Central Indochina dry forests
Central Indochina dry forests are a significant ecological zone characterized primarily by deciduous dipterocarp trees. Covering approximately 123,600 square miles (320,123 square kilometers), these forests thrive in a warm climate with an annual rainfall of less than 60 inches (1,524 millimeters) and experience a distinct dry season lasting four to five months. The tree species in these forests are adapted to periodic droughts, shedding leaves to minimize water loss. Notably, this forest type supports a diverse range of flora and fauna, including endangered species such as the Asian elephant and various bird species, but faces severe threats from habitat loss due to agricultural expansion, hunting, and climate change.
Once a continuous ecosystem extending into India, the region's forests have become fragmented, particularly in Thailand, Laos, and Vietnam. The persistent pressures from human activity, combined with frequent fires, have transformed what were once lush forests into degraded savanna woodlands. Although still rich in biodiversity, the central Indochina dry forests are experiencing substantial declines in species populations, with many animals, particularly mega-herbivores and large carnivores, now endangered or extinct. These forests are considered among the most threatened ecosystems globally, underscoring the urgent need for conservation efforts to protect their unique ecological heritage.
Central Indochina dry forests
Category: Forest Biomes.
Geographic Location: Southeast Asia.
Summary: Indochina’s central dry forests are known globally for their large vertebrate fauna, reminiscent of those found in African savannas. Land use practices are making this biome vulnerable to degradation.
Indochina boasts a wide variety of forest types. Highly diverse, subtropical moist forests are found in the north, while dry deciduous forests and dry evergreen forests are found in southeastern Indochina. The dry deciduous forests of central Indochina are the most dominant forest type of them all, covering an area of 123,600 square miles (320,123 square kilometers).
Historically, these forests formed a contiguous ecosystem extending into India—until habitat conversion to agriculture in Thailand began to isolate various segments from one another. Large blocks of this forest type remain in north, northeastern, and south-central Cambodia, whereas most of the similar natural habitat is cleared away in Thailand, Laos, and Vietnam. Despite long-term human pressures that have degraded much of Indochina’s natural ecosystems, the fragments of dry forest that remain contain an extraordinary diversity of flora and fauna.
These deciduous forests are adapted to a dry season that lasts several months, followed by several months of monsoon rains. The climate is warm; temperatures rarely fall below 68 degrees F (20 degrees C). Annual rainfall is less than 60 inches (1,524 millimeters). Unlike nearby semi-evergreen forests, these forests withstand both a drier and a longer dry season of approximately four to five months.
Fires occur frequently from December to early March, when conditions are driest. The forests grow on thin, sandy, or rocky soil in a landscape that is generally flat, with a few scattered hills and seasonally inundated grasslands crossed by temporary streams.
Forest Composition
Another name for the central dry forest is deciduous dipterocarp forest, so called because it is dominated by deciduous dipterocarp tree species. Deciduous refers to tree species that shed their leaves for part of the year; this adaptation helps the trees reduce water loss when water availability is low. This behavior is common among deciduous trees in North America during the fall. Unlike North American deciduous trees, however, dry-forest deciduous trees do not shed all of their leaves at the same time.
Dipterocarp refers to a family of tropical tree species that are highly diverse and widespread throughout southern and southeast Asia, with two species that extend into central Africa. Of the nearly 550 identified dipterocarp species, 48 are found in Indochina. Only six species of dipterocarps are deciduous; all six occur in Indochina. They are the most ecologically as well as economically important tree group in the Southeast Asian lowland forests; they are the most dominant species, and they provide valuable tropical hardwoods. The wood is known for strong, light, straight, and knot-free timber. Oily aromatic resins are harvested from all the Asian species. Cambodians collect the resins by cutting holes at a tree’s base and periodically burning the area to stimulate resin flow.
A combination of drought stress and frequent fires have created low, open forests with understories dominated by grasses, cycads, shrubs, or bamboo beneath a closed or semi-closed canopy. The deciduous dipterocarp tree trunks are smooth and unbranching, with cauliflower-like crowns that often emerge above the canopy. Large buttresses at the base of some trees allow them to grow tall despite the thin soil.
Compared with their evergreen relatives, deciduous dipterocarps are shorter, about 30 to 120 feet (9 to 37 meters), with broader, thicker leaves. They are adapted to frequent fires, with thick, corklike bark, and the ability to resprout from roots. With increased stress from human pressure and repeated fires, these fire-adapted deciduous dipterocarp forests become degraded and transformed into savanna woodlands. Such a community retains the most fire-resistant species, interspersed with grasses and shrubs.
Biodiversity and Threats
Despite decreases in populations and species, this region is still considered to be one of the most zoologically diverse regions of the world. Many bird species are found here, including the giant ibis, white-rumped ibis, white-shouldered ibis, lesser adjutant, Sarak’s crane, green peafowl, and long-billed vulture.
In the early 1900s, scientists and big-game hunters described the game abundance in northern and eastern Cambodia’s dry deciduous forests as second only to the game abundance in the Serengeti. Large populations of mega-herbivores such as the Asian elephant, wild water buffalo, gaur, baten, kouprey, Eld’s deer, and both the Javan and Sumatran rhinoceros grazed these forests. They were hunted by large carnivores, including tigers, clouded leopards, leopards, and wild dogs. By 1936, the American Museum of Natural History’s Fleischmann-Clark Indochina Expedition reported declines in the numbers of wild water buffaloes and widespread, indiscriminate shooting of Eld’s deer.
Habitat loss from agriculture and human settlement, as well as hunting for local and international trade, has all taken a large toll on animal populations here. Both rhinoceros species have become extinct in the area, and the kouprey is considered to be extinct globally, with an occasional sighting in remote areas of Cambodia. All the other mega-herbivores, as well as the tiger, are endangered.
The greatest threats to the region are from human activities and climate; the area is densely populated, and the impact of human habitation is widespread. For example, fire is used to clear large areas for building and other purposes, which decreases the natural habitat. Studies show that because of climate change, dry forests are among the most threatened in the world.
Bibliography
Buchadas, Ana. "Uncovering Major Types of Deforestation Frontiers Across the World's Tropical Dry Woodlands." Nature Sustainability, vol. 5, pp. 619-27, 5 May 2022, doi.org/10.1038/s41893%2D022%2D00886%2D9,-Subjects. Accessed 8 Aug. 2022.
Sivakamur, M. V. K., H. P. Das, and O. Brunini. Impacts of Present and Future Climate Variability and Change on Agriculture and Forestry in the Arid and Sub-Arid Tropics. Dordrecht, Netherlands: Springer, 2005.
Sterling, Jane Eleanor, Martha Maud Hurley, and Le Duc Minh. Vietnam: A Natural History. New Haven, CT: Yale University Press, 2006.
World Wild Fund for Nature. “Southeastern Asia: Thailand, Cambodia, Laos, and Vietnam.” 2012. http://worldwildlife.org/ecoregions/im0202.