The pattern of vegetation is of course strongly influenced by climatic variations. The alternation of seasons and moderate range of temperatures have favoured the growth of immense expanses of forest. There are two main types of forest, the dense evergreen forest and the open deciduous forest. The latter type is most frequently found in the north, at the foot of the western hills, in the upper Menam plain and on the Khorat plateau. The foliage is dense during the rainy season but disappears almost completely in the dry season, when the bare trees and the masses of dry leaves on the ground produce a rather monotonous effect which is accentuated by the greyish yellow colouring. The composition of the open forest naturally depends both on climate and on soil conditions. Where conditions are right a dense evergreen forest is produced, including species typical of the tropical rain forests. It is a kind of wooded savanna which extends into areas of poor soil (disintegrating sandstones), receives little water outside the monsoon season and is made up of woody species with a hard thick bark. In some of the less dry areas bamboo forests have grown up, no doubt as a result of deforestation - for example between the Khwae Yai and the Khwae Noi, between the rivers Ping and Yom, near Kamphaeng Phet and frequently on the Khorat plateau and south of Phetchaburi. The tropical forest is predominant, however, along the south-east coast, on the hills bordering the Khorat plateau, on the western ranges of hills and above all in the Malay peninsula, where large areas are covered with almost impenetrable jungle.

Characteristic features of this type of vegetation are the "layered" pattern of species and their incredible variety. The uppermost layer consists of giant trees, one of the most impressive of which is the yang (Dipterocarpus alatus); its great bare trunk rises to a height of 40 m and its crown of leathery leaves to between 40 and 55 m. The yang yields not only timber but oil. Another king of the tropical forest is the takhian, which may reach a height of 60 m; in addition to a very hard and resistant timber it yields dammar resin. The sandalwood tree also yields valuable wood and an oil which is used in perfumery.

There is a great diversity of trees of medium height. Among them are varieties of sterculia, whose enormous roots form "tables" between 1 and 2 m high. In swampy areas the pandanus, a palm-like tree, forms impenetrable thickets. In clearings the bamboo is predominant, and along watercourses a giant species is found which may grow to a height of 25 m and a thickness of 1 m; the bamboo is frequently used for collecting rainwater. Among the fruit-bearing trees of the tropical forest the most useful is the durian (Durio zibethinus), which does not grow beyond latitude 120 N. It may reach a height of 20 m. Its long narrow leaves have a silvery sheen and a brown or reddish inner surface; the yellow flowers are produced in clusters, which grow directly from the trunk or the larger branches. The fruit, which is as big as a man's head, is covered with pyramidal scales and contains five seeds the size of a hazelnut which are much prized for their creamy taste, though the husk has a penetrating smell of onions. In the plains the peasants grow the durian solely for its fruit.