Tree line

The tree-line or timberline is the edge of the habitat at which trees can grow. Beyond the tree-line, they are unable to grow as conditions are too bad.

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 and one-sided because of cold and winds.

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

• Arctic tree-line The farthest north in the Northern Hemisphere that trees can grow; farther north, it is too cold.
• Antarctic tree-line The farthest south in the Southern Hemisphere that trees can grow; farther south, it is too cold as well.
• Alpine tree-line The highest height where trees grow: higher up, it is too cold, or snow covers the ground for too much of the year. The climate above the tree-line is called an alpine climate.
• Exposure tree-line 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 of strong winds reduce tree growth.
• Desert tree-line The places where trees cannot grow as there is too little rainfall.
• Toxic tree-line The environment is too extreme for trees to grow. This can be caused by molten lava or hot rock (infrared radiation), any of several kinds of fumarole (steam, sulfur, acid), soil pH, low oxygen conditions, salt concentrations (as near the Dead Sea or Great Salt Lake), sulfate levels, other natural causes, or by man made pollution.
• Wetland tree-line The wettest ground on the margins of muskegs and bogs that trees can grow in, below which the ground is too saturated with water, excluding oxygen from the soil that tree roots need to grow. However, no such line exists for swamps, where trees, such as Bald cypress and the many mangrove species, are adapted to growing in permanently waterlogged soil.

Severe winter climate conditions at alpine tree-line causes stunted krummholz growth. Karkonosze, Poland.

At the tree line, tree growth is often not clear with the last trees forming low bushes. If it is caused by wind, these are known as krummholz, from the German for 'twisted wood'.

The tree line, like many other natural lines (lake boundaries, for example), appears well-defined from a distance, but upon close inspection, it is not that clear. Trees grow shorter until they simply stop growing.

Typical tree-line species

Dahurian Larch growing close to the Arctic tree-line in the Kolyma region, Arctic northeast Siberia.

Some typical tree-line tree species (note the predominance of conifers):

• Dahurian Larch (Larix gmelinii)
• Foxtail Pine (Pinus balfouriana)
• Great Basin Bristlecone Pine (Pinus longaeva)
• Rocky Mountains Bristlecone Pine (Pinus aristata)
• Potosi Pinyon (Pinus culminicola)
• Macedonian Pine (Pinus peuce)
• Whitebark Pine (Pinus albicaulis)
• Swiss Pine (Pinus cembra)
• Mountain Pine (Pinus mugo)
• Hartweg's Pine (Pinus hartwegii)
• Subalpine Larch (Larix lyallii)
• Arctic White Birch (Betula pubescens subsp. tortuosa)
• Snow Gum (Eucalyptus pauciflora)
• Antarctic Beech (Nothofagus Antarctica)

Alpine tree-lines

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. Given this caveat, here is a list of average tree-lines from locations around the globe:

Location	Approx. latitude	Approx. elevation of tree-line		Notes
		(m)	(ft)
Sweden	68°N	800	2600
Norway	61°N	1100	3600	Lower near the coast
Swiss Alps	46°N	2100	6900	Higher in the southern side of the Alps.
New Hampshire, USA	44°N	1220	4000	Some peaks with lower treelines due to fire and subsequent loss of soil.
Wyoming, USA	43°N	3000	9800
Rocky Mountain NP, USA	40°N	3500	11500	On warm SW slopes; lower (about 2400 m or 8000 ft) on NE slopes
Japanese Alps	39°N	2900	9500
Yosemite, USA	38°N	3200	10500	West side of Sierra Nevada
Yosemite, USA	38°N	3600	11800	East side of Sierra Nevada
Himalaya	28°N	4400	14400
Hawaii, USA	20°N	2800	9000	Precipitation low above the trade winds
Costa Rica	9.5°N	3400	11200
Mount Kilimanjaro, Tanzania	3°S	3000	9800
New Guinea	6°S	3900	12800
Andes, Peru	11°S	3900	12800	East side; on west side tree growth is restricted by dryness
Sierra de Córdoba, Argentina	31°S	2000	6560	Precipitation low above trade winds, also high exposure
Australian Alps, Australia	36°S	2000	6560	West side of Australian Alps
Australian Alps, Australia	36°S	1700	5580	East side of Australian Alps
South Island, New Zealand	43°S	1200	3940	Strong maritime influence serves to cool summer and restrict tree growth

Arctic and Antarctic 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; 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°Q	71°N	The North Atlantic current makes Arctic climates in this region warmer than other coastal locations at comparable latitude.
West Siberian Plain	75°R	66°N
Central Siberian Plateau	102°S	72°N	Extreme continental climate means the summer is warm enough to allow tree growth at higher latitudes, extending to 72°30'N at Ary-Mas (102° 27' E) in the Novaya River valley, a tributary of the 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 due to 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.
Quebec	72°W	56°N	Very strong influence of the Labrador Current on summer temperatures. 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 due to isolation from natural seed sources; a very few trees are surviving, but growing slowly, at Søndre Strømfjord, 67°N.
Tierra del Fuego, Chile and Argentina	69°W	55°S	Tree growth reaches very close to the southernmost point of South America, with the limit related to extreme exposure rather than climate.

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

References

• Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
• Ødum, S. 1979. Actual and potential tree-line in the North Atlantic region, especially in Greenland and the Faroes. Holarctic Ecology 2: 222-227.
• Ødum, S. 1991. Choice of species and origins for arboriculture in Greenland and the Faroe Islands. Dansk Dendrologisk Årsskrift 9: 3-78.
• Beringer, J., Tapper, N. J., McHugh, I., Lynch, A. H., Serreze, M. C., & Slater, A. 2001. Impact of Arctic treeline on synoptic climate. Geophysical Research Letters 28 (22): 4247-4250.