Included here is much of Thailand together with the western parts of Laos and Tenasserim (Burma).

Northern Thailandian Lower Montane Forest

These forests cover the moist valley basins and slopes at altitudes ranging from 1000-1800 m. Here the forest composition is dominated by the families Fagaceae, Magnoliaceae, Lauraceae and Theaceae, while members of the dominant lowland families like Annonaceae, Diperocarpaceae, Meliaceae, Sapindaceae and so on become far less conspicuous. The principal trees are Actinodaphne henryi, Canarium subulatum, Castanopsis acuminatissima, Lithocarpus auriculatus, Quercus brandisiana, Paramichelia baillonii and Schima wallichii. Structurally they can be divided into three tree-layers, but have poorly developed undergrowth and there are few woody lianas. Examples of some of the smaller trees are Adinandra integerrima, Lindera pulcherrima, Symplocos racemosa and the two endemics Chionanthus sutepensis (Oleaceae) and Glochidion acuminatum var. siamensis (Euphorbiaceae). The endemic palm Wallichia siamensis (Arecaceae) may also be encountered. In the shady areas the rare root parasite Sapria himalayana (Rafflesiaceae) may also be found. Like its larger relative Rafflesia, it has striking large orange-red flowers. In parasitizes the two herbaceous climbers Parthenocissus himalayana and Tetrastigma serrulatum. The few other climbers are Gnetum montanum, Myxopyrum smilacifolium and Ventilago calyculata. Epiphytic shrubs, on the other hand, such as Agapetes hosseana and Rhododendron veitchianum are frequently found, and there are many epiphytic herbaceous species including the endemic orchid Vanilla siamensis.  In areas affected by shifting cultivation, the secondary growth is often dominated by oaks such as Castanopsis fissa, while the herbaceous elements typically includes the two endemic species Hieracleum siamensis (Apiaceae) and Pedicularis siamensis (Scrophulariaceae). On exposed ridges and steep slopes where there is frequent soil erosion or where there are excessive anthropogenic impacts such as burning, cutting and grazing, the native pine Pinus kesiya becomes an important component.

Northern Thailandian Upper Montane Forest

At altitudes above 1800 m in the Northern Highlands this type of upland forest is now confined to just a few peaks such as Doi Inthanon. They are often no more than about 23 m in height and typically have a continuous, flat crowned canopy with no lower tree layers. At these altitudes most of the magnolias of lower levels have disappeared and a different assemblage of oaks are found including Castanopsis purperea, Lithocarpus aggregatus and Quercus glabricupula. Other characteristic trees are Acer laurinum, Beitschmiedia globularia, Cinnamomum tamala, Eury nitida, Gordonia dalylieshiana, Heliciopsis terminalis, Lindera thomsonii, Neolitsea foliosa, Schima wallichii and Symingtonia populnea. Among the smaller trees are Helicia formosana, Macropanax oreophilus, Myrsine semiserrata, Neocinnamomum caudatum, Osmanthus fragrans and Symplocos dryophila. Under the dense canopy, the undergrowth is generally sparse, but often includes a mossy ground layer, while in wet depressions bog mosses (Sphagnum) predominate. The few herbaceous elements include several endemic species such as Delphinium altissimum var. siamensis (Ranunculaceae) and Scabiosa siamensis (Dipsaceae). Shrubs, on the other hand, are mainly confined to the forest margins where the endemic Cornus oblonga var. siamica (Cornaceae) can be found. Large woody lianas are also absent, but there are a number of herbaceous vines such as Hedera himalaica, Jasminium dispermum, Streptoliron volubile and the endemic Amphicarpaea siamensis (Fabaceae). The typically crooked trees support a luxuriant epiphytic flora of flowering plants, ferns mosses and lichens including many orchids of the genera Bulbophyllum, Dendrobium, Eria, Luisia, Malaxis, Otochilus and Pholidota, comprising several endemic species.

References

Ogawa, H., Yoda, K. & Kira, T. 1961. A preliminary survey of the vegetation of Thailand. Nature and Life in Southeast Asia, 1: 21-157.

Rundel, P. W. & Boonpragob, K. 1995. Dry forest ecosystems of Thailand. In: Seasonally Dry Tropical Forests. Eds. S. H. Bullock, H. A. Mooney and E. Medina. Cambridge University Press.

Santisuk, T. 1988. An Account of the Vegetation of Northern Thailand. Franz Steiner Verlag Wiesbaden Gmbh Stuttgart.

Seidenfaden, G. & Smitinand, T. 1959. The orchids of Thailand. A preliminary list. The Siam Society, Bangkok.

Smitinand, T. 1966. The vegetation of Doi Chiengdao – a limestone massive in Chiengmai, North Thailand. Natural History Bulletin of the Siam Society, 21: 93-128.

Williams, L. 1965. Vegetation of Southeast Asia. Studies of Forest Types. 1963-1965. Agricultural Research Services. U.S. Department of Agriculture.