Tree line

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 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, they are unable to grow because of inappropriate environmental conditions (usually cold temperatures, insufficient air pressure, or lack of moisture). Some distinguish additionally a deeper timberline, where trees can develop trunks.

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'.

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.

Contents

Types

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

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. Usually associated with mountains, the climate above the tree line is called an alpine climate, and the terrain can be described as alpine tundra. Mountains of the Pacific Northwest of North America exhibit lower treelines on north-facing slopes than south-facing slopes, because increased shade means the deep snowpack takes longer to melt, which shortens the growing season for trees.
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 5000 ft (1500 m) elevation in the Desert Southwestern United States. The desert treeline tends to be higher 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.
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. The slopes of Mauna Loa above 10,000 ft in Hawaii are an example of this. 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.
Different tree species have different tolerances to drought and cold. Mountain ranges isolated by oceans or deserts may have restricted reportoires 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.
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.
The northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold to sustain trees. Extremely cold temperatures 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.
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 sub-antarctic 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.
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. Example are around Nikel in Russia and previously in the Erzgebirge.

Typical vegetation

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

North America

South America

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

Australia

Worldwide distribution

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. In addition, in some tropical or island localities, the lack of biogeographical access to species that have evolved in a sub-alpine environment, can result in lower tree lines than one might expect by climate alone.

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

Location Approx. latitude Approx. elevation of tree line Notes
(m) (ft)
Scotland 57°N 500 1,600 [1]
Sweden 68°N 800 2,600
Chugach Mountains, Alaska 61°N 700 2,300 Tree line around 1500 feet or lower in coastal areas
Norway 61°N 1,100 3,600 Much lower near the coast, down to 5-600 metres. At 71°N, in Finnmark county, the tree-line is below sea level (Arctic tree line).
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 [2]
Canadian Rockies 51°N 2,400 7,900
Mount Katahdin, Maine, USA 46°N 1,150 3,770
Eastern Alps, Austria, Italy 46°N 1,750 5,740 more exposure to Russian cold winds than Western Alps
Alps of Piedmont, Northwestern Italy 45°N 2,100 6,900
New Hampshire, USA 44°N 1,400 4,600 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 2600m 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) in the Uintas
Rocky Mountain NP, USA 40°N 3,500 11,500 On warm southwest slopes
3,250 10,660 On northeast slopes
Japanese Alps 39°N 2,900 9,500
Yosemite, USA 38°N 3,200 10,500 West side of Sierra Nevada[3]
3,600 11,800 East side of Sierra Nevada[3]
Sierra Nevada, Spain 37°N 2,400 7,900 Precipitation low in summer
Himalaya 28°N 4,400 14,400
Hawaii, USA 20°N 2,800 9,200 Geographic isolation and no local tree species with high tolerance to cold temperatures (see Mt. Kilimanjaro).
Pico de Orizaba, Mexico 19°N 4,000 13,000 [2]
Costa Rica 9.5°N 3,400 11,200
Mount Kilimanjaro, Tanzania 3°S 3,000 9,800 Example of a tropical location lacking biogeographical access to species that are evolved for living in a subalpine environment. Thus, the tolerance of the indigenous species is lower and it results in a lower tree line
New Guinea 6°S 3,900 12,800
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
Tasmania, Australia 41°S 1,200 3,900 Cold Winters, Strong Cold Winds and Cool Summers with occasional summer snow restrict tree growth
South Island, New Zealand 43°S 1,200 3,900 Strong maritime influence serves to cool summer and restrict tree growth
Torres del Paine, Chile 51°S 950 3,120 Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth[4]
Navarino Island, Chile 55°S 600 2,000 Strong maritime influence serves to cool summer and restrict tree growth[4]

Arctic 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. 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.
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 because of isolation from natural seed sources; a very few trees are surviving, but growing slowly, at Søndre Strømfjord, 67°N.

Antarctic tree lines

Kerguelen Island, Île Saint-Paul, South Georgia, South Orkney, and other Sub-Antarctic islands are all so heavily wind exposed and with a far too cold summer climate (tundra) that none have any indigenous tree species.

Antarctic Peninsula is the northernmost point in Antarctica and has the mildest weather. It is located 1,080 kilometres (670 mi) from Cape Horn on Tierra del Fuego (Tierra del Fuego contains trees). But, no trees live on 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.

See also

References

Footnotes

  1. ↑ Action For Scotland's Biodiversity (page 85)
  2. ↑ 2.0 2.1 Körner, Christian. "High Elevation Treeline Research". http://pages.unibas.ch/botschoen/treeline_elevation/index.shtml. Retrieved 2010-06-14. 
  3. ↑ 3.0 3.1 Schoenherr, Allan A. (1995). A Natural History of California. UC Press. ISBN 0-520-06922-6. 
  4. ↑ 4.0 4.1 Tree-ring growth patterns and temperature reconstruction from Nothofagus pumilio (Fagaceae) forests at the upper tree line of southern, Chilean Patagonia

Further Reading