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 winds.
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.
Typical tree-line species
Some typical tree-line tree species (note the predominance of conifers):
Table of 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:
Arno, S. F. & Hammerly, R. P. 1984. Timberline. Mountain and Arctic Forest Frontiers. The Mountaineers, Seattle. ISBN 0-89886-085-7