Alpine plants are so-called because they live in the alpine zone of mountains. In this area which lies between the timber-line and the zone of permanent snow fields where plant life is absent, dwarf shrubs and ‘cushion plants’ predominate in alpine meadows, on scree and in crevices of rocks. These plants live in the face of a climate so extreme that evolution has moulded their forms in a highly specialized and characteristic fashion to survive in conditions as harsh as may be found anywhere on the globe. Floods of rain, drought, baking heat, extreme cold, winds of intense ferocity, radiation and unstable soil conditions all have to be withstood if a plant is to survive at such levels of exposure. Any slight rise in the terrain which may afford shelter from the biting wind or temporary protective blanketing of snow in a hollow are used to maximum advantage in this inhospitable environment.
The alpine zone is typically found above around 2,400m (7,900ft) in central and southern European mountain systems, higher than this in mountains of warmer climes. In the East African mountain ranges, for instance, the alpine zone occurs between 4,000m and 5,000m (13,000ft and 16,500ft), while a typical mountain of tropical Indonesia or Malaysia has alpine shrubs above 3,800m (12,500ft) and alpineabove 4,500m (14,800ft). Here, at these upper limits of plant growth, the land is rock-littered and rolling, broken by peaks and cliffs, with stretches of bare wind-blown rock faces interspersed with snow pockets, with patches of brown vegetation, either dead or dormant. Wind whistles across the scene perpetually, sometimes at devastating speeds of i6okmph (ioomph) or more. Despite such difficult conditions for growth and reproduction, some of the most perfect and delicate blooms of all the are found in this zone. The tiny exquisite snow buttercup, Ranunculus nivalis, has been found budding under a 3m (10ft) snow bank, whilst the little bell-bloomed stalks of the alpine soldanella, Soldanella alpina, which melt their way up through their snow covering by radiating stored energy as heat, contrast sharply with the severity of their surroundings. Spring and summer, though of short duration, can transform the scene into a blaze of flowering colour.
Growth and shape
Survival at these heights depends largely on the adoption of one particular habit—low growth. The high winds of the alpine zone are often charged with rock particles and ice crystals; exposure to such winds invites damage to the delicate tissues of plants. Standing on an exposed mountainside, a man can be blown over by the force of the wind. Should he now sit down, he finds the effects of the wind’s buffeting are considerably reduced. If he lies flat on the ground, he would find that the wind has little or no effect upon him now. Alpines are often called ‘belly plants’ by scientists because they grow so low, out of the wind’s way, that an observer must lie on his stomach to investigate them. This characteristic habit is often reflected in their names—acaulis (without), prostrata (prostrate), procumbens (leaning forward), humilis (humble). The drag effect on the wind, which operates close to the ground, is increased by the presence of lumps and bumps in the surface which break up the flow of air, providing pockets of shelter from the wind. Even the tiniest irregularity can provide shelter for a plant which is small enough to grow within its lee.
As well as low growth, plants need to be supple so that they can bend before the blast. Rigid woody structures would soon be snapped off, and the only woody plants in alpine zones have pliable branches which can adopt all manner of shapes. The Rocky Mountain snow willow is such a plant. Though it hugs the ground, rising to only a few centimetres in height, it still manages to produce a profusion of catkins.
Cushion-shaped plants, inconspicuous ground-hugging domes, are another common feature of the alpines. Here closely-matted stalks clump together, providing an ideal form forwind resistance, and also acting as a heat-trap. Thus despite great diurnal fluctuation of temperature externally, within the cushion a buffering effect is experienced as heat from the sun during the day-time is absorbed and retained. Temperature measurements inside a cushion of Silene acaulis, the cushion pink or moss campion (a numerous and successful alpine of the subarctic, found in North America, Europe and Scandinavia), have shown that its interior may be as much as 10°C (50°F) warmer than the surrounding air.
