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.
Other valuable trees are the palms, three species of which are of particular importance. The coconut palm grows mainly in the coastal regions; but there are few villages anywhere in Thailand where the areca palm (producing the betel nuts which the peasants love to chew) and the sugar-palm (Borassus flabellifer) are not found.
The ipoh supplies the people of the forest with a powerful poison for their arrows. The rattan (Calamus) is a very common type of palm which yields the famous rattan cane; its long flexible lianas are put to many uses in the villages.
Apart from the trees the humid tropical forest has a proliferation of mosses, lianas and bushes in astonishing variety. There are many species of orchids, among which Dendrobium and Cymbidium are particularly notable. Orchids are used to decorate houses, hanging in little boxes from the roof, and many species are offered for sale in the Sunday market in Bangkok.
In the open forests the vegetation is less abundant, and the trees are of more modest size – between 7 and 15 m on hard dry soil. The king of these forests is teak (Tectona grandis), which grows singly or in small groups in the hills of northern Thailand, but only above 600 m. Its sturdy trunk can grow to a considerable height. Its wide oblong leaves fall during the dry season and begin to grow again in May and June; before they are fully out white flowers appear at the tips of the branches. The fruits, of the size of hazelnuts, fall to the ground when they are ripe (February-March).
Large areas round Nakhon Ratchasima and in western Thailand are covered with a monotonous dried-up forest carpeted with dusty soil in summer. The trees are stunted, with gnarled and bizarrely contorted branches. The commonest species is the bamboo, of various types, which form a typical “bamboo jungle”. Their woody stems can reach a height of some 40 m; they grow very rapidly, new shoots growing from the roots as the old canes die. The natives use the bamboo in many different ways: the thickest canes are used as timber or as water pipes, the thinner ones as stakes or fences or in the manufacture of furniture, musical instruments or domestic utensils, while the young shoots make excellent eating. Some of the bamboos flower only every 20 or 30 years, after which the plant dies. The bamboo tends to replace other species in forests, particularly rain forests, which have been cleared by man or by fire.
The pine, two species of which are found, grows mainly at higher altitudes. One species, with indented bark, occurs almost exclusively in northern Thailand; the other grows in the hills bordering the Khorat plateau but sometimes extends down into the Menam plain.
Climatic conditions (mainly the rainfall) have produced a specific type of grassland to the north-east of Nakhon Ratchasima, where continual flooding has eliminated the bamboos and favoured the growth of large areas of thung, a species of grass which grows on swampy ground. In summer the land takes on a steppe-like aspect, but in the rainy season the rivers overflow their beds, the steppe becomes green again and large lakes are formed. A few hills emerge as wooded islets, and hedges of bamboos grow along the rivers’ normal beds.
The mangrove forests characteristic of the tropics are found mainly in swamp-edged bays round the Gulf of Thailand. A strip almost 500 km long runs along the west coast of the Malay peninsula. At certain points in bays and estuaries which are sheltered by islands the rivers have deposited magnificent beds of sand, and this unstable soil, saturated with salt water and often muddy, provides ideal conditions for the growth of mangrove forest. At high tide only the crests of the trees emerge from the water; at low tide the protruding roots of the mangroves growing along the shore can be seen. The upper parts of the roots absorb oxygen. The long spike-like seeds are produced on the younger branches, and when they are about 50 cm long they fall and stick upright in the ground: set up in serried rows like this they form a barrier which may slow the movement of the tides. In spite of their dark leathery leaves mangroves need water, mainly on account of the heat, and the salt which is accumulated on the upper surface of the leaves is washed away by rain. In the more sheltered parts of the swamps various other woody species in addition to mangrove are found, and are used by the peasants to make charcoal. Some trees yield a fine brownish-red wood which is used in the manufacture of furniture.
In similar ecological conditions another type of short-stemmed palm, the nipa, is of common occurrence. It likes brackish water and forms colonies along the banks of rivers. They have usually been planted by man, since the nipa provides the raw material for wickerwork and basketmaking and also yields a much prized sugary sap.
Another tree commonly found in villages is the kapok, which can reach a height of 30 m. It has deeply indented hand-shaped leaves and white flowers, from which pods the size of a small cucumber hang on long stems. When the seeds are ripe the pods burst and release the “wool” in which the seeds are embedded. Near temples visitors will frequently see the bo or bodhi tree (Ficus religiosa), also known as the pipal. To the Buddhist it is a sacred tree, and its heart shaped leaves are found everywhere as a decorative theme. The tamarind has pretty yellow flowers of a velvety texture and a brown-fleshed fruit which makes a delicious drink. The flamboyant (Deploying regia) has large red flowers; at Bangkok it grows in profusion along the Sathorn canal. The jacaranda has vivid turquoise flowers, and its timber is much used in furniture. It is an import from South America.
Visitors will also encounter that astonishing tree, the banyan (Ficus bengalensis), whose branches descend vertically into the soil and form roots, which produce other branches; and as this process continues the tree extends farther and farther away from the main trunk. With its intertwining branches and its dense growth of foliage a single tree takes on the aspect of a copse rather than a tree. A particularly impressive specimen, of considerable age, can be seen by the roadside a short distance from Phimai.
The casuarina or kangaroo tree prefers a sandy soil. At first sight it looks like a pine; but what appear to be long needles are in fact delicate slender twigs. The leaves are reduced to small tufts between the knots on the branches. The tree reaches a respectable height, and its timber is much sought after for its exceptional hardness.
Of the numerous bushes to be found in Thailand some are well known in the west and have been acclimatised as decorative plants. The most notable are perhaps the oleander, jasmine and hibiscus. Gardenias form perfumed hedges. The oxora has thick clusters of white, red or orange flowers. The bougainvillea is found in many gardens. In ponds and ditches you will find the Indian lotus (Nelumbium) – not to be confused with the water-lily (Nymphaea), which is also common. Its leaves and round white flowers stick up out of the water, while the water-lily floats on the surface.