Tree line

The tree line is the edge of a habitat at which trees can grow and beyond which they cannot. 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]: 51 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]: 151[3]: 18}

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").^[4]: 58

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]: 55 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 tree line 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.^[11]

Variation

The tree line elevation at a location is generally set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging,^[6] or grazing^[12] 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 6 °C or 43 °F.^[13][6] 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.^[14]

Because of climate change, which leads to earlier snowmelt 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.^[15]

Other local factors can locally change the elevation of tree line, such as aspect of slope, rain shadow. Tree lines 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.^[16]: 109 In the southern hemisphere, the south-facing slopes have the shorter growing season. On coasts and isolated mountains, the tree line is often much lower than corresponding altitudes inland and in larger, more complex mountain systems. This is known as the Massenerhebung effect, and is caused by large mountain ranges retaining more heat and reducing wind velocity downwind, compared to isolated mountains.^[17] In addition, in some tropical or island localities, the lack of local drought- and cold-adapted species can result in lower tree lines than one might expect by climate alone.^[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.^[2]: 151 The climate above the tree line of mountains is called an alpine climate,^[17]: 21 and the habitat can be described as the alpine zone.^[19]

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".^[20] 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,^[20] to 1.4 °F (0.78 °C) per 1,000 feet (300 m) in the moister mountains of the eastern United States.^[21] Skin effects and topography can create microclimates that alter the general cooling trend.^[22]

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.^[20]

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,^[23] 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.^[24]

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 tree line than the northern faces because of increased sun exposure and aridity. Hawaii's tree line of about 7,900–9,500 ft (2,400–2,900 m) is above a temperature inversion which blocks moisture from reaching the highest slopes.^[18]

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.^[25] 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.^{[citation needed]}

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,^[26] in contrast with the telescoped alpine timberlines.^[20] 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:^[27][28] 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.^[27] The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.^[20][29]

Tree species near tree line

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

Australia

• Snow gum (Eucalyptus pauciflora)

Eurasia

• Dahurian larch (Larix gmelinii)
• Macedonian pine (Pinus peuce)
• Swiss pine (Pinus cembra)
• Mountain pine (Pinus mugo)
• Arctic white birch (Betula pubescens subsp. tortuosa)
• Rowan^[30] (Sorbus aucuparia)

North America

• Subalpine fir (Abies lasiocarpa)^[16]: 106
• Subalpine larch (Larix lyallii)^[31]
• Mountain hemlock (Tsuga mertensiana)
• Alaska yellow cedar (Chaemaecyparis nootkatensis)
• Engelmann spruce (Picea engelmannii)^[16]: 106
• Whitebark pine (Pinus albicaulis)^[31]
• Great Basin bristlecone pine (Pinus longaeva)
• Rocky Mountains bristlecone pine (Pinus aristata)
• Foxtail pine (Pinus balfouriana)
• Limber pine (Pinus flexilis)
• Potosi pinyon (Pinus culminicola)
• Black spruce (Picea mariana)^[1]: 53
• White spruce (Picea glauca)
• Tamarack (Larix laricina)
• Hartweg's pine (Pinus hartwegii)

South America

• Antarctic beech (Nothofagus antarctica)
• Lenga beech (Nothofagus pumilio)^[32]
• Alder (Alnus acuminata)
• Pino del cerro (Podocarpus parlatorei)
• Polylepis (Polylepis tarapacana)
• Eucalyptus (not native to South America but grown in large amounts in the high Andes).^[33]

Worldwide distribution

Alpine tree lines

Averaging over many locations and local microclimates, the tree line 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 tree line is roughly constant, between 3,500 and 4,000 metres (11,500 and 13,100 ft).^[34]

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, 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.^{[citation needed]} In northern inland Scandinavia, there is substantial maritime influence on high parallels that keep winters relatively mild, but with enough inland effect to have summers well above the threshold for the tree line. Here are some typical polar tree lines:

Antarctic tree lines

The southernmost trees in the world are on Isla Hornos (56°S), at the southern tip of South America. Trees do not exist on subantarctic islands nor in Antarctica. Therefore, there is no continental Antarctic tree line: the Southern Ocean acts as a tree boundary.^[47]

The subantarctic islands (South Georgia, Prince Edward, Crozet, Kerguelen, Heard and McDonald, and Macquarie Islands)^[48] lie in the Antarctic Circumpolar Current between 46.4° and 54.6°S. While the climate of these islands is cold and wet with long growing seasons, none of these islands have trees, due to the strong winds of the Roaring Forties and Furious Fifties.^[49][50]

Southern Rata forests exist on Enderby Island and Auckland Islands (both 50°S) and these grow up to an elevation of 370 m (1,200 feet) 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.^[51] The climate on these islands is not severe, but tree growth is limited by almost continual rain and wind. The summers are very cold, with an average January temperature of 9 °C (48 °F), while winters are a mild 5 °C (41 °F) but wet.