Many mountain plants occupy rock crevices. These chasmophytes, as they are called, escape competition from other plants, and the reach of grazing animals and man; the chasmophytic habitat can, in fact, provide a last refuge for plants under pressure from adversely changing. environmental conditions and the activities of man. It is remarkable how theircan obtain enough moisture and nourishment in the tiniest cracks in a rock or cliff.
Alpines grow extremely slowly. The addition of less than a centimetre of stem during a single season is quite normal in mosses and higher plants, lichens grow even more slowly. A typical cushion-plantputs out as few as two minute a year; a mat of mountain avens, Dryas octopctala, a metre across, was probably a seedling a century ago.
The little growth that these plants are capable of is concentrated in their short growing season into producing thethey need to reproduce and into the long which they need to collect moisture in the thin dry soils of these upper altitudes. Timing is all-important for alpine plants. During cold weather they lie dormant, conserving their energy, waiting for the summer season and the chance to reproduce. The arrival of the warmer weather heralds a burst of activity, as the plants race against time to produce their and —in Norway, a snow buttercup, Ranunculus nivalis was found to flower five days after the snow had melted off the plant and only seventeen days later, it bore ripe . But with summers so short, a is unable to mature and produce seeds of its own all in a single season, so most mountain plants are perennials, gradually building up their store of energy until it is sufficient for flower production. Of more than 300 species of flowering plants to be found above the timber-line in the Rockies, for example, only two are , and one of these produces flowers of only pinhead size. Many mountain plants must wait for three or four years before they can risk blooming. The glacier lily, which waits seven years before flowering, can die in the struggle to replace its leaves should an animal eat them early in the season, and any careless picking by climbers almost always spells death. A mat of Silene acaulis may take as long as ten years to establish itself sufficiently to produce its first flower, and twenty years before full bloom is achieved.
How alpine cushions form
The cushion is a characteristic growth form obstacle to wind, and also form a heat-trap from which branches radiate, each bearing of alpine plants: the tightly matted stalks inside which the sun’s heat is retained at small leafy shoots packed together in the close to the ground provide virtually no night. Cushions arise from a centralsmallest possible space.
Many alpine flowers are brilliantly coloured, such as the rich wine-colouring of the purple saxifrage Saxifraga oppositifolia, the deep golden yellow of the snow buttercup Ranunculus nivalis, and the vivid dark blue of the gentian, Gentiana nivalis. These intense colours absorb light and heat, and are possibly particularly attractive to insects, of which there are a surprising number in view of the rigours of the environment and tendency to get blown away by the wind (60 percent of insects found above the timber line are actually wingless, and many of those which do have wings seldom use them). There are many pale-coloured and white flowers also; possibly these stand out particularly well against the dark green heat-absorbent leaves.
The warmth-retaining interiors of the cushions like Silene acaulis are a haven to cold-gripped insects which, whilst crawling around within, effect. Many mountain plants are adapted for wind-pollination, an effective method in this zone of low plant growth and high wind velocities. Others are self-fertilizing, for example the grass Festuca ovina, while some species such as the mountain buttercup, Ranunculus moritana, can set seeds without being fertilized at all. The mountain grass Poa alpina has seeds which germinate while still attached to the parent plant, while some other grasses produce new shoots instead of seeds, as with Festuca vivipara. The drooping saxifrage Saxifraga cernua forms bulbils which drop off and winter beneath the snow, producing new plants the following spring.
Structural and physiological adaptations
Much of an alpine plant’s energy is directed towards its roots. Thin dry soils mean a wide root spread to obtain sufficient water, and blasting winds together with an unstable soil demand good root anchorage. Megarrhiza (large bulb or root) is a name which can be found associated with a number of alpine plants; a moss campion only a few centimetres high can have roots of a metre or more in depth. Long tap roots can often be found associated with plants growing in moving scree, where they act as anchors. Such plants, slithering steadily downhill with the superficial surface, can often be found with the point of origin of the root actually above the plant!
