Introduction. Communities designated “alpine” exist on mountains and high plateaus above treeline, a zone of stunted woody plants that marks the upper limits of tree growth. The elevation of treeline generally occurs at lower elevations as latitude increases, but can also reflect mountain mass, regional climate (arid versus humid), and continental versus maritime influences.
The study of the zonation of vegetation according to elevation is still informed and inspired by the 19th century writings of the German geographer Alexander von Humboldt, who traveled through South America and climbed some of the continent’s highest peaks, most notably Chimborazo in Ecuador. He illustrated his findings in now classic drawings of the distribution of plants on many of the world’s highest peaks, one of which is shown below.
In North America, the study of altitudinal zonation was pioneered in 1889 by C. Hart Merriam, who described six “Life Zones” on the San Francisco Peaks outside Flagstaff, AZ. He defined a life zone as a belt of vegetation and animal life that is similarly expressed with increases in altitude and increases in latitude. This concept works well in the western US where it was designed, but falls apart in the tropics, where high elevation communities do not at all resemble low elevation vegetation or (usually) fauna.
Climate. The climate of a particular mountain depends on location. However, a significant factor in all alpine regions is atmospheric pressure. Air density diminishes with increasing altitude, so that at 8,550 ft asl air pressure is 26 percent lower than at sea level and at 19,000 ft. asl is only 50 percent of sea level barometric pressure. Less dense air translates to less carbon dioxide available to plants for photosynthesis and less oxygen for animals. Ambient temperature also decreases with elevation at the normal lapse rate of 3.5° F/1000 ft increase in elevation. Little water vapor can be held in thin, cold air, so high mountains are usually dry. Low temperatures also inhibit evaporation, further contributing to general aridity.
The thin atmosphere typically subjects alpine life to intense solar radiation. It can also produce extreme diurnal temperature changes. Snow cover, however, will reflect solar radiation back to apace during the day but insulate the ground during the night.
In rugged terrain, wind can be an important element of the climate. Upslope winds rise in response to warming during the day, but reverse after sunset as the alpine area quickly cools and denser cold air drains down into the valleys below.
The actual climatic conditions on any given mountain vary with elevation, latitude, aspect, and general regional climate. An intricate mosaic of microclimates may develop over very short distances in response to shelter from or exposure to wind and drainage conditions.
Soils. Soil development is slow due in part to low soil temperatures that inhibit the activity of microorganisms. Frost action and downslope slippage and erosion disturb upper horizons. On-site weathering, landslides, and glacial deposition often produces a mix of very large and very fine particles. Peat bogs, shallow stony soils, and deep grassland soils may be found a relative short distance apart. Windblown materials trapped by plants add fine particles and nutrients. Early in the growing season soils may be saturated from snow melt; later in summer the top inch or so may dry out. Soils may be stony entisols or lithosols, deep inceptisols with clear development of A and B horizons, or histosols, the bog soils found in depressions and other poorly drained sites.
MID-LATITUDE ARCTIC-ALPINE LIFE ZONE VEGETATION
The arctic-alpine zone vegetation of the mid-latitudes bears many similarities with the true Arctic. In North America and Eurasia treeline is variable according to latitude. It is reached near 17,000 ft asl in the Himalayas and at sea level in the Arctic; usually it is higher on inland, continental peaks and on larger mountain masses, both of which retain heat later into summer, than on coastal or less massive mountain ranges. In the Northern Hemisphere, the vegetation near treeline consists of stunted evergreen trees deformed by the wind, members of the pine family (spruce, fir, or pines) known as krummholz. Broad-leaved, deciduous birches and alders may also occur.
Like the Arctic, high elevation environments in the mid-latitudes of the Northern Hemisphere experience short growing seasons, low mean annual temperatures, seasonal differences in sun angle, and seasonal temperature changes. Permafrost may or may not occur on the highest mountains, but disturbance comes from avalanches and rock slides as well as frost action. Arctic-alpine tundra vegetation similar to the Arctic Tundra occurs at elevations above treeline. Many plant species are the same as found in the Arctic, although the mountain flora may be richer in species. The similarities between alpine and arctic floras diminish with latitude and essentially disappear around 30° N. In North America, where the major mountain ranges run north-south, there are or recently have been direct connections with the Arctic tundra. In Eurasia, major mountain chains such as the Altai ,Himalayas, Tien Shan, and Alps, are oriented parallel to the Arctic Tundra and are isolated from it, resulting in the presence of few truly arctic species. Nonetheless the same growthforms dominate on both continents.
Common growthforms are perennial herbs, graminoids (grasses and sedges), rosettes, cushions, mats, and dwarfed shrubs. Rosettes and cushion plants are usually most prominent on windy ridges and in areas without a snow cover. Grasses and sedges dominate in moist habitats. The prostrate woody shrubs commonly are either evergreen heaths or deciduous willows. Lichens and mosses are common. Most alpine regions host 200-300 species of higher plants.
