Editing Tree line (section)

== Types ==
[[File:Distribution_of_Plants_in_a_Perpendicular_Direction_in_the_Torrid,_the_Temperate,_and_the_Rigid_Zones_1848_Alexander_Keith_Johnston.png|thumb|600px|right|This map of the "Distribution of Plants in a Perpendicular Direction in the Torrid, the Temperate, and the Frigid Zones" was first published 1848 in "The Physical Atlas". It shows tree lines of the [[Andes]], [[Tenerife]], [[Himalaya]], [[Alps]], [[Pyrenees]], and [[Lapland (Finland)|Lapland]].]]
[[File:Bistrishko-Branishte.jpg|thumb|Alpine tree line of [[mountain pine]] and [[European spruce]] below the [[subalpine zone]] of [[Bistrishko Branishte]], with the surmounting [[Golyam Rezen]] Peak, [[Vitosha Mountain]], [[Sofia]], [[Bulgaria]]]]

Several types of tree lines are defined in [[ecology]] and [[geography]]:

===Alpine===
[[File:Treeline In The Tararuas.JPG|thumb|An alpine tree line in the [[Tararua Range]]]]
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.<ref name=ecosystem/>{{rp|151}} The climate above the tree line of [[mountain]]s is called an [[alpine climate]],<ref name="plantlife">{{Cite book |last=Körner |first=C |title=Alpine plant life: functional plant ecology of high mountain ecosystems |publisher=Springer |year=2003 |isbn=978-3-540-00347-2}}</ref>{{rp|21}} and the habitat can be described as the [[alpine zone]].<ref name="romo">{{Cite web |title=Alpine Tundra Ecosystem |url=http://www.nps.gov/romo/naturescience/alpine_tundra_ecosystem.htm |access-date=2011-05-13 |website=Rocky Mountain National Park |publisher=National Park Service}}</ref> 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.<ref name="rockymtns">{{Cite book |last=Peet |first=R.K. |title=North American Terrestrial Vegetation |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-55986-7 |editor-last=Barbour |editor-first=M.G. |edition=2nd |chapter=Forests and Meadows of the Rocky Mountains |editor-last2=Billings |editor-first2=M.D.}}</ref>{{rp|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 [[volcano]]es in central Mexico, and on mountains in each of the 11 western states and throughout much of Canada and Alaska".<ref name="arno" /> 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 {{convert|3.5|F-change|C-change}} per {{convert|1000|ft|m}} of elevation gain in the dry mountains of the western United States,<ref name="arno" /> to {{convert|1.4|F-change|C-change}} per {{convert|1000|ft|m}} in the moister mountains of the eastern United States.<ref name="baker">{{Cite journal |last=Baker |first=F.S. |year=1944 |title=Mountain climates of the western United States |journal=[[Ecological Society of America|Ecological Monographs]] |volume=14 |issue=2 |pages=223–254 |doi=10.2307/1943534 |jstor=1943534|bibcode=1944EcoM...14..223B }}</ref> Skin effects and [[topography]] can create [[microclimate]]s that alter the general cooling trend.<ref name="gei">{{Cite book |last=Geiger |first=R. |url=https://archive.org/details/climatenearthegr032657mbp |title=The Climate near the Ground |publisher=[[Harvard University Press]] |year=1950 |location=Cambridge, MA}}</ref>

Compared with arctic tree lines, alpine tree lines may receive fewer than half of the number of degree days (above {{convert|10|C}}) 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.<ref name="arno" />

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,<ref name="sowell">{{Cite journal |last1=Sowell |first1=J.B. |last2=McNulty |first2=S.P. |last3=Schilling |first3=B.K. |year=1996 |title=The role of stem recharge in reducing the winter desiccation of ''Picea engelmannii'' (Pinaceae) needles at alpine timberline |journal=[[American Journal of Botany]] |volume=83 |issue=10 |pages=1351–1355 |doi=10.2307/2446122 |jstor=2446122}}</ref> 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 {{convert|400|m|ft}} lower than on exposed intervening shoulders.<ref name="shaw">{{Cite journal |last=Shaw |first=C.H. |year=1909 |title=The causes of timberline on mountains: the role of snow |journal=Plant World |volume=12 |pages=169–181}}</ref>

