Vegetation of Southeast Asia Studies of Forest Types 1963-1965

Broad-leaved Evergreen forest 1 seems to be the most appropriate.
The designation 'Evergreen forest 1 is not entirely applicable, because
it would also include Coniferous forest. 'Rain forest 1 , although
descriptive of t}he cause of the Type rather than of the Type
itself, is also acceptable because it is concise, readily understood,
and in general use in tropical regions.
Several factors are essential for the development of the Rain
forest. The major features are discussed briefly on the following
pages.
Climate: The prevailing climate in the Rain forest is marked
by relatively high temperatures. Variations in temperature are remarkably
slight on a diurnal, seasonal or geographical basis. According
to readings taken at typical sites, the mean annual temperature
is said to be about 79° K. (26°C.). Soil temperatures are equally
constant. Low temperature appears to be the major factor setting
the altitudinal limit of the Rain forest. It is essentially frostfree
except along its upper altitudinal border.
Rainfall: Rainfall is high. In areas with optimum Rain forest
there is no narked seeronal drought, but precipitation la scattered
throughout the year. Actually there are only limited areas in the
humid tropics which are truly non-seasonal. Wet and dry seasons
usually alternate. Tnis factor, in turn, may influence seasonal
rhythms in flowering, production of young leaves, and other phys\ological
processes. Unlike the temperature range, annual precipitation
is highly variable both diurnally and seasonally, as well as geographically.
In acme favorable distribution patterns, annual precipitation
of 63 inches (1,600 mm.) might be considered as absolute
minimum, whereas in the southwestern Peninsula of Thailand, 79 inches
(2,000 mm.) is regarded as a typical minimum. At Takuapa, in the
southwestern section of the Peninsula, the maximum may reach 10A
irches (i»,170 mm.), and in some years even more. Some areas with
annual precipitation above the minimum indicated may have drought
periods, especially in regions where there is high evaporation.
Therefore, the seasonal pattern is highly important, for true Rain
forest cannot exist in an area where there is a well-defined and a
prolonged dry period, lasting 2 or more months.
Humidity; In addition to high temperature and abundant rainfall,
Rain forest areas are characterized by uniformly high humidity. This
seems to be at or near the saturation point at night, and remains
high during most of the day, even during the dry season. This factor
may coinpensate for lower rainfall at certain periods in some areas.
Humidity tends to increase vith elevation. For example, along the
upper slopes and ridges of mountains, humidity may remain constantly
close to saturation.
Wind; This is an important factor.in humid areas.where its effect
on evaporation rate and resultant drying of the atmosphere could be