Included here is much of Thailand together with the western parts of Laos and Tenasserim (Burma). The seasonal forest have been divided into seasonal evergreen and tropical deciduous.

Thailandian Seasonal Evergreen Forest

These forests, which also include a few deciduous species, occupy northern sheltered, moist valleys and low hills to an altitude of about 900 m, and are widespread in the central highlands and on the south and southwest slopes of the Korat Plateau. They are well developed along large watercourses in open broad valleys where they often form distinct gallery forests. In these situations magnificent stands of lofty evergreen dipterocarps occur, especially Dipterocarpus turbinatus, while others may include Anisoptera costata, Dipterocarpus alatus, D. costatus, Hopea odorata, Shorea assamica, S. roxburgii, S. thorelii and Vatica cinerea. Although not as tall as the tropical evergreen forests of peninsula Thailand certain stands can reach heights of 35 m, but they have fewer dipterocarp species. In terms of structure they usually have three tree layers, while emergent trees are uncommon. Nevertheless, their canopies include a rich variety of species with at least 23 families represented. Some of the more typical include Ailanthus triphysa, Altringia excelsa, Antiaris toxicaria, Bischofia javanica, Dracontomelon dae, Irvingia malayana, Lagerstroemia balanse, Nyssa javanica, Pterogygota alata, Tetrameles nudiflora and Toona ciliata. The middle and lower tree layers also comprise many species including the two endemic or near endemic species Chisocheton siamensis (Meliaceae) and Mammea siamensis (Hypericaceae).  Many of the taller trees have large buttresses including the evergreen Dracontomelon dae and the deciduous Tetrameles nudiflora. Like tropical forests cauliflory and ramiflory are frequent (e.g. Baccarea ramiflora and species of Ficus), and fagelliflory can be found in some species (e.g. Oroxylum indicum and Parkia leiophylla). The well-developed shrub layer includes Alchornia rugosa, Ardisia vestita, Barleria strigosa, Canthium horridum, Clausena excavata, Dendrocnide stimulans, Lepisanthes rubiginosa, Melodorum fruticosum, Sterculia lanceolata, and the cycad Cycas micholititzii. Palms are also common especially in the more moist places and along watercourses. The principal species are Areca triandra, Arenga pinnata, Caryota mitis, Livistonia speciosa, Pinanga gracilis and the endemic Wallichia siamansis (Arecaceae). Lianas are plentiful with at least 20 families represented. Examples of endemic species are Artabotrys siamensis (Annonaceae) and Randia siamensis (Rubiaceae). Finally ground layer herbaceous species are characterized by genera such as Alpinia, Aglaonema, Amorphophallus, Arisaema, Boesembergia, Catimbium, Ctenolophon, Curcuma, Globba, Hedychium and Tacca.

Thailandian Tropical Deciduous Forest

These can be broadly divided into mixed deciduous forest and deciduous dipterocarp forest. The former usually reach their best development on soils derived from limestone, while the latter is characteristic of soils derived from sandstone and quartzite with extensive stands on the sandstone formations of the Khorat Plateau. However, in some areas, such as on some of the drier slopes, the forest include elements of both these forest types. Notable families in the mixed deciduous forest are Fabaceae, Combretacaea, Lythraceae and Verbenaceae, but these forests are usually evenly mixed and rarely have any single species dominance; the exception being the occasional dominant stands of teak (Tectona grandis) encountered on the fertile alluvium soils of the valley plains.  In general, these forests reach heights of 30 m or so and have three tree layers. Among the principal trees of the upper layer are Ailanthus triphysa, Anogeissus acuminata, Bombax ceiba, Butea monosperma, Cananga latifolia, Chukrasia velutina, Dillenia pentagyna, Eugenia cumini, Garuga pinnata, Gmelina arborea, Haldina cordifolia, Haloptelea integrifolia, Lagerstroemia calyculata, Spondias pinnata, Stereospermum colais, and the endemic Chionanthus velutinus (Oleaceae), Horsfieldia amygdalina var. macrocarpa, Knema andamanica subsp. peninsularis, Knema tenuinervia subsp. kanburiensis (Myristicaeae), Pithecellobium tenue (Fabaceae), Pterospermum grandiflorum and P. littorale (Sterculiaceae).

The middle and lower layers have equally rich assemblages of species including, for example, the three endemic or near endemic trees Cassa siamensis (Fabaceae) Grewia winitii (Tiliaceae) and Maerua siamensis (Capparidaceae). Undergrowth species are also numerous with many shrubs like Clausena excavata, Clerodendron serratum, Croton hutchinsonianus together with a variety of endemic or near endemics like Ardisia tristis (Myrsinaceae), Barleria siamensis (Acanthaceae), Capparis echinocarpa, C. siamensis (Capparidaceae) Enkleia siamensis (Thymelaeaceae), and many bamboos such as the endemic Thyrsostachys siamensis (Poaceae). Many of the climbers are also unique to these forests with endemics such as Alyxia thailandica, Ichnocarpus fulvus (Apocynaceae), Bauhinia winitii (Fabaceae), Jasminum annamense subsp and J. siamense (Oleaceae), Stephania crebra (Menispermaceae) and Tinospora siamensis (Meninspermaceae). The ground layer herbaceous species also comprise several endemics such as Fimbristylis prabatensis (Cyperaceae), Gentiana arenicola (Gentanaceae) and the hemi parasite Centranthera siamensis (Scrophulariaceae). The deciduous dipterocarp forests have a more open canopy with some trees reaching heights of 35 m, but in general they seldom exceed more than about 18 m. They can have either a two or three layered canopy, with the most characteristic upper layer species comprising several deciduous dipterocarps such as Dipterocarpus obtusifolius, D. tuberculatus, Shorea obtuse and the endemic or near endemic Shorea siamensis (Dipterocarpaceae). Other common trees include Canarium subulatum, Gluta usitata, Morinda pubescens, Sisyrolepsis muricata and the endemic Sindora siamensis (Fabaceae). The principal shrubs are Crotalaria bracteata, Ellipeiopsis cherrevensis, Holarrhena densiflora and the endemic Barbleria siamensis (Acanthaceae). Also frequently found in the shrub layer is the endemic cycad Cycas siamensis (Cycadaceae). The endemic Bauhinia strychnifolia (Anacardiaceae) is one of the many climbers. At ground layer, these forests are characterized by a conspicuous layer of graminoides including many dwarf bamboos and the endemic Eulalia siamansis (Poaceae).


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