The flora of Lower Siam
H.N. Ridley, Director of the Singapore Botanic Gardens, published this floristic survey in 1911 to delineate the botanical boundary between the Siamese and Malayan floras along the western coast of the Malay Peninsula. Drawing on his own fieldwork in Kedah, Perlis, and Setul (March 1910) and earlier collections by Koenig, Curtis, and Dr. Keith, Ridley argues that a sharp floristic transition occurs at approximately Alor Star, where geology, climate, and plant composition all shift abruptly from the humid Malayan type to a distinctly xerophytic Siamese one.
Summary
Ridley’s central concern is the identification and characterisation of a floristic boundary that had remained poorly defined until his 1910 expedition. He establishes that the transition is not gradual but coincides with a geological break: the granite chain of the Malay Peninsula terminates at Kedah Peak and Gunong Perak, beyond which sandstone and limestone formations dominate the landscape from Alor Star northwards to Renong. This geological shift is accompanied by a markedly more pronounced dry season, during which many trees lose their leaves and herbaceous plants on the limestone outcrops wither completely above ground. The resulting flora shows a great preponderance of xerophytic and spiny species, with a corresponding diminution of strictly hygrophytic forms, and a significant increase in Indian and Burmese elements absent from the Malayan flora to the south.
The article also serves as a synthesis of prior collecting efforts in the region. Ridley reviews the work of Koenig (1779, Lankawi islands), Curtis (1896, Kasum and Pungah), and Dr. Keith (1890–91, Bangtaphan on the East Coast), reproducing Curtis’s detailed narrative of his 1896 expedition in full because the original Garden Report had become scarce. Keith’s collections from Bangtaphan are highlighted as filling a critical gap between Tenasserim and the Malay Peninsula, and his observations on vegetation types, rainfall (63 inches at Bangtaphan versus 200 at Mergui), and the history of a 60-square-mile lalang grass area created by a typhoon and subsequent fires are incorporated to illustrate the East Coast’s drier character.
The limestone flora receives particular attention as a potential repository of northern relict species. Ridley notes that southern limestone outcrops (Batu Caves, Ipoh, Kota Glanggi) already show traces of the Tenasserim flora, but the absence of a dry period in those localities limits the survival of xerophytic northern elements. He anticipates that the geological age of the northern limestone—expected to be Permo-Carboniferous—would illuminate the origin and distribution of this flora, though that data was not yet available.
Key Findings
- The floristic boundary between Lower Siam and the Malay Peninsula lies at approximately Alor Star, coinciding with the replacement of granite by sandstone and the termination of the limestone range at Perlis (pp. 15–16).
- Kedah Peak and Gunong Perak, both granite, bear a strictly Malayan flora, confirming that the geological substrate is a key determinant of the transition (p. 15).
- Rainfall at Bangtaphan (East Coast, 11°15′ N) was recorded at 63 inches in 1890, compared with 200 inches at Mergui, indicating the East Coast is generally drier than the West (p. 18).
- A 60-square-mile area of lalang grass at Bangtaphan was created by a typhoon approximately 27 years before Keith’s 1892 account, followed by fire, which reduced the remaining jungle (p. 18).
- Dendrobium Farmerii, abundant in Mergui, reaches its apparent southern limit at Pongah (Tongkah), where it is highly prized by local collectors (p. 23).
- The Lankawi islands include Gunong Chinchang (2,413 ft) and Gunong Raya (2,942 ft), both apparently granite, with limestone reported on the north face of the former (p. 25).
Conclusion
Ridley concludes that the flora of Lower Siam constitutes a distinct botanical province, sharply demarcated from the Malayan flora by a combination of geological, climatic, and edaphic factors converging at Alor Star. The xerophytic character of the Siamese flora, its Indian-Burmese affinities, and the potential of the limestone outcrops to preserve northern relicts together point to a complex biogeographic history that remains to be fully resolved pending geological data and further exploration of the East Coast and interior mountains.
Context
- The article draws on herbarium collections at the Singapore Botanic Gardens (Keith’s Bangtaphan specimens with sketches) and the Kew herbarium (Curtis’s new species), as well as unpublished Garden Reports and personal correspondence.
- Historiographically, the paper represents a foundational floristic delimitation of the Siamese Peninsula that predated comprehensive regional floras and established the Alor Star boundary as a working reference point for subsequent botanical and biogeographic studies of the region.