Tree line

thumb|Tree line above [[St. Moritz, Switzerland. May 2009]]

thumb|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 in growth and one-sided because of cold and constant wind.

The tree line is the edge of a habitat at which trees can grow and beyond which they cannot. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions (usually low temperatures, extreme snowpack, or associated lack of available moisture). The tree line is sometimes distinguished from a lower timberline, which is the line below which trees form a forest with a closed canopy.

At the tree line, tree growth is often sparse, stunted, and deformed by wind and cold. This is sometimes known as (German for "crooked wood").

The tree line often appears well-defined, but it can be a more gradual transition. Trees grow shorter and often at lower densities as they approach the tree line, above which they are unable to grow at all.

Causes

Due to their vertical structure, trees are more susceptible to cold than more ground-hugging forms of plants. A series of warm summers in the 1940s seems to have permitted the establishment of "significant numbers" of spruce seedlings above the previous tree line in the hills near Fairbanks, Alaska. Survival depends on a sufficiency of new growth to support the tree. Wind can mechanically damage tree tissues directly, including blasting with windborne particles, and may also contribute to the desiccation of foliage, especially of shoots that project above the snow cover.

Variation

The tree line elevation at a location is generally set by the mean temperature, while the realized tree line may be affected by disturbances, such as logging, or grazing Most human activities cannot change the actual tree line, unless they affect the climate.

Because of climate change, which leads to earlier snowmelt and favorable conditions for tree establishment, the tree line in North Cascades National Park has risen more than in 50 years.

Other local factors can locally change the elevation of tree line, such as aspect of slope, rain shadow. Tree lines on north-facing slopes in the northern hemisphere are lower than on south-facing slopes, because the increased shade on north-facing slopes means the snowpack takes longer to melt. This shortens the growing season for trees. In the southern hemisphere, the south-facing slopes have the shorter growing season. On coasts and isolated mountains, the tree line is often much lower than corresponding altitudes inland and in larger, more complex mountain systems. This is known as the Massenerhebung effect, and is caused by large mountain ranges retaining more heat and reducing wind velocity downwind, compared to isolated mountains.

Types

thumb|600px|right|This map of the "Distribution of Plants in a Perpendicular Direction in the Torrid, the Temperate, and the Frigid Zones" was first published 1848 in "The Physical Atlas". It shows tree lines of the [[Andes, Tenerife, Himalaya, Alps, Pyrenees, and Lapland.]]

thumb|Alpine tree line of [[mountain pine and European spruce below the subalpine zone of Bistrishko Branishte, with the surmounting Golyam Rezen Peak, Vitosha Mountain, Sofia, Bulgaria]]

Several types of tree lines are defined in ecology and geography:

Alpine

thumb|An alpine tree line in the [[Tararua Range]]

An alpine tree line is the highest elevation that sustains trees; higher up it is too cold, or the snow cover lasts for too much of the year, to sustain trees. and the habitat can be described as the alpine zone.

The alpine tree line boundary is seldom abrupt: it usually forms a transition zone between closed forest below and treeless alpine zone above. This zone of transition occurs "near the top of the tallest peaks in the northeastern United States, high up on the giant volcanoes in central Mexico, and on mountains in each of the 11 western states and throughout much of Canada and Alaska". Skin effects and topography can create microclimates that alter the general cooling trend.

Compared with arctic tree lines, alpine tree lines may receive fewer than half of the number of degree days (above ) based on air temperature, but because solar radiation intensities are greater at alpine than at arctic tree lines the number of degree days calculated from leaf temperatures may be very similar. and prolonging a supply of moisture through the early part of the growing season. However, snow accumulation in sheltered gullies in the Selkirk Mountains of southeastern British Columbia causes the tree line to be lower than on exposed intervening shoulders.

In some mountainous areas, higher elevations above the condensation line, or on equator-facing and 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. Many south-facing ridges of the mountains of the Western U.S. have a lower tree line than the northern faces because of increased sun exposure and aridity. Hawaii's tree line of about is above a temperature inversion which blocks moisture from reaching the highest slopes. Extremely low temperatures, especially when prolonged, can freeze the internal sap of trees, killing them. In addition, permafrost in the soil can prevent trees from getting their roots deep enough for the necessary structural support.

Unlike alpine tree lines, the northern tree line occurs at low elevations. The Arctic forest-tundra transition zone in northwestern Canada varies in width, perhaps averaging and widening markedly from west to east, in contrast with the telescoped alpine timberlines. North of the arctic tree line lies the low-growing tundra, and southwards lies the boreal forest.

Two zones can be distinguished in the Arctic tree line: a forest–tundra zone of scattered patches of krummholz or stunted trees, with larger trees along rivers and on sheltered sites set in a matrix of tundra; and "open boreal forest" or "lichen woodland", consisting of open groves of erect trees underlain by a carpet of Cladonia spp. lichens.

