Thailand covers an extensive area of about 513,000 square kilometres in the middle of the Southeast Asian mainland, with very rich and diversified vegetation of about 15,000 plant species, ranging from the high mountains in the north-western hill tracts along the Tanasserim Range to the hill plateau of the north-eastern region, through the Central Plain into the eastern corner adjacent to Kampuchea; and into the peninsular region south of the Isthmus of Kra to the Malaysian border.