See also

• Montane ecosystems
• Ecotone: a transition between two adjacent ecological communities
• Edge effects: the effect of contrasting environments on an ecosystem
• Massenerhebung effect
• Snow line

References

1. ^ Elliott-Fisk, D.L. (2000). "The Taiga and Boreal Forest". North American Terrestrial Vegetation. 2nd ed. Barbour, M.G. & Billings, M.D. (eds.). Cambridge University Press. ISBN 978-0-521-55986-7.
2. ^ Jørgensen, S.E. (2009). Ecosystem Ecology. Academic Press. ISBN 978-0-444-53466-8.
3. ^ Körner, C. (2012). Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Springer. ISBN 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.
5. ^ "Why treelines?"
6. ^ 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
7. ^ 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.
8. ^ Coates, K.D.; Haeussler, S.; Lindeburgh, S; Pojar, R.; Stock, A.J. (1994). Ecology and silviculture of interior spruce in British Columbia. OCLC 66824523
9. ^ 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
10. ^ Viereck, L.A.; Van Cleve, K.; Dyrness, C. T. (1986). "Forest ecosystem distribution in the taiga environment". Forest Ecosystems in the Alaskan Taiga. Van Cleve, K.; Chapin, F.S.; Flanagan, P.W.; Viereck, L.A.; Dyrness, C.T. (eds.). New York, NY: Springer-Verlag. pp. 22–43. doi:10.1007/978-1-4612-4902-3_3. ISBN 978-1-4612-4902-3.
11. ^ Maher, CT; Nelson, CR; Larson, AJ (2019). "Winter damage is more important than summer temperature for maintaining the krummholz growth form above alpine treeline". J Ecol. 108 (3): 1074–1087. doi:10.1111/1365-2745.13315
12. ^ 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
13. ^ 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
14. ^ Paulsen, Jens & Körner, Christian (2014). "A climate-based model to predict potential treeline position around the globe". Alpine Botany. 124 (1): 1–12. Bibcode:2014AlBot.124....1P. doi:10.1007/s00035-014-0124-0. S2CID 8752987
15. ^ "Climate Change Resource Brief - North Cascades National Park". U.S. National Park Service. January 30, 2018. Retrieved May 13, 2025.
16. ^ Peet, R.K. (2000). "Forests and Meadows of the Rocky Mountains". North American Terrestrial Vegetation. 2nd ed. Barbour, M.G. & Billings, M.D. (eds.). Cambridge University Press. ISBN 978-0-521-55986-7.
17. ^ Körner, C (2003). Alpine plant life: functional plant ecology of high mountain ecosystems. Springer. ISBN 978-3-540-00347-2.
18. ^ Leuschner, Christoph (April 1996). "Timberline and Alpine Vegetation on the Tropical and Warm-Temperate Oceanic Islands of the World: Elevation, Structure and Floristics". Vegetatio. 123 (2): 193–206. doi:10.1007/BF00118271
19. ^ "Alpine Tundra Ecosystem". Rocky Mountain National Park. National Park Service. Retrieved 2011-05-13.
20. ^ Arno, S.F. (1984). Timberline: Mountain and Arctic Forest Frontiers. Seattle, WA: The Mountaineers. ISBN 978-0-89886-085-6.
21. ^ Baker, F.S. (1944). "Mountain climates of the western United States". Ecological Monographs. 14 (2): 223–254. Bibcode:1944EcoM...14..223B. doi:10.2307/1943534. JSTOR 1943534
22. ^ Geiger, R. (1950). The Climate near the Ground. Cambridge, MA: Harvard University Press
23. ^ Sowell, J.B.; McNulty, S.P.; Schilling, B.K. (1996). "The role of stem recharge in reducing the winter desiccation of Picea engelmannii (Pinaceae) needles at alpine timberline". American Journal of Botany. 83 (10): 1351–1355. doi:10.2307/2446122. JSTOR 2446122
24. ^ Shaw, C.H. (1909). "The causes of timberline on mountains: the role of snow". Plant World. 12: 169–181.
25. ^ Pienitz, Reinhard; Douglas, Marianne S. V.; Smol, John P. (2004). Long-term environmental change in Arctic and Antarctic lakes. Springer. p. 102. ISBN 978-1-4020-2126-8.
26. ^ Timoney, K.P.; La Roi, G.H.; Zoltai, S.C.; Robinson, A.L. (1992). "The high subarctic forest–tundra of northwestern Canada: position, width, and vegetation gradients in relation to climate". Arctic. 45 (1): 1–9. doi:10.14430/arctic1367. JSTOR 40511186
27. ^ Löve, Dd (1970). "Subarctic and subalpine: where and what?". Arctic and Alpine Research. 2 (1): 63–73. doi:10.2307/1550141. JSTOR 1550141
28. ^ Hare, F. Kenneth & Ritchie, J.C. (1972). "The boreal bioclimates". Geographical Review. 62 (3): 333–365. Bibcode:1972GeoRv..62..333H. doi:10.2307/213287. JSTOR 213287
29. ^ R.A., Black & Bliss, L.C. (1978). "Recovery sequence of Picea mariana–Vaccinium uliginosum forests after burning near Inuvik, Northwest Territories, Canada". Canadian Journal of Botany. 56 (6): 2020–2030. Bibcode:1978CaJB...56.2020B. doi:10.1139/b78-243
