The topography and drainage of Thailand in part define the country as four widely recognized geographical regions: the North (N), Northeast (NE), Center (C), and South (S) (Map 1). The four regions have no administrative significance, but each differs from the others in population, basic resources, natural features, and level of social and economic development. Thailand is administratively divided into seventy-three districts (new provinces) (Map 2) (LePoer 1989). Botanically, Thailand is included in the Indochinese subdivision of the continental southeast Asiatic region according to the recent floristic divisions of the earth (Good 1974). Thailand can be divided into seven floristic regions (Smitinand 1958) (Map 3). They are: the Northern (N), Northeastern (NE), Eastern (E), Central (C), SoutheasternSE), Southwestern (SW), and the "Peninsula" (PEN). Bryologically, Thailand is often divided into 18 natural regions (old provinces) (Map 4) (Hansen 1961; Touw 1968; Noguchi 1972, 1973). To illustrate the relationships among the geographical regions (4), floristic divisions (7), natural regions (18), and administrative provinces (73) of Thailand, I have incorporated the data into a classification as shown in table 1.
The followings are descriptions of floristic features of each division, including natural regions and districts (Note: roman numerals before natural regions and alphanumeric numbers before districts are corresponding to those shown in the maps). Geographical units used for recording the distribution of mosses in Thailand are shown in Map 3 and Map 4.
1) Northern division (N): This division is under the Indo-Myanmaran floristic tendency. The region is rich in high mountains, having the highest peak (Doi Inthanon, alt. 2576 m) in Thailand. The geological formation is generally of sandstone or granite. Types of forests are ranging from dry dipterocarp forest below 500 m, dry evergreen forest between 500 to 1000 m, and to dry hill evergreen forests above 1000 m (Smitiand 1958). Both historically and in recent times, northern Thailand has received the most attention of botanists who collected bryophytes. The division includes four natural regions and 18 provinces.
2) Northeastern division (NE): This region corresponds to the Indochinese flora, but the Indo-Myanmaran elements can also be found here. Floristic affinities are close to southwestern China as well. The region is the high plateau of Thailand. Types of forests are from dry deciduous to mixed deciduous forests with large tracts of dry evergreen forests growing intermittently. Pine forests are present from above 1000 m. Dipterocarp forests are the common feature of the region. A significant number of bryophytes have been collected from this region. This division includes two natural regions and 10 provinces.
3) Eastern division (E): This region is under the influence of the central and southern Indochinese flora. Dry dipterocarp forests are the main feature of the region. Savannas are also common. Pine forests can be seen side by side or mixed with dry dipterocarp forests. No significant bryophyte collections have been made from this region. This division includes two natural regions and 7 provinces.
4) Central division (C): This division corresponds with the southern half of Bangkok plain or central valley. The region is mostly under cultivation. Virgin forests have almost disappeared. The dipterocarps, once formed the evergreen forests in the region, are scattered here and there nowadays. Little bryological field work has been carried out in this area. The division includes three natural regions and 12 provinces.
5) Southeastern division (SE): This region is under the influence of both southern Vietnamese and Malayan floras. The dipterocarps together with Solenospermum and Parkia form the predominating species in the tropical rain forests (Smitinand 1958). Savannas are also widespread in the plains. The common low shrub species in the area is Dillenia hookeri, which grows in common, scattered clumps. Rich mangrove swamps and tidal forests are the dominant feature of the coastal line and along the estuaries of the main rivers. Field work involving bryophyte collecting has been relatively well undertaken in this region. The division includes two natural regions and 7 provinces.
6) Southwestern division (SW): This region is predominated by limestone formation and corresponds with the Lower Myanmaran flora. Evergreen forests are usually present in remote part of the region near the border between Myanmar and Thailand. Bamboo forests and savanna scrubs are the common features of the plains. The bamboo forests often gradually change into mixed deciduous and dry dipterocarp forests. Only a small number of bryophytes have been collected here mostly by collectors traveling from northern to peninsular Thailand. This division includes one natural region and 5 provinces.
7) The peninsular division (PEN): This region is mostly under the influence of the Malayan flora. The Malayan elements are fairly common in the region although in northern part of the region Myanmaran elements are notably joined together. The forests are mainly of the tropical rain forest type and are made up of many dipterocarp species. Mangrove swamps in the region are very rich and are the main mangrove-forests of the country. They almost cover the whole western coast. A large number of bryophyte collections have been made from this region. The division includes four natural regions and 14 provinces.
| GE | Floristic Divisions | Natural Regions | Provinces |
| N | Northern (N) | PAYAP (I) | 2. Chiang Mai; 3. Chiang Rai; 1. Mae Hong Son |
| MAHARAT (II) | 7. Lampang; 6. Lamphun; 5. Nan; 4. Phayao; 8. Phrae | ||
| NAKHON SAWAN (III) | 31. Chainat; 12. Kamphaeng Phet; 19. Nakhon Sawan; 9. Tak; 30. Uthai Thani | ||
| PHITSANULOK (IV) | 21. Phetchabun; 20. Phichit; 13. Phitsanulok; 10. Sukhothai; 11. Uttaradit | ||
| NE | Northeastern (NE) | UDAWN (V) | 23. Khon Kaen; 14. Loei; 27. Mukdahan; 18. Nakhon Phanom; 16. Nong Khai; 17. Sakon Nakon; 15. Udon Thani |
| ROI ET (VI) | 25. Kalasin; 24. Maha Sarakham; 26. Roi Et | ||
| Eastern (E) | UBON (VII) | 37. Sisaket; 36. Surin; 29. Ubon Ratchathani; 28. Yasothon | |
| RACHASIMA (VIII) | 35. Buriram; 22. Chaiyaphum; 34. Nakhon Ratchasima | ||
| C | Central (C) | AYUTHIA (IX) | 40. Ang Thong; 33. Lop Buri; 41. Phra Nakhon Si Ayutthaya; 42. Sara Buri; 32. Sing Buri |
| KRUNGTEP (X) | 52. Krung Thep Mahanakhon; 45. Nonthaburi; 46. Pathum Thani; 53. Samut Prakan | ||
| NAKAWN CHAISI (XI) | 44. Nakhon Pathom; 51. Samut Sakhon; 39. Suphan Buri | ||
| Southeastern (SE) | CHANTHABURI (XII) | 57. Chanthaburi; 56. Rayong; 58. Trat | |
| PRACHINBURI (XIII) | 55. Chachoengsao; 54. Chon Buri; 47. Nakhon Nayok; 48. Prachin Buri | ||
| Southwestern (SW) | RACHABURI (XIV) | 38. Kanchanaburi; 49. Phetchaburi; 59. Prachuap Khiri Khan; 43. Rat Buri; 50. Samut Songkhram | |
| S | Peninsula (PEN) | SURAT (XV) | 60. Chumphon; 63. Surat Thani |
| PHUKET (XVI) | 65. Krabi; 62. Phangnga; 64. Phuket; 61. Ranong; 69. Satun; 67. Trang | ||
| NAKHON SI THAMMARAT (XVII) | 66. Nakhon Si Thammarat; 68. Phatthalung; 70. Songkhla | ||
| PATTANI (XVIII) | 73. Narathiwat; 71. Pattani; 72. Yala |