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 {{convert|8000|ft|m|abbr=on}} is also above the condensation zone and results due to a lack of moisture.{{cn|date=July 2023}}

===Exposure===
On [[coast]]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]]s reduce tree growth. In addition, the lack of suitable soil, such as along [[scree|talus slopes]] or exposed rock formations, prevents trees from gaining an adequate foothold and exposes them to drought and sun.{{cn|date=September 2024}}

===Arctic===
[[File:Rivière aux Mélèzes 1.JPG|thumb|right|Treeline visible in lower left, northern Quebec, Canada, while trees also grow in the sheltered river valleys|alt=An aerial photo viewing down to Earth with rivers visible. Ground is covered by snow, with trees in the lower left and in the valleys of the rivers.]]
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.<ref>{{Cite book |last1=Pienitz |first1=Reinhard |title=Long-term environmental change in Arctic and Antarctic lakes |last2=Douglas |first2=Marianne S. V. |last3=Smol |first3=John P. |publisher=Springer |year=2004 |isbn=978-1-4020-2126-8 |page=102}}</ref> 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.{{cn|date=July 2023}}

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 {{convert|145|km|mi}} and widening markedly from west to east,<ref name="timoney">{{Cite journal |last1=Timoney |first1=K.P. |last2=La Roi |first2=G.H. |last3=Zoltai |first3=S.C. |last4=Robinson |first4=A.L. |year=1992 |title=The high subarctic forest–tundra of northwestern Canada: position, width, and vegetation gradients in relation to climate |journal=Arctic |volume=45 |issue=1 |pages=1–9 |doi=10.14430/arctic1367 |jstor=40511186 |doi-access=free}}</ref> in contrast with the telescoped alpine timberlines.<ref name="arno">{{Cite book |last=Arno |first=S.F. |title=Timberline: Mountain and Arctic Forest Frontiers |publisher=The Mountaineers |year=1984 |isbn=978-0-89886-085-6 |location=Seattle, WA}}</ref> North of the arctic tree line lies the low-growing [[tundra]], and southwards lies the [[Taiga|boreal forest]].

Two zones can be distinguished in the Arctic tree line:<ref name="love">{{Cite journal |last=Löve |first=Dd |year=1970 |title=Subarctic and subalpine: where and what? |journal=[[Arctic, Antarctic, and Alpine Research|Arctic and Alpine Research]] |volume=2 |issue=1 |pages=63–73 |doi=10.2307/1550141 |jstor=1550141}}</ref><ref name="hare2">{{Cite journal |last1=Hare |first1=F. Kenneth |last2=Ritchie |first2=J.C. |year=1972 |title=The boreal bioclimates |journal=[[Geographical Review]] |volume=62 |issue=3 |pages=333–365 |doi=10.2307/213287 |jstor=213287|bibcode=1972GeoRv..62..333H }}</ref> 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]]s.<ref name="love" /> The proportion of trees to lichen mat increases southwards towards the "forest line", where trees cover 50 percent or more of the landscape.<ref name="arno" /><ref name="black">{{Cite journal |last1=R.A. |first1=Black |last2=Bliss |first2=L.C. |year=1978 |title=Recovery sequence of ''Picea mariana–Vaccinium uliginosum'' forests after burning near Inuvik, Northwest Territories, Canada |journal=[[Canadian Journal of Botany]] |volume=56 |issue=6 |pages=2020–2030 |doi=10.1139/b78-243|bibcode=1978CaJB...56.2020B }}</ref>

===Antarctic===
{{Further|Antipodes Subantarctic Islands tundra|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 {{convert|5|C|F}} support trees and woody plants, and those below {{convert|5|C|F}} do not.<ref>{{WWF ecoregion|id=aa1101|name=Antipodes Subantarctic Islands tundra}}</ref>
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).<ref>{{WWF ecoregion|id=nt0402|name=Magellanic subpolar Nothofagus forests}}</ref> For example, the northern halves of [[Hoste Island|Hoste]] and [[Navarino Island|Navarino]] Islands have [[Nothofagus antarctica]] forests but the southern parts consist of moorlands and tundra.