Southeastern Indochina dry evergreen forests
The Southeastern Indochina Dry Evergreen Forests ecoregion is globally outstanding for the large vertebrate fauna it harbors within large intact landscapes. Among the impressive large vertebrates are the Indo-Pacific region's largest herbivore, the Asian elephant (Elephas maximus), and largest carnivore, the tiger (Panthera tigris). The list includes the second known population of the critically endangered Javan rhinoceros (Rhinoceros sondaicus)-comprising a handful of animals in Vietnam's Cat Loc reserve-Eld's deer (Cervus eldi), banteng (Bos javanicus), gaur (Bos gaurus), clouded leopard (Pardofelis nebulosa), common leopard (Panthera pardus), Malayan sun bear (Ursus malayanus), and the enigmatic khting-vor (Pseudonovibos spiralis), known to science only by a few horns. But the ecoregion's conservation priority does not rest merely on its charismatic biodiversity. Importantly, it also represents a rare instance of a nonmontane ecoregion with large expanses of intact habitat that can allow viable populations of these species to survive over the long term. Unfortunately, all is not well in this haven, for plans to log Cambodia's forests, where most of the large habitat blocks lie, will result in large-scale habitat loss and fragmentation. Therefore, the ecoregion has been placed on the critical list.
Location and General Description
The Southeastern Indochina Dry Evergreen Forests ecoregion occurs in a broad band across northern and central Thailand into Laos, Cambodia, and Vietnam. Dry evergreen forest is more appropriately called semi-evergreen forest because a significant proportion of canopy tree species are deciduous at the height of the dry season. Extensive areas of this ecoregion in southern Laos and northeastern Cambodia occur with a largely deciduous forest canopy. The Southeastern Indochina Dry Evergreen Forests ecoregion occurs in humid and subhumid climatic regions where mean annual rainfall is generally between 1,200 and 2,000 millimeters, and a significant dry period of 3-6 months occurs each year. Semi-evergreen forest is the predominant forest cover in this ecoregion, but it often occurs in mosaics of deciduous dipterocarp or mixed deciduous forest communities. The distribution of dry evergreen forest habitats across a landscape catena is largely a function of gradients of soil moisture availability, with soil parent material unimportant. Therefore, it is common to see dry evergreen forest on both calcareous and crystalline rock substrates but with a deep soil profile and good capacity for soil moisture storage as a common characteristic.
Semi-evergreen forests represent a highly heterogeneous group of communities whose classification together derives from the parallel climatic and edaphic regimes that promote their formation. These communities have a characteristic tall and multilayered forest structure similar to that of lowland evergreen rain forest but grow in areas of lower and more seasonal rainfall regimes. In contrast to evergreen rain forests, species diversity is lower and the canopies and understory more open. Although dry evergreen forests have floristic relationships to lowland Indo-Malaysian forest communities, they exhibit a unique floristic structure that is rich in species endemic to mainland southeast Asia. With the broad distribution of these types of communities across Thailand and Laos, however, there are few local endemics.
The canopy of semi-evergreen forests generally is multilayered and reaches about 30-40 meters (m), with an open structure. Dipterocarps are a major component of the forest structure and form emergent tree canopies with such species as Dipterocarpus alatus, D. costatus, Hopea odorata, Shorea guiso, S. hypochra, and Anisoptera costata. Other giant emergents are species of Ficus, Tetrameles nudiflora, and Heritiera javanica, each forming large buttresses. In broad valley areas, semi-evergreen forest often occurs as a fringing gallery forest along streams, grading out into deciduous dipterocarp forest on drier sites with shallower soils. Unusually large trees such as Dipterocarpus turbinatus, D. alatus, and D. costatus (at the head of valleys) once formed dense stands in these forests, with a variety of other dipterocarps in ravines and scattered in such habitats. These species begin to drop out at about 700 meters elevation and are replaced by montane species.
Bamboos are common in the forest, particularly as colonizers of open gaps after disturbance. Palms are present, most notably along watercourses, but less abundant and diverse than in lowland tropical rain forest habitats. Lianas are abundant, but understory formations are less complex than in lowland rain forest, and species richness is also lower.
Semi-evergreen forests generally are intolerant of fire. In comparison to species in adjacent dry deciduous dipterocarp communities, woody species in dry evergreen forest resprout poorly after fire. Similarly, dry evergreen forest species are sensitive to drought, apparently because of their less well-developed root systems.
The ecoregion is globally outstanding for species richness, especially for the large vertebrate assemblage and associated ecological processes. The known mammal fauna of 160 species includes tiger, Asian elephant, douc langur (Pygathrix nemaeus), red-cheeked gibbon (Hylobates gabriellae), pileated gibbon (Hylobates pileatus), wild dog, Malayan sun bear, clouded leopard, common leopard, gaur, banteng, Javan rhinoceros, Eld's deer, and southern serow (Naemorhedus sumatraensis). The douc langur and the murid Maxomys moi are near endemics, whereas the northern smooth-tailed tree shrew (Dendrogale murina) is endemic (Table 1). Many of these species are threatened.
|Table 1. Endemic and Near-Endemic Mammal Species.|
|An asterisk signifies that the species' range is limited to this ecoregion.|
Asia's largest predator, the tiger, was once widespread throughout the Indochina bioregion. But hunting and loss of the prey base and habitat have fragmented its distribution, confining the viable populations to the few areas where large areas of intact habitat still remain, known as TCUs. These forests overlap with three high-priority (Level I) TCUs. Even within these TCUs, hunting and habitat loss continue to take their toll on this endangered species, which evokes images of the wild spaces in Asia that used to exist not very long ago. This ecoregion is one of the few in the Indochina bioregion that still has intact habitat and prey populations that provides hope for the tiger's long-term survival.