In North America, arctic-alpine vegetation occurs in the west along the Cascades-Sierra Nevada range and the Rocky Mountain cordillera. The older Appalachian mountains are much lower in elevation, but regional climate is such that several peaks reach above treeline and have arctic-alpine zones. Among them are Mt, Katahdin (Maine), the Presidential range of the White Mountains (NH), and Mt. Marcy (NY). Alpine tundra communities also occur in eastern Canada on the higher peaks of the Gaspé Peninsula and the uplands of southern Labrador.
Perennial plants of the alpine flora are able to begin photosynthesizing at temperatures just above freezing. (Lowland plants usually begin growth at 40°-55° F.) Early season growth depends on energy and nutrients stored in roots or tubers and which will be replenished during the summer. At the beginning of the growing season many plants look red due to anthocyanin pigments, which convert light to heat and promote cold hardiness. During the summer, the red hues are masked by chlorophyll, but they reappear at the end of the growing season. Hairs on leaves and buds help trap heat and also help prevent sunburn of delicate tissues.
Plants reproduce both sexually and vegetatively. They may take 10-15 years before flowering for the first time. Vegetative or clonal development occurs via rhizomes, stolons, offset rosettes, or adventitious roots growing off buried stems. Many species are long-lived. Some clones of willow, alpine azalea, and cottongrass (a sedge) may be thousands of years old.
MID-LATITUDE ARCTIC-ALPINE LIFE ZONE ANIMAL LIFE
Unlike the Arctic, alpine tundra is home to several burrowing animals such as voles and, in North America, pocket gophers. Many small mammals are restricted to areas where deep, insulating snow accumulates over their winter shelter. Red-backed voles find such protection and remain active year-round under the krummholz; deer mice and pika shelter in rock crevices. Marmots and ground squirrels hibernate. Sure-footed ungulates such as mountain goat, mountain bighorn sheep, and, in Alaska, Dall’s sheep, live in small groups and scale very steep slopes to graze on summer pastures high in the mountains, but move to lower elevations in winter. Their counterparts in Eurasia are chamois and ibex in Europe, and thar, markhor, and blue sheep (Bharal) in the western Himalayas.
The availability of some small mammals all winter means weasels and martens can hunt all year long. Larger predators include brown (grizzly) bears and wolves. In Central and South Asia, the rare and elusive snow leopard is an inhabitant of the arctic-alpine life zone.
Few birds are year-round residents of the alpine tundra, the main exception being the White-tailed Ptarmigan of western North American mountains. Summer residents such as rosy finches, Water Pipit, and Rock Wren breed above treeline, but migrate away from the area in winter. Avian predators hunting in the arctic-alpine zone include Golden Eagle, Prairie Falcon, and Red-tailed Hawk.
Reptiles and amphibians are virtually absent from this life zone. Invertebrates are represented by abundant soil-dwelling springtails (order Collembola) and flying arthropods such as bumblebees, flies, and butterflies, all important pollinators of alpine plants.
SOUTHERN HEMISPHERE MID-LATITUDE ALPINE LIFE ZONE
Zonation of vegetation along slopes from high rocky ridges to the lowlands is displayed in the mid-latitudes of the Southern Hemisphere, but zones harbor entirely different species than found in Northern Hemisphere counterparts. Low shrubs are characteristic in all alpine habitats be they in the southern Andes of South America, the high plateau of Lesotho in Africa, or the Southern Alps of South Island, New Zealand.
The alpine areas in the mid-latitudes are in the Andes in central Argentina and parts of Chile (27°-39° S) and occur from about 14,000 ft asl in the north down to about 5,500 ft asl in the south. The mountains are snow-covered for several months in winter. Tree line is marked by tall shrubs that give way to small shrubs, needlegrass tussocks, and cushion plants.Small perennial forbs, rosettes, cushions, and shor grasses grow in the subnival zonebelow snowline. Endemic shrubs include the legume Adesmia pinifolia and daisy family evergreens Chuquiragui ruscifolia, and C. echegarayi. Among endemic cushion plants are two in the family Apiaceae– Azorella cryptoantha and Laretia acaulis, and a rush, Oxychloe bisexualis. Animlas are generally related to species of the Andean dry puna and Patagonian steppe. Endemic mammals include the endangered wild chinchilla, the Patagonian chinchilla mouse, and the Andean field mouse.
The broad, dissected plateau of Lesotho is a treeless region at 11,000 ft asl. The ground is waterlogged in summer. Austral winter brings nightly freezing and associated features such as needle ice, solifluction, and patterned ground. Alpine heath communities dominate in which low shrubs stand about 24 inches tall. Shrubs and strawflower cushions are scattered across extensive grasslands of tufted fescues, wiregrasses, and meadowgrasses in the genus Pentaschistis. Bogs form in swales.
