Tree line

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Tree line above St. Moritz, Switzerland. May 2009
In this view of an alpine tree line, the distant line looks particularly sharp. The foreground shows the transition from trees to no trees. These trees are stunted in growth and one-sided because of cold and constant wind.

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).[1]: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.[2]:151[3]: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'.[4]: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.[4]:55

Types[edit]

There are several types of tree lines defined in ecology and geology:

Alpine[edit]

An alpine tree line in the Tararua Range

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.[2]:151 Usually associated with mountains, the climate above the tree line is called an alpine climate,[5]:21 and the terrain can be described as alpine tundra.[6] 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.[7]:109 This is reversed in the southern hemisphere.

Desert[edit]

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.[8] The desert treeline tends to be lower 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.

Desert-alpine[edit]

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.

Double[edit]

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.[9]:37

Exposure[edit]

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[edit]

The northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold all year round to sustain trees.[10] 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.

Antarctic[edit]

The southernmost latitude in the Southern Hemisphere where trees can grow; further south, it is too cold to sustain trees. It is a theoretical concept that does not have any defined location. No trees grow in Antarctica or the subantarctic islands. This tree line would be the southernmost point in the environment at which trees can no longer grow, except there are no landmasses that have a true treeline analogous to the arctic treeline.

Other[edit]

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.

Typical vegetation[edit]

Severe winter climate conditions at alpine tree line causes stunted krummholz growth. Karkonosze, Poland.
Dahurian Larch growing close to the Arctic tree line in the Kolyma region, Arctic northeast Siberia.

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

Eurasia[edit]

North America[edit]

South America[edit]

View of a Magellanic Lenga forest close to the tree line in Torres del Paine National Park, Chile.

Australia[edit]

Worldwide distribution[edit]

Alpine tree lines[edit]

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).[14]

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

Location Approx. latitude Approx. elevation of tree line Notes
(m) (ft)
Finnmarksvidda, Norway 69°N 500 1,600 At 71°N, near the coast, the tree-line is below sea level (Arctic tree line).
Abisko, Sweden 68°N 650 2,100 [14]
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[15]:85
Canadian Rockies 51°N 2,400 7,900
Tatra Mountains 49°N 1,600 5,200
Olympic Mountains WA, USA 47°N 1,500 4,900 Heavy winter snowpack buries young trees until late summer
Swiss Alps 47°N 2,200 7,200 [16]
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
Sikhote-Alin, Russia 46°N 1,600 5,200 [17]
Alps of Piedmont, Northwestern Italy 45°N 2,100 6,900
New Hampshire, USA 44°N 1,350 4,400 [18] Some peaks have even lower treelines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua.
Wyoming, USA 43°N 3,000 9,800
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 [14] On warm southwest slopes
3,250 10,700 On northeast slopes
Japanese Alps 36°N 2,900 9,500
Yosemite, CA, USA 38°N 3,200 10,500 [19] West side of Sierra Nevada
3,600 11,800 [19] East side of Sierra Nevada
Sierra Nevada, Spain 37°N 2,400 7,900 Precipitation low in summer
Khumbu, Himalaya 28°N 4,200 13,800 [14]
Hawaii, USA 20°N 3,000 9,800 [14] Geographic isolation and no local tree species with high tolerance to cold temperatures.
Pico de Orizaba, Mexico 19°N 4,000 13,100 [16]
Costa Rica 9.5°N 3,400 11,200
Mount Kilimanjaro, Tanzania 3°S 3,950 13,000 [14]
New Guinea 6°S 3,850 12,600 [14]
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[20]
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[citation needed]
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[citation needed]
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[21]
Navarino Island, Chile 55°S 600 2,000 Strong maritime influence serves to cool summer and restrict tree growth[21]

Arctic tree lines[edit]

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.

Antarctic tree lines[edit]

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.

Trees growing along the north shore of the Beagle Channel, 55°S.

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.[citation needed]

See also[edit]

  • Ecotone: a transition between two adjacent ecological communities
  • Edge effect: the effect of contrasting environments on an ecosystem
  • Massenerhebung effect
  • Tundra: an area where tree growth is inhibited by low temperatures and short growing seasons

References[edit]

  1. ^ a b Elliott-Fisk, D.L. (2000). "The Taiga and Boreal Forest". In Barbour, M.G.; Billings, M.D. North American Terrestrial Vegetation (2nd ed.) (Cambridge University Press). ISBN 978-0-521-55986-7. 
  2. ^ a b Jørgensen, S.E. (2009). Ecosystem Ecology. Academic Press. ISBN 978-0-444-53466-8. 
  3. ^ Körner, C.; Riedl, S. (2012). Alpine Treelines: Functional Ecology of the Global High Elevation Tree Limits. Springer. ISBN 9783034803960. 
  4. ^ a b Zwinger, A.; Willard, B. E. (1996). Land Above the Trees: A Guide to American Alpine Tundra. Big Earth Publishing. ISBN 1-55566-171-8. 
  5. ^ Körner, C (2003). Alpine plant life: functional plant ecology of high mountain ecosystems. Springer. ISBN 3-540-00347-9. 
  6. ^ "Alpine Tundra Ecosystem". Rocky Mountain National Park. National Park Service. Retrieved 2011-05-13. 
  7. ^ a b c Peet, R.K. (2000). "Forests and Meadows of the Rocky Mountains". In Barbour, M.G.; Billings, M.D. North American Terrestrial Vegetation (2nd ed.) (Cambridge University Press). ISBN 978-0-521-55986-7. 
  8. ^ Bradley, Raymond S. (1999). Paleoclimatology: reconstructing climates of the Quaternary 68. Academic Press. p. 344. 
  9. ^ Baldwin, B.G. (2002). The Jepson desert manual: vascular plants of southeastern California. University of California Press. ISBN 0-520-22775-1. 
  10. ^ Pienitz, Reinhard; Douglas, Marianne S. V.; Smol, John P. (2004). Long-term environmental change in Arctic and Antarctic lakes. Springer. p. 102. 
  11. ^ Chalupa, V. (1992). "Micropropagation of European Mountain Ash (Sorbus aucuparia L.) and Wild Service Tree [Sorbus torminalis (L.) Cr.]". In Bajaj, Y.P.S. High-Tech and Micropropagation II. Biotechnology in Agriculture and Forestry 18. Springer Berlin Heidelberg. pp. 211–226. doi:10.1007/978-3-642-76422-6_11. ISBN 978-3-642-76424-0. 
  12. ^ a b "Treeline". The Canadian Encyclopedia. Retrieved 2011-06-22. 
  13. ^ 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. doi:10.1111/j.1466-8238.2010.00598.x. 
  14. ^ a b c d e f g Körner, Ch (1998). "A re-assessment of high elevation treeline positions and their explanation". Oecologia 115 (4): 445–459. doi:10.1007/s004420050540. 
  15. ^ "Action For Scotland's Biodiversity". 
  16. ^ a b Körner, Ch. "High Elevation Treeline Research". Retrieved 2010-06-14. 
  17. ^ "Physiogeography of the Russian Far East". 
  18. ^ "Mount Washington State Park". New Hampshire State Parks. Archived from the original on 2013-04-03. 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." 
  19. ^ a b Schoenherr, Allan A. (1995). A Natural History of California. UC Press. ISBN 0-520-06922-6. 
  20. ^ 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. doi:10.1111/j.1365-2699.2005.01191.x. 
  21. ^ a b 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 (Santiago) 75 (2). doi:10.4067/S0716-078X2002000200008. 

Further reading[edit]