Editing Tree line (section)

===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}}