Treeline in the alpine region of South Island, New Zealand (41°-47° S) is approximately 3,300 ft asl and snowline approximately 6,550 ft asl. The transition from forest into alpine vegetation is usually abrupt as southern beeches (Nothofagus spp.) are suddenly replaced by alpine tundra. Climate is maritime and annual precipitation on the windward side of mountains may total more than 160 inches. Nightly freezing is experienced during the winter, but in general conditions are less severe than in continental alpine zones elsewhere in the mid-latitudes. Nonetheless, typical alpine growthforms–rosettes, cushions, and mats–have evolved here. The most distinctive plants are so-called “vegetable sheep,” mound-forming rosettes of composites in the genera Raoulia and Haastii. A unique aspect of the New Zealand alpine region is an abundance of succulents on dry, sunny scree slopes. Plant species often have congeners in other parts of the world, but many are restricted to New Zealand or to New Zealand and some Antarctic islands. Many endemic birds breed in the alpine, most notably the large flightless rail, Takahē (Porphyrio hochstetteri) and the Kea (Nestor nobilis), a parrot.
TROPICAL ALPINE LIFE ZONES
Plant and animal communities existing above treeline in the tropics bear little resemblance to those in the mid-latitudes and are subject to a suite of different environmental conditions. The alpine zone that stretches in disjunct patches in the northern Andes from Venezuela to northern Peru (with a few outliers in Costa Rica) is called páramo. Another expression of the biome, known as puna, occurs on the Altiplano in the central Andes. On the highest East African mountains is an afroalpine zone, where plants show convergent evolution viz à viz growthforms with those of the South American páramo. In Indonesia and New Guinea, the communities above treeline are simply known as tropical alpine. A high elevation tropical zone also occurs on the highest volcanoes in Hawaii.
Treeline in the tropics is often indistinct. Where the alpine zone develops above rainforest or cloud forest, it may be marked by clumps of shrubs and stunted trees festooned with epiphytes and growing in a tussock grassland. On dry slopes, succulents and shrubs merely give way to an alpine flora.
Since tropical alpine zones exist in patches isolated from each other on mountain tops, the taxa present in a given area vary. However, there are similarities in climate, general environmental conditions, and the growthforms assumed by plant life. All are essentially steppes of tussock grasses with other growthforms scattered according to local climatic and soil conditions.
Climate. Daylength and temperatures vary little in high elevation tropical areas, but diurnal changes can be dramatic. While there is no prolonged period of cold to induce dormancy, “winter” may occur every night. Plants must be able to withstand freezing temperatures suddenly and at any time of year. Furthermore, daytime may find mountain tops shrouded in clouds formed by noon as warm moist air rises from rainforests in the lowlands or from the Trade Winds blowing off nearby oceans. This depresses temperatures, reduces light, and reduces the extreme daily fluctuations of temperature which occur under clear skies.
Rainy seasons alternate with less rainy seasons or dry seasons up to six months long depending upon geographic location. At equatorial sites, there are usually two periods of maximum precipitation, each corresponding with an equinox, when the Equatorial Low is overhead. The drier seasons then come near the solstices when the Low is most distant. Beyond the equatorial region, the Trade Winds influence the precipitation regime and there is usually only one period of maximum precipitation, which correlates with the Equatorial Low moving to the Tropic in the opposite hemisphere. [Generally in the tropics, maximum annual precipitation totals happen at mid-elevations in cloud forests. Above the cloud forest, totals decrease.]
Soils. All alpine regions tend to have young, poorly developed soils and scree or talus on steeper slopes. Level areas accumulated sediments and organic matter from decaying plants so have deeper soils. There is no permafrost, but nightly frost action disturbs the soil profile.
Vegetation. Five growthforms are characteristic of tropical alpine life zones around the world. Each dominates in a particular environment determined by soil and moisture availability: tussock grasses, cushion plants, sclerophyllous (hard, thickened) shrubs with small leaves, ground-level rosettes, and giant rosettes. The foliage of grasses, cushions, and low, stalkless rosettes create mild microclimates at ground level. Grass tussocks produce an insulating blanket of dead grass around their growth points which also stores moisture. Many plants are covered in hairs that shield growing tissues from intense solar radiation and also serve to trap humid air, reducing transpiration during dry days. Many are silvery, a color adaptation that helps to reflect sunlight. Leathery leaves and often waxy coatings are other mechanisms to prevent water loss via transpiration.Giant rosettes do not grow everywhere but are unique, photogenic elements of tropical alpine floras. In the Andes are Espletia and Puya; in Africa lobelias, giant groundsels, and giant senecios. Tree ferns develop into this growthform on New Guinea, while in Hawaii the silverswords do. Giant rosettes grow to 3-30 feet tall and have sheaths of dead leaves 4-12 inches thick to insulate the stems against cold; temperatures inside the stem are thus able to remain above freezing. The covering of dead foliage also traps moisture, and stores and releases nutrients to young leaves. Flowers on giant rosettes are often encased in bracts or dense hairs to protect them from the cold.
Animal life. Mammals in tropical alpine zones are often small, secretive, and have dull-colored pellage. Birds with large wing spans or small body size are at advantage in thin air, so very large condors (South America) and Old World Vultures (Africa) and very small hummingbirds (South America) and sunbirds (Africa) are characteristic members of the avian fauna. Hummingbirds and sunbirds are important pollinators of alpine plants.