=Forest%20line%20has%20moved%20up,pack%2C%20and%20earlier%20snow%20melt. ↩

  19. Jørgensen, S.E. (2009). Ecosystem Ecology. Academic Press. ISBN 978-0-444-53466-8. 978-0-444-53466-8 ↩

  20. Körner, C (2003). Alpine plant life: functional plant ecology of high mountain ecosystems. Springer. ISBN 978-3-540-00347-2. 978-3-540-00347-2 ↩

  21. "Alpine Tundra Ecosystem". Rocky Mountain National Park. National Park Service. Retrieved 2011-05-13. https://www.nps.gov/romo/learn/nature/alpine_tundra_ecosystem.htm ↩

  22. Peet, R.K. (2000). "Forests and Meadows of the Rocky Mountains". In Barbour, M.G.; Billings, M.D. (eds.). North American Terrestrial Vegetation (2nd ed.). Cambridge University Press. ISBN 978-0-521-55986-7. 978-0-521-55986-7 ↩

  23. Arno, S.F. (1984). Timberline: Mountain and Arctic Forest Frontiers. Seattle, WA: The Mountaineers. ISBN 978-0-89886-085-6. 978-0-89886-085-6 ↩

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