Vegetation of Southeast Asia Studies of Forest Types 1963-1965

system based predominantly on structure, rather than a knowledge of
the taxonomic identity of the plants composing the respective communities.
No casual factors need be considered, such as particular
edaphic or climatic conditions. The principal objective is to devise
a means of recording and plotting vegetation.
• Criteria For Classification
In the United States the prevailing practice is to separate forest
types by factors of composition, whereas European Geologists and
foresters rely more upon climatic and edaphic factors In the United
States a 'forest type 1 has been defined as a descriptive term used to
group stands of similar characters as regards con-.position and development,
owing to given physical and biological factors, by which they
may be differentiated from other groups or stands. The term suggests
a repetition of the same characters under similar conditions.
In order to establish a broad classification of tropical vegetation,
applicable to the vegetation of Thailand or to Southeast Asia in general,
important criteria for consideration are:
Habitat: This involves the primary factors of the environment,
namely climatic, physiographic, edaphic (soil and subsoil) and biotic
(felling, clearing, grazing, or burning).
The origin of the vegetation of any given habitat is very complicated.
This is well illustrated by the range of plant associations
occurring in Thailand, Vietnam or Puerto Rico. The presence or absence
of a plant species is determined not by a single environmental
factor, but a series of combinations is usually involved. Some of
these are favorable for the plant, others may be unfavorable. A plant
rarely, if ever, grows in an environment where all conditions are
favorable for its optimum development. A number of local or environmental
combinations exist, such as local variations in rainfall,
atmospheric and soil moisture, range of daily or seasonal temperatures,
velocity of wind, character of soils and intensity of light. In
addition, erosion and rock decomposition, and the destruction or
changing of old habitats may change the environment of the plant.
The primary cause for the presence of a plant association may depend
on factors which are active now or have been operative in the
past. This is well illustrated in Thailand, as elsewhere in Southeast
Asia, where changes have taken place in the distribution of
plant life. The destruction of the original growth over large areas,
as a result of man's activities during a period of many centuries,
has resulted in the modification of the original plant associations,
and the development, for example, of bamboo brakes and other secondary
growth.
Each plant species requires a more or less definite combination of
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