30. ^ Chalupa, V. (1992). "Micropropagation of European Mountain Ash (Sorbus aucuparia L.) and Wild Service Tree [Sorbus torminalis (L.) Cr.]". High-Tech and Micropropagation II. Vol. 18. Biotechnology in Agriculture and Forestry. Bajaj, Y.P.S. (ed.). Springer Berlin Heidelberg. pp. 211–226. doi:10.1007/978-3-642-76422-6_11. ISBN 978-3-642-76424-0.
31. ^ "Treeline". The Canadian Encyclopedia. Archived 2010-12-03 at the Wayback Machine. Retrieved 2011-06-22.
32. ^ Fajardo, A; Piper, FI; Cavieres, LA (2011). "Distinguishing local from global climate influences in the variation of carbon status with altitude in a tree line species". Global Ecology and Biogeography. 20 (2): 307–318. Bibcode:2011GloEB..20..307F. doi:10.1111/j.1466-8238.2010.00598.x. hdl:10533/134794
33. ^ Dickinson, Joshua C. (1969). "The Eucalypt in the Sierra of Southern Peru". Annals of the Association of American Geographers. 59 (2): 294–307. doi:10.1111/j.1467-8306.1969.tb00672.x. ISSN 0004-5608. JSTOR 2561632
34. ^ Körner, Ch (1998). "A re-assessment of high elevation treeline positions and their explanation". Oecologia. 115 (4): 445–459. Bibcode:1998Oecol.115..445K. CiteSeerX 10.1.1.454.8501. doi:10.1007/s004420050540. PMID 28308263. S2CID 8647814
35. ^ "Action For Scotland's Biodiversity"
36. ^ Körner, Ch. "High Elevation Treeline Research". Archived 2011-09-27 at the Wayback Machine. Retrieved 2010-06-14.
37. ^ "Physiogeography of the Russian Far East"
38. ^ "Mount Washington State Park". New Hampshire State Parks. Archived 2013-04-03 at the Wayback Machine. Retrieved 2013-08-22. Tree line, the elevation above which trees do not grow, is about 4,400 feet in the White Mountains, nearly 2,000 feet below the summit of Mt. Washington.
39. ^ "Georgia's natural resources and conservation" (in Georgian). geostat.ge. National Statistic Office of Georgia. Retrieved 2023-04-13.
40. ^ Schoenherr, Allan A. (1995). A Natural History of California. UC Press. ISBN 978-0-520-06922-0.
41. ^ "台灣地帶性植被之區劃與植物區系之分區". Archived 2014-11-29 at the Wayback Machine.
42. ^ "Mount Kinabalu National Park". www.ecologyasia.com. Ecology Asia. 4 September 2016. Retrieved 6 September 2016.
43. ^ "Alpine trees | ANU Research School of Biology"
44. ^ Lara, Antonio; Villalba, Ricardo; Wolodarsky-Franke, Alexia; Aravena, Juan Carlos; Luckman, Brian H.; Cuq, Emilio (2005). "Spatial and temporal variation in Nothofagus pumilio growth at tree line along its latitudinal range (35°40′–55° S) in the Chilean Andes". Journal of Biogeography. 32 (5): 879–893. Bibcode:2005JBiog..32..879L. doi:10.1111/j.1365-2699.2005.01191.x. S2CID 51845387
45. ^ Sottile, Gonzalo D.; Echeverría, Marcos E.; Tonello, Marcela S.; Marcos, María A.; Bamonte, Florencia P.; Rayó, Cecilia; Mancini, María V. (2020). "Dinámica de la vegetación andina del lago Argentino (50° S, 72° O) desde el retiro de los glaciares (ca. 12.000 años cal AP)" (in Spanish). Andean Geology. 47 (3): 599–627. Bibcode:2020AndGe..47..599S. doi:10.5027/andgeoV47n3-3303. hdl:11336/141218
46. ^ Aravena, Juan C.; Lara, Antonio; Wolodarsky-Franke, Alexia; Villalba, Ricardo; Cuq, Emilio (2002). "Tree-ring growth patterns and temperature reconstruction from Nothofagus pumilio (Fagaceae) forests at the upper tree line of southern, Chilean Patagonia". Revista Chilena de Historia Natural. 75 (2): 00008. Bibcode:2002RvCHN..7500008A. doi:10.4067/S0716-078X2002000200008. hdl:11336/40918
47. ^ Buma, B; Holz, A; Díaz, IA; Rozzi, R (September 2020). "The world's southernmost tree and the climate and windscapes of the southernmost forests". Ecography. 44 (1): 14–24. doi:10.1111/ecog.05075
48. ^ Smith, VR & Lewis Smith, RI (1987). "The biota and conservation status of subantarctic islands". Environment International. 13 (1): 95–104. Bibcode:1987EnInt..13...95S. doi:10.1016/0160-4120(87)90047-X
49. ^ Fitzgerald, Nicholas B (2019). Vegetation Change on subantarctic Macquarie Island. University of Tasmania.
50. ^ French, DD & Smith, VR (1985). "A comparison between Northern and Southern Hemisphere tundras and related ecosystems". Polar Biology. 5 (1): 5–21. Bibcode:1985PoBio...5....5F. doi:10.1007/BF00446040
51. ^ Morwood, Maddy (4 Sep 2022). "How the world's loneliest tree is helping scientists advance climate change research". Australian Broadcasting Company.

