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Plants on the Himalayas

Plants on mountain sides above 3000 m have to face severe climatic conditions. They are, therefore, greatly modified to combat the combined effects of cold and heat, dampness and aridity, strong winds and sunshine.

Water supply is the greatest influence on life in high mountainous regions. High above in the mountains, the soil is covered by a deep mantle of snow for many months through the long winter. Streams are frozen solid and violent snow storms are a matter of routine. Naturally, the soil is often frozen from October to May and this means that plant life is impossible as the roots are unable to absorb water.

It may be as late as May/June before the higher summits are free of snow. But many slopes and summits may never be free from snow. Here plants life is difficult. By October the snow has usually recommenced and sharp frosts occur well before this date. In the short season, June to late September, the plant must produce leaves and flowering stems, its flowers must be pollinated, its seeds must be ripened and these must be distributed.

But even during summer, plants have to deal with vastly fluctuating conditions on a day to day basis. The day temperatures become much higher and in sunny places (the slopes that face the sun) the soil temperature may reach about 45 degree Centigrade and this makes for very rapid evaporation from the surface layers of the soil. The night temperatures, however, are below freezing point well into June so that for long periods the soil is frozen at night. These large daily extremes of temperature are very trying for plants, causing a very irregular supply of water to the leaves and stems and consequently disturbing transpiration. For this reason, alpine plants (plants that grow above the tree-line) have evolved special devices to control transpiration and consequent loss of water.

Although much rain and snow falls on the mountains most of it is lost through the running off of water from steep slopes; the stony soil is able to absorb little water and owing to its porous nature holds very little. The soils are hence dry. Again, not all places on mountains receive good rain. Rainfall can be torrential on the windward side and too scarce on the leeward side.

The fierce winds on mountain sides also cause rapid evaporation and contribute to the dryness of high mountain soils. One reason that mountains are windy is that the velocity of wind increases with altitude. Another is that winds pick up speed when forced through narrow places. Like a stream encountering a constricted channel, a wind confronted by two adjoining peaks may blow with extra violence through the gorge between. The lower atmosphereic pressure on the mountain tops also increases the evaporation rate.

Other factors influencing plant life in the alpine regions are the intensity of sunlight, rarity of the atmosphere, and less dust; but these effects are much more obscure than those of the other influences.

In order to combat such conditions alpine plants are highly specialized. They are thus almost all perennials, possessing an underground stem or rhizome in which food substances are stored during the summer. They pass the winter in a dormant state, their upper stems and leaves dying down. When spring arrives, the plant can commence growth immediately, using its stored food until such time that the roots can absorb water and nutrient salts from the soil.
To combat drought most alpines have a low, bushy habit; their leaves are usually very small and often have thick, hairy coats, coatings of wax, or a thick leathery epidermis; the root system is usually very well developed and in many cases the underground portion is much larger than the aerial portion of the plant. It is thus obvious that trees cannot exist at all in this zone and that tall herbaceous plants will be rare.

In spite of adverse conditions many species of plants have successfully survived on the high mountains. Wherever a pocket of soil occurs or along the sides of springs and rivulets, the competition between different species and also between members of the same species is very severe and the struggle to survive becomes merciless. Only the fittest can hope to carry on under these conditions.

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