Tree species near tree line

thumb|[[Larix gmelinii|Dahurian larch growing close to the Arctic tree line in the Kolyma region, Arctic northeast Siberia]]

thumb|View of a [[Magellanic subpolar forests|Magellanic lenga forest close to the tree line in Torres del Paine National Park, Chile]]

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

Australia

Snow gum (Eucalyptus pauciflora)

Eurasia

Dahurian larch (Larix gmelinii)
Macedonian pine (Pinus peuce)
Swiss pine (Pinus cembra)
Mountain pine (Pinus mugo)
Arctic white birch (Betula pubescens subsp. tortuosa)
Rowan (Sorbus aucuparia)

North America

Subalpine fir (Abies lasiocarpa)
Mountain hemlock (Tsuga mertensiana)
Alaska yellow cedar (Chaemaecyparis nootkatensis)
Engelmann spruce (Picea engelmannii)
Alder (Alnus acuminata)
Pino del cerro (Podocarpus parlatorei)
Polylepis (Polylepis tarapacana)
Eucalyptus (not native to South America but grown in large amounts in the high Andes).

Worldwide distribution

Alpine tree lines

thumb|right|Tree line elevation by latitude

Averaging over many locations and local microclimates, the tree line rises when moving 1 degree south from 70 to 50°N, and per degree from 50 to 30°N. Between 30°N and 20°S, the tree line is roughly constant, between .

|Northern Quebec

|data-sort-value="56"|56°N

| The cold Labrador Current originating in the arctic makes eastern Canada the sea-level region with the most southern tree-line in the northern hemisphere.

|Southern Urals

|data-sort-value="55"|55°N

|Canadian Rockies

|data-sort-value="51"|51°N

|Tatra Mountains

|data-sort-value="49"|49°N

|Olympic Mountains, Washington, United States

|data-sort-value="47"|47°N

|Heavy winter snowpack buries young trees until late summer

|Swiss Alps

|data-sort-value="47"|47°N

|Alps of Piedmont, Northwestern Italy

|data-sort-value="45"|45°N

|New Hampshire, United States

|data-sort-value="44"|44°N

| Some peaks have even lower tree lines because of fire and subsequent loss of soil, such as Grand Monadnock and Mount Chocorua.

|Wyoming, United States

|data-sort-value="43"|43°N

|Caucasus Mountains

|data-sort-value="42"|42°N

|Rila and Pirin Mountains, Bulgaria

|data-sort-value="42"|42°N

| Up to on favorable locations. Mountain Pine is the most common tree line species.

|Pyrenees Spain, France, Andorra

|data-sort-value="42"|42°N

| Mountain Pine is the tree line species

|Steens Mountain, Oregon, US

|data-sort-value="42"|42°N

|Wasatch Mountains, Utah, United States

|data-sort-value="40"|40°N

|Higher (nearly in the Uintas)

| rowspan="2" |Rocky Mountain NP, CO, United States

| rowspan="2" data-sort-value="40"|40°N

| West side of Sierra Nevada

| Strong winds and poor soil restrict further grow of trees.

|Hawaii, United States

|data-sort-value="20"|20°N

|Costa Rica

|data-sort-value="9.5"|9.5°N

|Mount Kinabalu, Borneo

|data-sort-value="6.1"|6.1°N

|Mount Kilimanjaro, Tanzania

|data-sort-value="-3"|3°S

| Upper limit of forest trees; woody ericaeous scrub grows up to 3900m

|New Guinea

|data-sort-value="-6"|6°S

|Andes, Laguna del Laja, Chile

|data-sort-value="-37"|37°S

|Temperature rather than precipitation restricts tree growth

|Mount Taranaki, North Island, New Zealand

|data-sort-value="-39"|39°S

|Strong maritime influence serves to cool summer and restrict tree growth

| Northeast Tasmania, Australia

| data-sort-value="-41" |41°S

|Although sheltered on the leeward side of the island, summers are still cool for the latitude.

| Southwest Tasmania, Australia

| data-sort-value="-43" |43°S

|Exposed to the westerly storm track, summer is extraordinarily cool for the latitude, with frequent summer snow. Springtime receives an extreme amount of cold, heavy precipitation; winds are likewise extreme.

|Fiordland, South Island, New Zealand

|data-sort-value="-45"|45°S

|Very snowy springs, strong cold winds and cool summers with frequent summer snow restrict tree growth

|Lago Argentino, Argentina

|data-sort-value="-50"|50°S

|Nothofagus pumilio

|Torres del Paine, Chile

|data-sort-value="-51"|51°S

|Strong influence from the Southern Patagonian Ice Field serves to cool summer and restrict tree growth

|Navarino Island, Chile

|data-sort-value="-55"|55°S

|Strong maritime influence serves to cool summer and restrict tree growth

The subantarctic islands (South Georgia, Prince Edward, Crozet, Kerguelen, Heard and McDonald, and Macquarie Islands) lie in the Antarctic Circumpolar Current between 46.4° and 54.6°S. While the climate of these islands is cold and wet with long growing seasons, none of these islands have trees, due to the strong winds of the Roaring Forties and Furious Fifties.

Southern Rata forests exist on Enderby Island and Auckland Islands (both 50°S) and these grow up to an elevation of in sheltered valleys. These trees seldom grow above in height and they get smaller as one gains altitude, so that by they are waist-high. These islands have only between 600 and 800 hours of sun annually. Campbell Island (52°S) further south is treeless, except for one stunted spruce, probably planted in 1907. The climate on these islands is not severe, but tree growth is limited by almost continual rain and wind. The summers are very cold, with an average January temperature of , while winters are a mild but wet.

Causes

Variation

Types

Alpine

Tree species near tree line

Australia

Eurasia

North America

Worldwide distribution

Alpine tree lines

See also

References

Further reading