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 variants:
- Arctic tree-line The furtherst north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees.
- Antarctic tree-line The furtherst south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees.
- Alpine tree-line The highest altitude 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.
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'.
Climatologist Wladimir Köppen demonstrated a relationship between the Arctic and Antarctic tree lines and the 10 °C summer isotherm; i.e., places where the average temperature in the warmest calendar month of the year is below 10 °C cannot support forests. However, Otto Nordenskiöld theorized that winter conditions also play a role: His formula is W = 9 - 0.1 C, with W denoting the average temperature in the warmest month and C the average of the coldest month, both in degrees Celsius (this would mean, for example, that if a particular location had an average temperature of -20 °C in its coldest month, the warmest month would need to average 11 °C or higher for trees to be able to survive there). Nordenskiöld's line tends to run to the north of Köppen's near the west coasts of the Northern Hemisphere continents, south of it in the interior sections, and at about the same latitude along the east coasts of both Asia and North America.
In the Southern Hemisphere, all of Tierra del Fuego lies outside the polar region in Nordenskiöld's system, but part of the island (including Ushuaia, Argentina) is reckoned as being within the Antarctic under Köppen's.
Some typical tree-line tree species (note the predominance of conifers):
Reference
Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7
See also climate.
