User:Hike395/altitudinal zonation

Article titled either Altitudinal zonation or Ecology of mountains

Factors
A variety of environmental factors determine the boundaries of altitudinal zones found on mountains, ranging from direct effects of temperature and precipitation to indirect characteristics of the mountain itself, as well as biological interactions of the species. The cause of zonation is complex, due to many possible interactions and overlapping species ranges. Careful measurements and statistical tests are required prove the existence of discrete communities along an elevation gradient, as opposed to uncorrelated species ranges.

Temperature
Decreasing air temperature usually coincides with increasing elevation, which directly influences the length of the growing season at different altitudes on the mountain. For mountains located in deserts, extreme high temperatures also limit the ability of large trees to grow near the base of mountains. In addition, plants can be especially sensitive to soil temperatures and can have specific elevation ranges that support healthy growth.

Humidity
The humidity of certain zones, including precipitation levels, atmospheric humidity, and potential for evapotranspiration, varies with altitude and is a significant factor in determining altitudinal zonation. The most important variable is precipitation at various altitudes. As warm, moist air rises up the windward side of a mountain, the air temperature cools and loses its capacity to hold moisture. Thus, the greatest amount of rainfall is expected at mid-altitudes and can support deciduous forest development. Above a certain elevation the rising air becomes too dry and cold, and thus discourages tree growth. Although rainfall may not be a significant factors for some mountains, atmospheric humidity or aridity can be more important climatic stresses that affect altitudinal zones. Both overall levels of precipitation and humidity influence soil moisture as well. One of the most important factors that controls the lower boundary of the encinal or forest level is the ratio of evaporation to soil moisture.

Soil composition
The nutrient content of soils at different altitudes further complicates the demarcation of altitudinal zones. Soils with higher nutrient content, due to higher decomposition rates or greater weathering of rocks, better support larger trees and vegetation. The altitude of better soils varies with the particular mountain being studied. For example, for mountains found in the tropical rain forest regions, lower elevations exhibit fewer terrestrial species because of the thick layer of dead fallen leaves covering the forest floor. At this latitude more acidic, humose soils exist at higher elevations in the montane or subapline levels. In a different example, weathering is hampered by low temperatures at higher elevations in the Rocky Mountain of the western United States, resulting in thin coarse soils.

Biological forces
In addition to physical forces, biological forces may also produce zonation. For example, a strong competitor can force weaker competitors to higher or lower positions on the elevation gradient. The importance of competition is difficult to assess without experiments, which are expensive and often take many years to complete. However, there is an accumulating body of evidence that competitively dominant plants may seize the preferred locations (that is warmer sites or deeper soils). Two other biological factors can influence zonation: grazing and mutualism. The relative importance of these factors is also difficult to assess, but the abundance of grazing animals, and the abundance of mycorrhizal associations, suggests that these elements may influence plant distributions in significant ways.

Solar radiation
Light is another significant factor in the growth of trees and other photosynthetic vegetation. The earth’s atmosphere is filled with water vapor, particulate matter, and gases that filter the radiation coming from the sun before reaching the earth’s surface. Hence, the summits of mountains and higher elevations receive much more intense radiation than the basal plains. Along with the expected arid conditions at higher elevations, shrubs and grasses tend to thrive because of their small leaves and extensive root systems. However, high elevations also tend to have more frequent cloud cover, which compensates for some of the high intensity radiation.

Massenerhebung effect
The physical characteristics and relative location of the mountain itself must also be considered in predicting altitudinal zonation patterns. The Massenerhebung effect describes variation in the tree line based on mountain size and location. This effect predicts that zonation of rain forests on lower mountains may mirror the zonation expected on high mountains, but the belts occur at lower altitudes. A similar effect is exhibited in the Santa Catalina Mountains of Arizona, where the basal elevation and the total elevation influence the altitude of vertical zones of vegetation.

Other factors
In addition to the factors described above, there are a host of other properties that can confound predictions of altitudinal zonations. These include: frequency of disturbance (such as fire or monsoons), wind velocity, type of rock, topography, nearness to streams or rivers, history of tectonic activity, and latitude.

Merriam
Altitudinal zonation was first described by C. Hart Merriam in 1889, who used the term life zone to define areas with similar plant and animal communities. Merriam observed that the changes in these communities with an increase in latitude at a constant elevation are similar to the changes seen with an increase in elevation at a constant latitude.

The life zones Merriam identified are most applicable to western North America, being developed on the San Francisco Peaks, Arizona and Cascade Range of the northwestern USA. He tried to develop a system that is applicable across the North American continent, but that system is rarely referred to.

