Tree line

The 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 because of inappropriate environmental conditions (usually cold temperatures, insufficient air pressure, or lack of moisture).

At the tree line, tree growth is often very stunted, with the last trees forming low, densely matted bushes. If it is caused by wind, it is known as krummholz formation, from the German for 'twisted wood'.

The tree line, like many other natural lines (lake boundaries, for example), appears well-defined from a distance, but upon sufficiently close inspection, it is a gradual transition. Trees grow shorter towards the inhospitable climate until they simply stop growing.

Types of tree lines

There are several types of tree lines defined in ecology and geology:

* Alpine tree line The highest elevation which sustains trees; higher up, it is too cold or snow cover persists for too much of the year, to sustain trees. Usually associated with mountains, the climate above the tree line is called an alpine climate, and the terrain can be described as alpine tundra. Mountains of the Pacific Northwest of North America exhibit lower treelines on north-facing slopes than south-facing slopes, because increased shade means the deep snowpack takes longer to melt, which shortens the growing season for trees.
* Desert tree line The driest places that trees can grow; drier desert areas having insufficient rainfall to sustain trees. These tend to be called the "lower" tree line and occur below about 5000 ft (1500 m) elevation in the Desert Southwestern United States. The desert treeline tends to be lower on pole-facing slopes than equator-facing slopes, because the increased shade on a pole-facing slope keeps those slopes cooler and prevents moisture from evaporating as quickly, giving trees a longer growing season and more access to water.
* Desert-Alpine tree line In some mountainous areas, higher elevations above the condensation line or on south-facing in the northern hemisphere and north-facing in the southern hemisphere, or leeward slopes can result in low rainfall and increased exposure to solar radiation. This dries out the soil, resulting in a localized arid environment unsuitable for trees. The slopes of Mauna Loa above 10,000 ft in Hawaii are an example of this. Many south-facing ridges of the mountains of the Western U.S. have a lower treeline than the northern faces because of increased sun exposure and aridity.
* Double tree line Different tree species have different tolerances to drought and cold. Mountain ranges isolated by oceans or deserts may have restricted reportoires of tree species with gaps that are above the alpine tree line for some species yet below the desert tree line for others. For example several mountain ranges in the Great Basin of North America have lower belts of Pinyon Pines and Junipers separated by intermediate brushy but treeless zones from upper belts of Limber and Bristlecone Pines.
* Exposure tree line 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 strong winds reduce tree growth. In addition, the lack of suitable soil, such as along talus slopes or exposed rock formations prevent trees from gaining an adequate foothold and expose them to drought and sun.
* Arctic tree line The furthest north in the Northern Hemisphere that trees can grow; further north, it is too cold to sustain trees. Extremely cold temperatures can result in freezing of the internal sap of trees, killing those trees. In addition, permafrost in the soil can prevent trees from getting their roots deep enough for the necessary structural support.
* Antarctic tree line The furthest south in the Southern Hemisphere that trees can grow; further south, it is too cold to sustain trees. It is a theoretical concept that does not have any defined location. No trees occur on Antarctica or the sub-antarctic islands, and there are no land masses to the north that have a true treeline.
* Other tree lines The immediate environment is too extreme for trees to grow. This can be caused by geothermal exposure associated with hot springs, such as at Yellowstone, or near volcanoes, high soil acidity near bogs, high salinity associated with playas or salt lakes, or ground that is too saturated by ground water which excludes oxygen from the soil, which most tree roots need for growth. The margins of muskegs and bogs are common examples of these types of open areas. However, no such line exists for swamps, where trees, such as Bald cypress and the many mangrove species, are adapted to growing in permanently waterlogged soil.

Typical vegetation

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

*Subalpine fir ("Abies lasiocarpa")
*Subalpine Larch ("Larix lyallii")
*Engelmann Spruce ("Picea engelmannii")
*Whitebark Pine ("Pinus albicaulis")
*Great Basin Bristlecone Pine ("Pinus longaeva")
*Rocky Mountains Bristlecone Pine ("Pinus aristata")
*Foxtail Pine ("Pinus balfouriana")
*Limber Pine ("Pinus flexilis")
*Dahurian Larch ("Larix gmelinii")
*Potosi Pinyon ("Pinus culminicola")
*Macedonian Pine ("Pinus peuce")
*Swiss Pine ("Pinus cembra")
*Mountain Pine ("Pinus mugo")
*Hartweg's Pine ("Pinus hartwegii")
*Arctic White Birch ("Betula pubescens" subsp. "tortuosa")
*Polylepis ("Polylepis tarapacana")
*Snow Gum ("Eucalyptus pauciflora")
*Antarctic Beech ("Nothofagus Antarctica")
*Lenga Beech ("Nothofagus pumilio")
*Black spruce ("Picea mariana")

Worldwide distribution

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. In addition, in some tropical or island localities, the lack of biogeographical access to species that have evolved in a sub-alpine environment, can result in lower tree lines than one might expect by climate alone.

Given this caveat, here is a list of approximate tree lines from locations around the globe:

Arctic tree lines

Like the alpine tree lines shown above, polar tree lines are heavily influenced by local variables such as aspect of slope and degree of shelter. In addition, permafrost has a major impact on the ability of trees to place roots into the ground. When roots are too shallow, trees are susceptible to windthrow and erosion. 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:

Antarctic tree lines

Kerguelen Island, Île Saint-Paul, South Georgia, and other Sub-Antarctic islands are all so heavily wind exposed and marginal in climate, that none have any indigenous tree species, although many such islands receive enough rainfall that they would otherwise be capable of hosting temperate rain forest. However, these are not directly related to the Antarctic tree line, but are related to exposure.

ee also

*Ecotone: a transition between two adjacent ecological communities
*Edge effect: the effect of contrasting environments on an ecosystem
*Massenerhebung effect
*Tundra: an area where tree growth is inhibited by low temperatures and short growing seasons

References

*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.