Vegetation of Southeast Asia Studies of Forest Types 1963-1965

the section of the Peninsula south of Prachuap-KMrikhan. These
aroas correspond roughly to the principal physiographic provinces
indicatedt
(1) Northern Highlands
General features: This region is hilly, in parts mountainous,
and is the area in which teak forest flourishes (Figs. 37, 38). It'
is bounded on the north by the Dan Lao range, which forms the dividing
line between the Salween and Mekong river basins. On the west, the
Thanon Thong Chai range extends southward from the northwest frontier
to link up with the Tenasserim range. This, in turn, continues southward
to form the backbone of the Peninsula. On the south this region
borders the northern limit of the vast alluvial plain of the Chao
Phraya basin, while on the east it is flanked by the ranges of Luang
Phrabang and the Phetchabun ranges. Within this region the series of
hills and mountain ranges, running remarkably parallel to one another,
form the headwaters of the principal rivers - Ping, Wang, Yom - converging
near the town of Paknampo to form the Chow Phraya, the largest
river in Thailand, which meanders through the thickly popxilated central
plain.
Climate: Because of the mountainous nature of the North,
the variations in elevation impose a so-called mountain climate, characterized
by extremes in temperatures (Fig. 5). The lowest recorded
temperature is 37° F. (2.8°C.) in January, and the highest
103° (39.5°C.). Rainfall is generally moderate but of long duration,
being more or less periodic in distribution. Under the influence 'of
the southwest monsoon, a wet season prevails from the middle of May
to October. The coolest and driest season lasts from October through
February, and the hot season extends from March to mid-May. Fog is
most frequent in March and April when it sometimes lasts through tLe
entire day. Tne period from October to February, before the peak of
the hoi, season, is considered the jest for making ground surveys and
aerial photographs.
(2) The Oontral or Chao Phraya Plain
General features; This vast plain occupies the central part
of Thailand. It is bordered by a piedmont belt on the west, east and
north, arid by the Gulf of Thailand on the south. This central plain
is about 187 miles (300 kilometers) long from north to south and
ranges from 30 to 93 miles (50 to 150 kilometers) in width from east
to west. It is a deltaic plain, built up ( by the accumulation of alluvial
Materials, eroded by streams and rivers > from the northern highlands.
Dotting the plain are numerous isolated hills whose bases have been
buried by alluvium. The rocks are similar to those of the bordering
mountains. The plain commences at about latitude 18° north where the
rivers flowing from the north emerge from their valleys and f:L> ally
unite at Paknampo, to form the Chao Phraya. At Chainat, head of the
delta, about 30 miles (50 km,) farther south, the Chao Phraya
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