The life zones that Merriam identified, along with characteristic plants, are as follows:
 * Lower Sonoran (low, hot desert): creosote bush, Joshua tree
 * Upper Sonoran (desert steppe or chaparral): sagebrush, scrub oak, Colorado pinyon, Utah juniper
 * Transition (open woodlands): ponderosa pine
 * Canadian (fir forest): Rocky Mountain Douglas fir, quaking aspen
 * Hudsonian (spruce forest): Engelmann spruce, Rocky Mountains bristlecone pine
 * Arctic-Alpine (alpine meadows or tundra): lichen, grass

The Canadian and Hudsonian life zones are commonly combined into a Boreal life zone.

This system has been criticized as being too imprecise. For example, the scrub oak chaparral in Arizona shares relatively few plant and animal species with the Great Basin sagebrush desert, yet both are classified as Upper Sonoran. However it is still sometimes referred to by biologists (and anthropologists) working in the western United States. Much more detailed and empirically based classifications of vegetation and life zones now exist for most areas of the world, such as the list of world ecoregions defined by the World Wide Fund for Nature, or the list of North American ecoregions defined by the Commission for Environmental Cooperation.

Holdridge
In 1947, Leslie Holdridge published a life zone classification using indicators of:
 * mean annual biotemperature (logarithmic)
 * annual precipitation (logarithmic)
 * ratio of annual potential evapotranspiration to mean total annual precipitation.

Biotemperature refers to all temperatures above freezing, with all temperatures below freezing adjusted to 0 °C, as plants are dormant at these temperatures. Holdridge's system uses biotemperature first, rather than the temperate latitude bias of Merriam's life zones, and does not primarily use elevation. The system is considered more appropriate to the complexities of tropical vegetation than Merriam's system.

At moderate elevations in mountains, the rainfall and temperate climate encourages dense montane forests to grow. Holdridge defines the climate of montane forest as having a biotemperature of between 6 and 12 C, where biotemperature is the mean temperature considering temperatures below 0 C to be 0 C. Above the elevation of the montane forest, the trees thin out in the subalpine zone, become twisted krummholz, and eventually fail to grow. The elevation where trees fail to grow is called the tree line. The biotemperature of the subalpine zone is between 3 and 6 C.

Above the tree line the ecosystem is called the alpine zone or alpine tundra, dominated by grasses and low-growing shrubs. The biotemperature of the alpine zone is between 1.5 and 3 C. Many different plant species live in the alpine environment, including perennial grasses, sedges,  forbs, cushion plants, mosses, and lichens. Alpine plants must adapt to the harsh conditions of the alpine environment, which include low temperatures, dryness, ultraviolet radiation, and a short growing season. Alpine plants display adaptations such as rosette structures, waxy surfaces, and hairy leaves. Because of the common characteristics of these zones, the World Wildlife Fund groups a set of related ecoregions into the "montane grassland and shrubland" biome.

Climates with biotemperatures below 1.5 C tend to consist purely of rock and ice.

Biotic zones in mountains
The details of the altitudinal zonation in mountains depends on the latitude and other local factors.

Montane forests
Montane forests occur around the world. The elevation of these forests varies across the globe, particularly by latitude. The upper limit of montane forests, the forest line or timberline, is often marked by a change to hardier species that occur in less dense stands. For example, in the Sierra Nevada of California, the montane forest has dense stands of lodgepole pine and red fir, while the Sierra Nevada subalpine zone contains sparse stands of whitebark pine.

The lower bound of the montane zone may be a "lower timberline" that separates the montane forest from drier steppe or desert region.

Montane forests differ from lowland forests in the same area. The climate of montane forests is colder than lowland climate at the same latitude, so the montane forests often have species typical of higher-latitude lowland forests. Humans can disturb montane forests through forestry and agriculture. On isolated mountains, montane forests surrounded by treeless dry regions are typical "sky island" ecosystems.

Temperate climate


Montane forests in temperate climate are typically one of temperate coniferous forest or temperate broadleaf and mixed forest, forest types that are well known from northern Europe, northern United States, and southern Canada. The trees are, however, often not identical to those found further north: geology and climate causes different related species to occur in montane forests.

Montane forests around the world tend to be more species-rich than those in Europe, because major mountain chains in Europe are oriented east-west, which blocked species migration in the last ice age.

Montane forests in temperate climate occur in Europe (the Alps, Carpathians, Caucasus and more), in North America (Cascade Range, Klamath-Siskiyou, Appalachians and more), south-western South America, New Zealand and Himalaya.

Mediterranean climate
Montane forests in mediterranean climate are warm and dry except in winter, when they are relatively wet and mild. These forests are typically mixed conifer and broadleaf forests, with only a few conifer species. Pine and Juniper are typical trees found in Mediterranean montane forests. The broadleaf trees show more variety and often evergreen, e.g., evergreen Oak.

This type of forest is found in the Mediterranean Basin, North Africa, Mexico and the southwestern US, Iran, Pakistan and Afghanistan.

