Science Fair Project Encyclopedia
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.
Typical tree-line species
Some typical tree-line tree species (note the predominance of conifers):
- Foxtail pine (Pinus balfouriana)
- Bristlecone pine (Pinus aristata, Pinus longaeva)
- Potosí 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)
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|
|Sweden||68 ° N||400||1300|
|Swiss Alps||46 ° N||2400||7900|
|Wyoming, USA||43 ° N||3000||9800|
|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|
|Himalayas||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|
|Finland||25 ° E||68 ° N|
|West Siberian Plain||75 ° E||66 ° N|
|Central Siberian Plateau||95 ° E||70 ° N||Extreme continental climate means summer is warm enough to allow tree growth at higher latitudes - extending to 72 ° N is some valleys.|
|Russian Far East (Kamchatka and Chukotka)||160 ° E||60 ° N||Oyashio Current and strong winds affect summer temperatures to prevent tree growth. Aleutian Islands almost completely treeless.|
|Alaska||152 ° W||68 ° N|
|Northwest Territories, Canada||132 ° W||69 ° N|
|Nunavut||95 ° W||61 ° N||Influence of very cold Hudson Bay moves treeline southwards.|
|Québec||72 ° W||56 ° N||Very strong influence of Labrador Current on summer temperatures. In parts of Labrador, the treeline can be as far south as 53 ° N.|
|Patagonia, Chile||51 ° S||Extremely strong influence of Antarctic Circumpolar Current moderates temperatures, but makes summer too cold very easily.|
Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
Beringer, Jason; Tapper, Nigel J.; McHugh, Ian; Lynch, Amanda. H.; Serreze, Mark. C. & Slater, Andrew; Impact of Arctic treeline on synoptic climate; Geophysical Research Letters, Vol. 28, No.. 22, pp. 4247-4250, November 15, 2001.
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