The Continental Interior

South-East Asia has an oceanic front in all directions except to the north. Its northerly land boundary commences as a panhandle between South and Central Asia and then on to the southern extremity of North-East Asia. The contiguous areas constituting the continental interior include the highlands of Myanmar, Thailand, Laos, and northern Vietnam. The relief pattern is that of a longitudinal ridge and furrow in Myanmar and an undulating plateau eastwards. These are related to their structural difference: the former being a zone of tertiary folds and the latter of block-faulted massifs of greater antiquity.

The basin of the Irrawady (Elephant River), forming the heartland of Myanmar, is ringed by mountains on three sides. The western rampart, linking Patkai, Chin, and Arakan, has been dealt with in the South Asian context. The northern ramparts, Kumon, Kachin, and Namkiu of the Tertiary fold, all trend north-south parallel to the Hengduan Range and are the highest in South-East Asia; and this includes Hkakabo Raz (5,881m) in the extreme north. East of the Irrawady lies the Shan Plateau which consists of mostly block-faulted massifs of the Mesozoic age which are rich in minerals. The topography has an undulating surface carved by the tributaries of the Sittang and Salween rivers. The hills are mature and rounded and approach 2,500m only in the extreme north. The population is predominantly Shan and Karen. Southwards, the Pegu Yoma extends 435 km as an outlier range between the Irrawady and Sittang rivers. Its high est peak, Popa Hill (1,519m), is an extinct volcano. The mountains of Myanmar are rich in tropical hardwood forests. Various hill tribes practise shifting cultivation on the uplands (Table 3).

The mountains of northern Thailand adjoining Myanmar and Laos are extensions of the Shan Plateau. They form a horseshoe at the headwaters of the Menam river. Those to the west are old ranges of granitic intrusions and upturned limestones. Doi Inthanon (2,595m), south of Chiang Mai, is the highest point. Evergreen forests predominate with mixed types below 1,800m and moist types above. Logging is making deep inroads in the area. The inhabitants above 1,000m are mostly Hmong who subsist on maize and poppy. Poppy growing has been controlled since the late 1970s and people have been encouraged to extract pine resin. Although there are several national parks in the Thai highlands, there is very little involvement of the local people as they are regarded as enemies of the forest (Ganjanapan 1998). The northern plateau is composed of ancient granite ridges. The plateau has a series of north-south ridges drained by four tributaries of the Menam River: from the west to east they are the Me Ping, Me Wang, Me Yom, and Me Nan. The climate is humid and precipitation ranges from 1,000-2,000 mm in summer. The winter is dry. Cultivation extends up to 1,200m upland with paddy as an important crop (Roder 1997). Swidden cultivation in secondary forest or shrub vegetation may be for two years, followed by a fallow period for as much as eight years. The eastern rim along the border of Laos is low in elevation. The natural vegetation is of the tropical monsoon rain type. The area has been the domain of more than a dozen ethnically different tribes such as the Akha, Hmong, Lisu, and Yao. They practise various forms of swidden agriculture and opium poppy is a major cash crop. For example, the Lisu cultivating at elevations ranging from 1,000 - 4,000m grow poppy after maize, while the Karen lower down (at 800- 1,200m) combine maize and upland rice on patches that are abandoned after three to six years of cultivation (Hurni 1982). Indigenous terms, such as raj (swidden), suan (garden), and naa (irrigated field), indicate the diversity of land use. The mountain watersheds of northern Thailand have extremely low runoff efficiency, e.g., the recorded surface runoff is only a fifth of the precipitation received (Alford 1992). Time-series' data showing little change in sediment regimes since the 1950s provide no conclusive evidence that swidden agriculture is a major factor in land degradation.

The northern part of Laos, along the eastern flank of the Mekong River, is mostly rugged highlands of sandstone and limestone. West of Luang Prabang, the relief form is made up of parallel ranges aligned south-west/north-east. Eastwards, the Xiang Khoang Plateau forms a compact block with a radial drainage pattern. Some of the highest peaks in Laos occur in this area—including the Phou Bea (2,820m) near the Plain of Jars. The general elevation of the plateau is 1,200m compared to 2,000m in the northern mountains. Towards the south, the Annamite chain is rugged and densely forested. The local tribes include the Miao in the east, the Moi at higher elevations above 900m, the Lolo in the north-west, and the Thai at lower elevations (below 1,000m). Ray, a local version of shifting cultivation is common in these highlands.

Vietnam's border with Laos and China in the extreme north is marked by a series of ranges. They are all aligned north-west/south-east, maintaining their trend in Yunnan from where they descend. Most of these are low ranges, approaching 3,000m on the central range between the Red (Songkoi) and Black (Songbo) rivers. The rock formation is mainly igneous overlain by limestones and sandstones. Fluvial erosion has created varied shapes on heavy limestone formations. With a tropical monsoon climate and an average annual rainfall of 1,500m, forests are an intermixture of deciduous and evergreen species. Much of these have been reduced to secondary types due to overexploitation through burning and clearing. The montagnards include the Hmong, Muong, Hani, and Yi.