Subtropical and tropical climate


In the tropics, montane forests can consist of broadleaf forest in addition to coniferous forest. One example of a tropical montane forest is a cloud forest, which gains its moisture from clouds and fog. Cloud forests often exhibit an abundance of mosses covering the ground and vegetation, in which case they are also referred to as mossy forests. Mossy forests usually develop on the saddles of mountains, where moisture introduced by settling clouds is more effectively retained. Depending on latitude, the lower limit of montane rainforests on large mountains is generally between 1500 and 2500 m while the upper limit is usually from 2400 to 3300 m.

Subalpine zone


The subalpine zone is the biotic zone immediately below the tree line around the world. In tropical regions of Southeast Asia the tree line may be above 4000 m, whereas in Scotland it may be as low as 450 m. Species that occur in this zone depend on the location of the zone on the Earth, for example, snow gum in Australia, or subalpine larch, mountain hemlock and subalpine fir in western North America.

Trees in the subalpine zone often become krummholz, that is, crooked wood, stunted and twisted in form. At tree line, tree seedlings may germinate on the lee side of rocks and grow only as high as the rock provides wind protection. Further growth is more horizontal than vertical, and additional rooting may occur where branches contact the soil. Snow cover may protect krummholz trees during the winter, but branches higher than wind-shelters or snow cover are usually destroyed. Well-established krummholz trees may be several hundred to a thousand years old.

Meadows may be found in the subalpine zone. Tuolumne Meadows in the Sierra Nevada of California, is an example of a subalpine meadow.

Example subalpine zones around the world include the French Prealps in Europe, the Sierra Nevada and Rocky Mountain subalpine zones in North America, and subalpine forests in the eastern Himalaya, western Himalaya, and Hengduan mountains of Asia.

Alpine grasslands and tundra


Alpine grasslands and tundra lie above the tree line, in a world of intense radiation, wind, cold, snow, and ice. As a consequence, alpine vegetation is close to the ground and consists mainly of perennial grasses, sedges, and forbs. Annual plants are rare in this ecosystem and usually are only a few inches tall, with weak root systems. Other common plant life-forms include prostrate shrubs, graminoids forming tussocks, and cryptogams, such as bryophytes and lichens.

Plants have adapted to the harsh alpine environment. Cushion plants, looking like ground-hugging clumps of moss, escape the strong winds blowing a few inches above them. Many flowering plants of the alpine tundra have dense hairs on stems and leaves to provide wind protection or red-colored pigments capable of converting the sun's light rays into heat. Some plants take two or more years to form flower buds, which survive the winter below the surface and then open and produce fruit with seeds in the few weeks of summer. Non-flowering lichens cling to rocks and soil. Their enclosed algal cells can photosynthesize at any temperature above 0 C, and the outer fungal layers can absorb more than their own weight in water.

The adaptations for survival of drying winds and cold may make tundra vegetation seem very hardy, but in some respects the tundra is very fragile. Repeated footsteps often destroy tundra plants, leaving exposed soil to blow away, and recovery may take hundreds of years.

Alpine meadows form where sediments from the weathering of rocks has produced soils well-developed enough to support grasses and sedges. Alpine grasslands are common enough around the world to be categorized as a biome by the World Wildlife Fund. The biome, called "Montane grasslands and shrublands", often evolved as virtual islands, separated from other montane regions by warmer, lower elevation regions, and are frequently home to many distinctive and endemic plants which evolved in response to the cool, wet climate and abundant sunlight.

The most extensive montane grasslands and shrublands occur in the Neotropic páramo of the Andes Mountains. This biome also occurs in the mountains of east and central Africa, Mount Kinabalu of Borneo, highest elevations of the Western Ghats in South India and the Central Highlands of New Guinea. A unique feature of many wet tropical montane regions is the presence of giant rosette plants from a variety of plant families, such as Lobelia (Afrotropic), Puya (Neotropic), Cyathea (New Guinea), and Argyroxiphium (Hawaii).

Where conditions are drier, one finds montane grasslands, savannas, and woodlands, like the Ethiopian Highlands, and montane steppes, like the steppes of the Tibetan Plateau.

Examples of zonation

 * Ecology of the Rocky Mountains
 * California montane chaparral and woodlands, an ecoregion.
 * Ecology of the Sierra Nevada
 * Sierra Nevada lower montane forest
 * Great Basin Desert
 * East African montane forests
 * Afromontane, a series of high-elevation regions in Africa
 * Angolan montane forest-grassland mosaic, an ecoregion.
 * Australian Alps montane grasslands, an ecoregion.
 * South Western Ghats montane rain forests, an ecoregion
 * Life zones of central Europe
 * Life zones of the Mediterranean region
 * Life zones of Peru