The ecoregion also harbors the critically endangered Javan rhinoceros, only the second population of this species known on Earth, the first being in the Ujung Kulon peninsula in western Java, Indonesia. The Indochinese population, discovered by scientists in the early 1990s, is limited to a handful of individuals confined to the small Cat Loc reserve in southern Vietnam. Their long-term survival is doubtful. However, most of this ecoregion's intact forests have not been scientifically explored, and a rhinoceros was allegedly captured from southern Laos, near the border with Cambodia, in the late 1980s. Therefore, discovery of additional populations is not improbable, especially because other species are known from body parts or reports.
The 455 bird species known from the ecoregion include two near-endemic species and one endemic species, the endangered orange-necked partridge (Arborophila davidi) (Table 2). Other species of conservation importance include the critically endangered giant ibis (Pseudibis gigantea) and the endangered white-winged duck (Cairina scutulata). The ecoregion overlaps slightly with BirdLife International's Endemic Bird Area, South Vietnamese Lowlands (144), near the Vietnam-Cambodia border.
|Table 2. Endemic and Near-Endemic Bird Species.|
|Phasianidae||Orange-necked partridge*||Arborophila davidi*|
|Phasianidae||Germain's peacock-pheasant||Polyplectron germaini|
|Timaliidae||Grey-faced tit-babbler||Macronous kelleyi|
|An asterisk signifies that the species' range is limited to this ecoregion.|
About two-thirds of the original forest in this ecoregion has been cleared or seriously degraded, especially in Vietnam and Thailand, but the habitat is relatively intact in Cambodia. A few large forest blocks also remain in Thailand and Laos. The thirty-one protected areas in this ecoregion cover 22,230 kilometers2 (18 percent) of the ecoregion (Table 3). These include seven reserves-Kulen Promtep, Thap Lan, Khao Yai, Xe Piane, Phou Xiang Thong, Beng Per, and Virachey-which are larger than 1,000 kilometers2. Overall, the protected areas in this ecoregion are large, with an average size of almost 750 kilometers2.
|Table 3. WCMC (1997) Protected Areas That Overlap with the Ecoregion.|
|Protected Area||Area (km2)||IUCN Category|
|Xe Bang Nouane||680||VIII|
|Huai Sa La||400||IV|
|Phou Xiang Thong||1,070||VIII|
|Phnom Nam Lyr||230||IV|
|Bien Lac-Nui Ong||300||IV|
|Bu Gia Map||270||IV|
|Nui Ba Ra||20||UA|
|Lo Go-Sa Mat||140||IV|
|Nam Bai Cat Tien||560||II|
|Nui Ba Den||40||UA|
|Duong Minh Chau||60||DE|
|Khao Sam Lan National Park||40||?|
|Khao Sam Lan||45||?|
|Ecoregion numbers of protected areas that overlap with additional ecoregions are listed in brackets.|
Types and Severity of Threats
Most of the forests in Vietnam have already been replaced by plantations. Shifting agriculture has further degraded some areas of this ecoregion. But the greatest threats now are from large-scale logging concessions that have been granted to multinational companies by the Cambodia government; therefore, the conservation status has been changed from relatively stable to critical.
Hunting to supply the huge wildlife trade has created empty forests throughout most of the ecoregion. From small, homemade crossbows used to kill small mammals for local consumption to bombs hidden in baited traps to kill tigers and pitfall traps for elephants, hunting has taken a very heavy toll on wildlife. The ravages of war and conflict have also had lasting effects; mines and bombs scattered across the landscape and the easy availability of automatic weapons that have replaced the crossbows have had deadly consequences.
Justification of Ecoregion Delineation
The extensive Indochina biounit (10) identified by MacKinnon in his analysis of conservation units across the Indo-Pacific region comprises three subunits that represent the tropical lowland plains, subtropical hills, and temperate montane areas. The largest of these subunits, Central Indochina (10a), is a mix of tropical moist forests and tropical dry forests. In keeping with our rules for defining ecoregions, we separated the distinct habitats of regional extent to delineate eight ecoregions using the overall framework of MacKinnon's biounit. The distinct narrow bands of dry evergreen forests along the low hills that wind through Indochina represent one of these ecoregions, the Southeastern Indochina Dry Evergreen Forests.
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
- Dinerstein, E., E. Wikramanayake, J. Robinson, U. Karanth, A. Rabinowitz, D. Olson, T. Mathew, P. Hedao, M. Connor, G. Hemley, and D. Bolze. 1997. A Framework for Identifying High Priority Areas and Actions for Conservation of Tigers in the Wild. Washington,DC: WWF-US and Wildlife Conservation Society.
- Stattersfield, A. J., M. J. Corsby, A. J. Long, and D. C. Wege. 1998. Global Directory of endemic bird areas. Cambridge, UK: Birdlife International.
- FAO. 1981. Tropical Forest Resources Assessment Project: Forest Resources of Tropical Asia. Rome: FAO and UNEP.
- IUCN 1991. The Conservation Atlas of Tropical Forests: Asia and the Pacific. London and Basingstoke: Macmillan Press Ltd. ISBN: 0333539923.
- MacKinnon, J. 1997. Protected areas systems review of the Indo-Malayan realm. Canterbury, UK: The Asian Bureau for Conservation (ABC) and The World Conservation Monitoring Center (WCMC)/ World Bank Publication. ISBN: 2880326095.
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