Leaves show adaptations to mountain conditions, many alpincs having adopted the ‘evergreen habit’ with one set of hard-earned leaves used for more than one season. Leaves are often thick and waxy, or furry as in the Swiss edelweiss, Leontopodium alpinum, to cut down on surface evaporation. Hairs also serve to retain heat; the snow willow Salix repens has fuzz-covered buds, the hairs being white and attached to a black core, so that light and heat penetrating the translucent hairs are absorbed by the black interior and retained there as escape is prevented by reflection. The willow bud acts, in effect, like a miniature.
A number of alpine plants are protected from frost damage by having a rich cell fluid which acts like anti-freeze, for example the mountain crowfoot Ranunculus glacialis. Some plants are so well adapted to cold and a minimum of sunlight that they are able to start growing while still lying deep beneath a snow covering as with Ranunculus nivalis and Soldanella alpina. Snow is, in fact, a great protector and insulator; extreme temperature variations are not felt within it and, provided it melts sufficiently at one point to allow sunshine to penetrate and the plant to photosynthesize sufficient starch to carry it through the winter, it can successfully nurture plant life.
Zonation of vegetation
At any latitude, as one goes up a mountain, provided that the vertical rise is sufficient, there is a zonation of climates which reflects the major climatic zonation throughout the world from the equator to the poles. Correspondingly, there is a zonation of vegetation belts as one rises up the mountain-side, so that a mountain compresses into a short space conditions and resulting vegetation types which on the flat would be spaced out over thousands of miles. Obviously there are local variations depending on the situation of the mountain. In certain tropical mountains, rain forest gives way to various levels of montane forest vegetation with progressively lower canopy levels, above which come heaths of tussocky grasses and sedges with gigantic woody treelike forms of genera which in temperate climes are small herbaceous plants. African examples of this gigantism include the tree, a gigantic member of the groundsel family which grows to a rosette of cabbagelike leaves and a flowering spike, and giant lobelias which can be as much as 6m (20ft) tall. Gnarled heather trees occur too, draped in lichens and drooping over beds of mosses and liverworts. This phenomenon of gigantism occurs both in equatorial African and South
American mountain systems, though the plants involved are of different families and genera. Above this zone comes the alpine zone of low-growing flowering plants and lichens, and finally the zone of permanent snow and ice is reached.
Despite minor local variation, the harsh environmental factors which a plant must face at high altitudes are the same the world over, and as a consequence widely-separated plant species, which are quite un-rclated to one another and whose lowland-dwelling relatives do not look at all alike, have evolved into similar forms as adaptations to similar environmental pressures. This is true of many of the un-related alpine plants which have adopted the widespread cushion form. Such ‘convergent evolution’ is responsible for the strong parallels which can be drawn between the high altitude flora of two very separate mountain systems, for example the Andes and the East African ranges. Convergent evolution from such dissimilar origins is not hard to understand when one remembers that evolution is accelerated by spontaneous mutations which are more likely to occur under the following conditions: extremes of heat or cold, radiation, oxygen deficiency. All these factors are present at high altitudes and thus the higher one goes, the greater the potential for genetic change will be.
Alpine plants also show another noteworthy feature: many of them also occur in arctic regions north of the tree boundary. Plants such as the snow gentian, Gentiana nivalis, the mountain avens, Dryas octopetala, the rockfoil
Saxifraga stellaris and the moss campion, Silene acaulis, belong in this arctic—alpine category. The Rockies share, in fact, as many as 65 species of flowering plants with the North American arctic. It is thought that this disjunct distribution arose when advancing ice forced the arctic flora southwards, and upon its retreat certain of the returning arctic species encountering mountain ranges became established at high altitudes, finding conditions there as suitable for their particular adaptations as farther north on the flat. Both environments are comparable in their severity; in both the survival of living things is poised on a knifeedge, a delicate balance between death and survival.