Mountains are unique habitats because they create a climate of their own. Above sea level, the mean annual temperature falls by approximately 3° or 4° F. per 1,000-foot (2° or 3° C per 300-meter) increase in elevation up to an altitude of 5 to 10 miles (8 to 16 kilometers). These zones have parallels in those that are found with increasing latitude, and the top of a high mountain in the tropics may be similar in average temperature to arctic tundra. Averages, however, can be misleading because an alpine climate shows more daily and seasonal variation than an arctic one.
The alpine tundra is the area near the top of a mountain, which is above where trees can grow (the treeline). Plants in this area show many forms of adaptation to their environmental conditions which include elevation, temperature, wind, rainfall, and snow cover.
Adaptations to temperature, wind, and snow
At very low temperatures, plant enzymes do not function efficiently and biochemical processes generally slow down. Alpine plants, such as the saxifrages (Saxifraga spp.), form dense cushions or tussocks that lie close to the surface of the ground, where they are protected from the wind and where the microclimate is warm. Alpine plants, such as the mountain crowfoot (Ranunculusglacialisl, can tolerate low temperatures due to greater concentrations of sugars.
The Ruwenzori mountains dividing western Uganda and Zaire support many plants that show unusual adaptations to their climate. In the forests in the subalpine zone, temperatures alter sharply between day and night. As the temperature drops, some giant species of lobelia (Lobelia spp.) fold in their leaves, creating their own microclimate inside them. Temperatures of 41° F. (5° C) have been recorded inside these leafy spheres, while the temperature outside was below freezing. Rain, which is trapped in the plant’s rosette during the day and warmed by the sun, also helps to keep the plant warm during the night.
Other plants, particularly on the tropical mountains, protect themselves from frost with a build-up of dead leaf material around their stems and by leaves that fold in at night to cover the growing tip. The species of groundsel (Senecio spp.) found in the African equatorial mountains avoid tissue damage in this way. Alpine plants are sensitive to summer heat, such as the three-leaved rush (Juncus tri-fidus), which cannot survive in conditions where the mean annual maximum temperature exceeds 72° F. (22° C). Occasionally, plants requiring higher average temperatures for growth survive on south-facing slopes but not in the valley below. The reason for this is temperature inversion, in which cold air falls to the valley floor and warm air remains above it.
In winter, strong winds in the alpine tundra, which can travel at speeds of more than 100 miles (161 kilometers) per hour and may carry lacerating ice crystals, restrict plant growth and demand special adaptations. Shrubs, such as the dwarf birch (Betula nanaJ, grow close to the ground where they are less likely to be damaged by wind. Some species of willows, such as the violet willow ISalix daphnoides) of the Flimalayas, have supple branches and stems that yield to wind pressure without breaking.
The growing season in the alpine tundra is short because snow may persist late into the summer and can be a permanent feature of north-facing hollows. Snow patches shrink gradually during the warmer months to reveal a vegetation rich in bryophytes, such as the liverwort Anthelia juratzkana and club mosses like the fir club moss (Lycopodium seiagoi.
Snow cover can be advantageous to plants, protecting them from very low temperatures and from some wind damage. It also provides a source of water for growth early in the spring, which is necessary where soils are shallow and easily drained. Many alpine plants have adaptations normally associated with warm, dry climates, such as succulent leaves, as in moss campion (.Silene acaulis), hairy surfaces, as in the edelweiss (Leonto-podium alpinum), and thick, waxy leaves, as in many of the saxifrages. Not only do these adaptations prevent the loss of water, but they also help to reflect the strong light, preventing the plant from overheating. Some species push their buds up through the melting layers to flower—for example, the alpine snowbell (Soldanella alpina).
Growth and pollination
In the alpine tundra, annual plants are rare because the growing season is too short for them to complete their life cycle and produce ripe seeds. In the Alps, there are usually less than two months of the year that are frostfree. Most herbaceous plants that grow on mountains are long-lived perennials, which cope with the rigors of the climate by producing only a small amount of growth each year.
Low temperatures may result in a lack of suitable insects for pollination. There is much competition between plants for available pollinators, and flowers have developed adaptations to make themselves particularly attractive. The flowers of many alpine plants are large and showy and some, such as those of the mountain avens (Dryas octopetala), take the form of a parabolic reflector that focuses the sun’s rays to the central part of the flower, thereby raising its temperature and enabling the insects to work more actively.
Distribution of species
Some alpine plants have a wide global distribution, even though individual populations are isolated. The purple saxifrage (Saxifraga oppositifolia), for example, is found in a complete circle around the Arctic and in the mountains of Britain, the Alps, the Hindu Kush, and the Rocky Mountains of North America. Fossil evidence has shown that this species was present in many of the intervening lowlands during the cold conditions of the last glaciation and has become fragmented in its distribution only during the last 10,000 years.
Mountains, particularly in tropical areas, can be regarded as climatic islands, which are centers of independent evolutionary development. The African mountains, such as the Ru-wenzori, are rich in endemic species—that is, those species that are restricted in their distribution to one particular area. This area, for example, is known for its giant plants. It is likely that these sites were once in closer contact with one another, perhaps during periods of lower global temperature; subsequent isolation has, thus, allowed evolution to proceed independently, giving rise to new, endemic species. Most alpine species have limited powers of seed dispersal, so the tracts of other vegetation between mountains represent impenetrable barriers to their movement.