Ecological zones 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 2500 millimeters). Most of the southeast, east, and northern parts of the country comprise the "dry zone, which receives between 1200 and 1900 mm 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—600 to 1200 mm per year—concentrated within the short period of the winter monsoon. The natural vegetation of the dry zone has 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 20th 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