The borderline for trees
The tree-line or timberline ( I call it borderline ) 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:
The furthest north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees.
The furthest south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees.
The highest elevation that trees can grow on mountains; higher up, it is too cold, soil too poor, or snow cover too much of the year to sustain trees.
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 high winds reduce tree growth.
The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees.
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 the tree-line, tree growth is often very stunted, affected by wind, with the last trees forming low, densely 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.
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