Tree-line

From Freepedia

Image:Tree line.jpg 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 due to inappropriate environmental conditions. There are several types:

  • Arctic tree-line The furthest north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees.
  • Antarctic tree-line The furthest south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees.
  • Alpine tree-line The highest elevation that trees can grow on mountains; higher up, it is too cold to sustain trees.
  • Exposure tree-line On coasts, and on isolated mountains, the tree-line is often much lower than in corresponding altitudes inland and in larger, more complex mountain systems, because high wind speeds adversely affect tree growth.
  • Desert tree-line The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees.
  • 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 water-logged soil.

At tree-line, tree growth is often very stunted, affected by wind, with the last trees forming low, dense matted bushes. These are known as krummholz, from the German for 'twisted wood'. The tree line, like many other natural lines (lake boundaries, for example), looks sharp from a distance, but upon sufficiently close inspection, it becomes a more gradual transition. Trees grow shorter towards the inhospitable climate until they simply stop growing.

The climate above the tree-line is called an alpine climate. Please see that article for more details on what climatic factors cause trees to fail to grow.

Contents

Typical tree-line species

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

Table of 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 400 1300
Swiss Alps 46°N 2400 7900
New Hampshire, USA 44°N 1220 4000
Wyoming, USA 43°N 3000 9800
Rocky Mountain NP, USA 40°N 3500 11500 On warm southwest-facing slopes; lower (about 2400 m or 8000 ft) on northeast-facing 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

Table of arctic and antarctic tree-lines

Like alpine tree-lines shown above, polar tree-lines are heavily influenced by local variables such as 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°E 71°N The North Atlantic current makes arctic climates in this region warmer than other coastal locations at comparable latitude.
West Siberian Plain 75°E 66°N
Central Siberian Plateau 102°E 70°N Extreme continental climate means the summer is warm enough to allow tree growth at higher latitudes, extending to 72°30'N at Arimas (102°27'E) in the Novaya River valley.
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
Northwest Territories, Canada 132°W 69°N
Nunavut 95°W 61°N Influence of the very cold Hudson Bay moves treeline southwards.
Québec 72°W 56°N Very strong influence of the Labrador Current on summer temperatures. In parts of Labrador, the treeline can be 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 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.

Image:BeagleChannelGlacier.JPG

Reference

  • 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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