Further reading

• Arno, S.F. & Hammerly, R.P. (1984). Timberline. Mountain and Arctic Forest Frontiers. Seattle: The Mountaineers. ISBN 978-0-89886-085-6.
• Beringer, Jason; Tapper, Nigel J.; McHugh, Ian; Chapin, F. S. III et al. (2001). "Impact of Arctic treeline on synoptic climate". Geophysical Research Letters. 28 (22): 4247–4250. Bibcode:2001GeoRL..28.4247B. doi:10.1029/2001GL012914
• Ødum, S (1979). "Actual and potential tree line in the North Atlantic region, especially in Greenland and the Faroes". Holarctic Ecology. 2 (4): 222–227. Bibcode:1979Ecogr...2..222O. doi:10.1111/j.1600-0587.1979.tb01293.x
• Ødum, S (1991). "Choice of species and origins for arboriculture in Greenland and the Faroe Islands". Dansk Dendrologisk Årsskrift. 9: 3–78.
• Singh, C.P.; Panigrahy, S.; Parihar, J.S.; Dharaiya, N. (2013). "Modeling environmental niche of Himalayan birch and remote sensing based vicarious validation". Tropical Ecology. 54 (3): 321–329. Archived from the original on July 21, 2015.
• Singh, C.P.; Panigrahy, S.; Thapliyal, A.; Kimothi, M.M.; Soni, P.; Parihar, J.S. (2012). "Monitoring the alpine treeline shift in parts of the Indian Himalayas using remote sensing". Current Science. 102 (4): 559–562. Archived 2013-05-16 at the Wayback Machine.
• Panigrahy, Sushma; Singh, C.P.; Kimothi, M.M.; Soni, P.; Parihar, J.S. (2010). "The Upward Migration of Alpine Vegetation as an Indicator of Climate Change: Observations from Indian Himalayan region using Remote Sensing Data". Bulletin of the National Natural Resources Management System. 35: 73–80. Archived from the original on November 24, 2011.
• Singh, C.P. (2008). "Alpine ecosystems in relation to climate change". ISG Newsletter. 14: 54–57.
• Ameztegui, A; Coll, L; Brotons, L; Ninot, JM (2016). "Land-use legacies rather than climate change are driving the recent upward shift of the mountain tree line in the Pyrenees". Global Ecology and Biogeography. 25 (3): 263. Bibcode:2016GloEB..25..263A. doi:10.1111/geb.12407. hdl:10459.1/65151