The natural vegetation of Nepal follows the pattern of climate and altitude. A tropical, moist zone of deciduous vegetation occurs in the Tarai and the Churia Range. These forests consist mainly of khair (Acacia catechu), a spring tree with yellow flowers and flat pods; sissoo (Dalbergia sissoo), an East Indian tree yielding dark brown durable timber; and sal (Shorea robusta), an East Indian timber tree with foliage providing food for lac insects (which deposit lac, a resinous substance used for the manufacture of shellac and varnishes, on the treeās twigs). On the MahÄbhÄrat Range, at elevations between 5,000 and 10,000 feet, vegetation consists of a mixture of many species, chiefly pines, oaks, rhododendrons, poplars, walnuts, and larch. Between 10,000 and 12,000 feet, fir mixed with birch, as well as rhododendron, abound. In the mid-mountain region of Nepal a fairly dense population has cleared all but the most inaccessible parts of the forest, which are restricted to areas of steep slopes and rocky terrain. Similarly, all readily accessible parts of valuable sal forest in the Tarai have been devastated by overcutting and depletive practices. The vast forested area below the timber line in the Great Himalaya Range bears some of the most valuable forests in Nepal, containing spruce, fir, cypress, juniper, and birch. Alpine vegetation occupies higher parts of the Great Himalaya Range. Just below the snow line, between 14,000 and 15,000 feet, grassy vegetation affords favorable grazing ground in summer.