Geoscience Reference
In-Depth Information
During the Northern Hemisphere's winter, a high-pressure center over Tibet
sends relatively cool, dry air to mainland Southeast Asia and northern Sumatra.
During the summer, a center of high pressure over the Indian Ocean sends warm,
moist air northeast over much of the same area. This creates a seasonal monsoon
climate over much of mainland Southeast Asia, including Myanmar, Thailand,
Cambodia, Laos, and Vietnam. The seasonal shifts in the ITCZ control the timing
of the monsoons.
Annual rainfall in the Asian-Pacific seasonal forests is quite similar to that of
the rainforest but concentrated while the sun is highest in the sky. Average tempera-
tures range from 72
C), with the highest temperatures occurring just
before the rainy season. Total annual rainfall reaches 40-70 in (1,000-1,800 mm),
with higher totals on the windward sides of the mountains. During the rainy season,
consistent cloud cover blocks incoming solar radiation, causing temperatures to
decrease.
Changes in the monsoonal cycle can lead to prolonged drought. Periods of hot,
dry weather can create an environment vulnerable to fire. Many of the seasonal for-
ests are not resistant to such fires and can be converted to scrub and grasslands in
an environment of high fire frequency. El Ni
-87
F(22
-31
no events can affect this region signifi-
cantly, effectively weakening the monsoon and pushing it toward the Equator. This
creates a period of prolonged drought, often leading to extensive forest fires.
Tropical cyclones (typhoons, hurricanes) affect seasonal forests between 10
~
and 20
N and S latitude. These include the forests of Bangladesh, the Philippines,
and much of Melanesia and Australia. The occurrences of cyclones into forested
areas create stands of disturbed forest, where fast-growing, sun-loving, often-invasive
pioneer species tend to dominate.
Soils
The soils of the Asian-Pacific seasonal forests are similar to those of the rainforest,
but the percentage of area covered by each soil type differs significantly. Deeply
weathered, infertile oxisols make up a large percentage of soils in the seasonal for-
est of the Asian and Indian mainland. The majority of soils on older islands in the
region are ultisols. Other forests soil types on more recent volcanic islands include
inceptisols, alfisols, and entisols. Most forests with these soils have been used for
agricultural production, as they tend to be richer, well-drained soils of volcanic or
alluvial origin.
Vegetation in the Asian-Pacific Tropical Seasonal Forests
Like the rainforest, floristically this region is called Malesia and is separated into
two more or less distinctive subregions. West Malesia includes India, Southeast
Asia,
the Philippines,
the Malayan Archipelago, Brunei, and the islands of
