The pattern of life in Sri Lanka depends directly on the availability of rainwater. The mountains and the southwestern part of the country, known as the "wet zone," receive ample rainfall (an annual average of 250 centimeters). Most of the southeast, east, and northern parts of the country comprise the "dry zone, which receives between 120 and 190 centimeters of rain annually. Much of the rain in these areas falls from October to January; during the rest of the year there is very little precipitation, and all living creatures must conserve precious moisture. The arid northwest and southeast coasts receive the least amount of rain--60 to 120 centimeters per year-- concentrated within the short period of the winter monsoon.
The natural vegetation of the dry zone is adapted to the annual change from flood to drought. The typical ground cover is scrub forest, interspersed with tough bushes and cactuses in the driest areas. Plants grow very fast from November to February when rainfall is heavy, but stop growing during the hot season from March to August. Various adaptations to the dry conditions have developed. To conserve water, trees have thick bark; most have tiny leaves, and some drop their leaves during this season. Also, the topmost branches of the tallest trees often interlace, forming a canopy against the hot sun and a barrier to the dry wind. When water is absent, the plains of the dry zone are dominated by browns and grays. When water becomes available, either during the wet season or through proximity to rivers and lakes, the vegetation explodes into shades of green with a wide variety of beautiful flowers. Varieties of flowering acacias are well adapted to the arid conditions and flourish on the Jaffna Peninsula. Among the trees of the dry-land forests are some valuable species, such as satinwood, ebony, ironwood, and mahogany.
In the wet zone, the dominant vegetation of the lowlands is a tropical evergreen forest, with tall trees, broad foliage, and a dense undergrowth of vines and creepers. Subtropical evergreen forests resembling those of temperate climates flourish in the higher altitudes. Montane vegetation at the highest altitudes tends to be stunted and windswept.
Forests at one time covered nearly the entire island, but by the late twentieth century lands classified as forests and forest reserves covered only one-fifth of the land. The southwestern interior contains the only large remnants of the original forests of the wet zone. The government has attempted to preserve sanctuaries for natural vegetation and animal life, however. Ruhunu National Park in the southeast protects herds of elephant, deer, and peacocks, and Wilpattu National Park in the northwest preserves the habitats of many water birds, such as storks, pelicans, ibis, and spoonbills. During the Mahaweli Garga Program of the 1970s and 1980s in northern Sri Lanka, the government set aside four areas of land totalling 190,000 hectares as national parks.
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