Editing Tree line (section)

===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>