- Tree line
The tree line or timberline 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).
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
krummholzformation, from the German for 'twisted wood'.
The tree line, like many other natural lines (
lakeboundaries, for example), appears well-defined from a distance, but upon sufficiently close inspection, it is a gradual transition. Trees grow shorter towards the inhospitable climate until they simply stop growing.
Types of tree lines
There are several types of tree lines defined in
* Alpine tree line The highest
elevationwhich 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 climateabove the tree line is called an alpine climate, and the terrain can be described as alpine tundra. Mountains of the Pacific Northwestof North America exhibit lower treelines on north-facing slopes than south-facing slopes, because increased shade means the deep snowpacktakes longer to melt, which shortens the growing season for trees.
* Desert tree line The driest places that trees can grow; drier
desertareas having insufficient rainfallto 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 lower 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.
* Desert-Alpine tree line In some mountainous areas, higher elevations above the
condensationline or on south-facing in the northern hemisphere and north-facing in the southern hemisphere, or 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 Loaabove 10,000 ft in Hawaiiare 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.
* Double tree line 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 Basinof North Americahave lower belts of Pinyon Pines and Junipers separated by intermediate brushy but treeless zones from upper belts of Limber and Bristlecone Pines.
* 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 strong winds reduce tree growth. In addition, the lack of suitable soil, such as along talus slopes or exposed rock formations prevent trees from gaining an adequate foothold and expose them to drought and sun.
* Arctic tree line The furthest
northin the Northern Hemispherethat trees can grow; further north, it is too cold to sustain trees. Extremely cold temperatures can result in freezing of the internal sap of trees, killing those trees. In addition, permafrostin the soil can prevent trees from getting their roots deep enough for the necessary structural support.
* Antarctic tree line The furthest
southin the Southern Hemispherethat 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 occur on Antarcticaor the sub-antarctic islands, and there are no land masses to the north that have a true treeline.
* Other tree lines The immediate environment is too extreme for trees to grow. This can be caused by geothermal exposure associated with hot springs, such as at
Yellowstone, or near volcanoes, high soil acidity near bogs, high salinity associated with playasor salt lakes, or ground that is too saturated by ground water which excludes oxygenfrom 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 mangrovespecies, are adapted to growing in permanently waterlogged soil.
Some typical Arctic and alpine tree-line tree species (note the predominance of
Subalpine fir("Abies lasiocarpa")
Subalpine Larch("Larix lyallii")
Engelmann Spruce("Picea engelmannii")
Whitebark Pine("Pinus albicaulis")
Great Basin Bristlecone Pine("Pinus longaeva")
Rocky Mountains Bristlecone Pine("Pinus aristata")
Foxtail Pine("Pinus balfouriana")
Limber Pine("Pinus flexilis")
Dahurian Larch("Larix gmelinii")
Potosi Pinyon("Pinus culminicola")
Macedonian Pine("Pinus peuce")
Swiss Pine("Pinus cembra")
Mountain Pine("Pinus mugo")
Hartweg's Pine("Pinus hartwegii")
*Arctic White Birch ("Betula pubescens" subsp. "tortuosa")
*Snow Gum ("Eucalyptus pauciflora")
*Antarctic Beech ("Nothofagus Antarctica")
*Lenga Beech ("Nothofagus pumilio")
Black spruce("Picea mariana")
Alpine tree lines
The alpine tree line at a location is dependent on local variables, such as aspect of slope,
rain shadowand 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:
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,
permafrosthas 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 valleysat latitudes where they could not grow on a more exposed site. Maritime influences such as ocean currentsalso play a major role in determining how far from the equator trees can grow. Here are some typical polar treelines:
Antarctic tree lines
Kerguelen Island, Île Saint-Paul, South Georgia, and other Sub-Antarctic islands are all so heavily wind exposed and marginal in climate, that none have any indigenous tree species, although many such islands receive enough rainfall that they would otherwise be capable of hosting temperate rain forest. However, these are not directly related to the Antarctic tree line, but are related to exposure.
Ecotone: a transition between two adjacent ecological communities
Edge effect: the effect of contrasting environments on an ecosystem
Tundra: an area where tree growth is inhibited by low temperatures and short growing seasons
*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.
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Look at other dictionaries:
tree line — n [singular] the ↑timberline … Dictionary of contemporary English
tree line — tree′ line n. ecl geo timberline … From formal English to slang
tree line — ☆ tree line n. TIMBERLINE … English World dictionary
tree line — noun line marking the upper limit of tree growth in mountains or northern latitudes • Syn: ↑timber line, ↑timberline • Hypernyms: ↑line * * * noun : timberline * * * timberline. * * * tree line … Useful english dictionary
tree line — /ˈtri laɪn/ (say tree luyn) noun → timber line … Australian English dictionary
tree line — noun Date: 1893 timberline … New Collegiate Dictionary
tree line — timberline. * * * … Universalium
tree line — noun A division in altitude above which trees tend not to grow on mountains … Wiktionary
tree-line — noun the edge of the habitat at which trees are capable of growing; the timberline … Wiktionary
tree line — timberline, altitude above which plants and trees cannot grow … English contemporary dictionary