The terminology used to describe these forest types and their sub-groups can be confusing and contradictory dependent on who is the audience and who is the speaker as many terms can cause confusion between their scientific meaning and their use in general conversation or as descriptions. It is further complicated in that many of the forest types and sub types occur within the same geographical, area as elevations, soil types, and localized conditions change. Indeed one single national park or wildlife sanctuary can have multiple different forest types present. For example Ta Phraya National Park contains mixed deciduous, dry evergreen and dry deciduous forest as well as bamboo forest in some of its disturbed secondary areas.
It is easier to comprehend the forest types if one breaks them down into the following groups:
Deciduous Forests and Evergreen Forests
The simple difference between these two groups is that in a deciduous forest almost all species of flora in the forest will shed their leaves in the long dry season months. In an evergreen forest there is a higher rainfall and species will fall and replace leaves all year and the forest remains green all year round.
That's not to say that Evergreen Forests (esp. Mixed Evergreen) do not drop more leaves during the dry season - they do, just that Deciduous will drop virtually ALL leaves from their dominant species and lose their green colour.
Deciduous Forests can be split (mainly) into 3 groups: Mixed Deciduous, Dry Deciduous and Bamboo Forest. In Thailand 70% of all forests are considered Deciduous Forests.
Evergreen Forests can be split (mainly) into 5 groups: Moist Evergreen, Dry Evergreen, Hill Evergreen, Mangrove Forest, and Pinus Forest. In Thailand 30% of forests are considered Evergreen Forests.
The dominant forest type in Thailand. Is recognizable by a fairly well spaced growth of trees that allows sunlight to penetrate to the ground and allow grasses and other ground growths. The trees are broad leafed (such as teak) with a distinct shedding on leaves in the dry season which can last 4 months. Huai Kha Khaeng has good examples of mixed deciduous forest.
Also known as Dry Dipterocarp Forest. A drier version of Mixed Deciduous that has a distinct leaf fall after coloration change in the leaves (i.e. think of autumn leaves in Europe or N. America)and is drier. Trees rarely exceed 15 meters, and are well spaced with lots of ground cover evident. The trees are limited largely to 4 species of dipterocarp that can survive in the poor and dry soil conditions of this forest type. Found mainly in N.E. Thailand.
Bamboo is found interspersed in many other types of forest and as a pioneer species it is quick growing and fast to colonize disturbed forest sites, both natural and man-made. As such and due to the logging excesses in Thailand in the past many bamboo forests have become established in such disturbed sites. Bamboo is actually a grass and not a tree.
Also known as Tropical Evergreen, large tracts this type of forest only exist south of Ranong in Thailand although some small pockets do exist in Central Thailand (indeed it is being considered to name a forest type that is between Moist Evergreen and Mixed Evergreen in Central Thailand). Your typical "rainforest" and a product of a short dry season and high yearly rain fall of over 2 meters. Canopy layers are very evident and little light reaches the forest floor.
Also known as Dry Evergreen, it differs from Moist Evergreen by having a longer dry season (3 to 4 months) and a more conspicuous shedding of foliage in the dry season. It occurs north of Rayong. Khao Yai and Thap Lan have large mixed evergreen tracts. Canopy layers are very evident and little light reaches the forest floor. yearly rainfall is less than 2 meters.
Also known as Cloud Forest, normally not found below 1000 metres and temperatures are usually cool throughout the year. Trees are normally less tall than other areas and covered in mosses, lichens and ferns. Doi Inthanon and Khao Luang are good examples at their higher elevations of this forest type.
Estuarine forests containing mangroves exist in coastal areas of brackish water and salt water throughout Thailand. Ranong and Pranburi are good examples of intact mangrove forests.
Forests containing primarily pines and conifers. They can grow between 200 and 1800 meters above sea level but are are usually found at higher elevations and on exposed hill tops. Primarily found in the North of Thailand but an isolated natural stands exists in Phu Toei National Park also.
This covers the main types of forests present in Thailand but not all as there are others such as savannah and swamp forest in her rich portfolio.
Perhaps we should introduce two more definitions that are important in describing forests in Thailand, those being definitions that are very important in describing the forest condition. They are Primary forest and Secondary forest:
Primary forest refers to untouched, pristine forest that exists in its original condition.
Secondary forest is forest that has been disturbed in some way, naturally or unnaturally.
Now you should be able to describe most of what you encounter, I hope.