valleys and upland areas that border the central plain. Most rivers, including the Nan, Ping, Wang, and Yom, unite in the lowlands of the lower-north region and the upper-central region. The Ping River and the Nan River unite to form the Chao Phraya River. The northeastern part is drained by rivers flowing into the Mekong basin, like the Kok and Ing. Traditionally, these natural features made possible several different types of agriculture, including wet-rice farming in the valleys and shifting cultivation in the uplands. The forested mountains also promoted a spirit of regional independence. Forests, including stands of teak and other economically useful hardwoods that once dominated the north and parts of the northeast, had diminished by the 1980s to 130,000 km2. In 1961 they covered 56% of the country, but by the mid-1980s forestland had been reduced to less than 30% of Thailand's total area. Northeastern Thailand The northeast, with its poor soils, also is not favoured agriculturally. However, sticky rice, the staple food of the region, which requires flooded, poorly drained paddy fields, thrives and where fields can be flooded from nearby streams, rivers and ponds, often two harvests are possible each year. Cash crops such as sugar cane and manioc are cultivated on a vast scale, and to a lesser extent, rubber. Silk production is an important cottage industry and contributes significantly to the economy. The region consists mainly of the dry Khorat Plateau which in some parts is extremely flat, and a few low but rugged and rocky hills, the Phu Phan Mountains. The short monsoon season brings heavy flooding in the river valleys. Unlike the more fertile areas of Thailand, the northeast has a long dry season, and much of the land is covered by sparse grasses. Mountains ring the plateau on the west and the south, and the Mekong delineates much of the northern and eastern rim. Some varieties of traditional medicinal herbs, particularly of the Genus Curcuma, family Zingiberaceae, are indigenous to the region. Central Thailand The "heartland", central Thailand, is a natural self-contained basin often termed "the rice bowl of Asia". The complex irrigation system developed for wet-rice agriculture in this region provided the necessary economic support to sustain the development of the Thai state from the 13th century Sukhothai Kingdom to contemporary Bangkok. Here the rather flat unchanging landscape facilitated inland water and road transport. The fertile area was able to