The flora of the limestone hills of the Malay Peninsula
M.R. Henderson, a botanist associated with the Singapore Botanic Gardens, published this comprehensive floristic survey in 1939, drawing on his own field collections from the late 1920s through 1938 and a systematic review of the herbarium at the Singapore Botanic Gardens. The article establishes that the limestone hills of the Malay Peninsula support a distinct lowland flora of approximately 745 species, a significant proportion of which is restricted to or characteristic of calcareous substrates, and that this flora differs from that of other geological formations primarily in degree rather than in the presence of wholly novel plant groups.
Summary
Henderson’s central aim is to assemble a reliable checklist of the flowering plants and gymnosperms growing on the limestone outcrops of the Peninsula and to characterize how this flora compares with that of surrounding non-calcareous terrain. A major methodological challenge he addresses is the unreliability of older herbarium records: many collectors, including Ridley and Kunstler, labelled specimens with localities such as “Batu Caves” or “near Gunong Pondok” without specifying whether the plant was actually on the limestone or merely at its base. Henderson therefore excluded all dubious records unless corroborated by later collections with explicit notes of limestone habitat (pp. 13–14). His own fieldwork, supplemented by that of Kiah, Symington, and others, covered the principal limestone areas from Setul in the far north through Perlis, Langkawi, Kedah, Kelantan, Pahang, Perak, and Selangor.
The article distinguishes three broad types of limestone hill—very dry with little soil and exposed rock; wet with gullies filled by rich soil; and well-covered with stiff red clay—and shows that vegetation structure varies markedly between them. Dry hills (exemplified by Kota Glanggi in Pahang) are characterized by stunted, gnarled trees, abundant rock-dwelling orchids, and an absence of mosses and Pandanus; wet hills (exemplified by Gua Panjang in Kelantan) support dense mossy growth, Corysanthes, Ophiorrhiza, and numerous Pandanus (pp. 11–12). Henderson argues that the limestone flora is fundamentally a lowland assemblage: no alpine or montane species occur on it, and families typical of moist or swampy lowland forest (such as Dipterocarpaceae, Myristicaceae, and Lauraceae) are poorly represented, while families adapted to well-drained rocky conditions (Gesneriaceae, Euphorbiaceae, Rubiaceae, Urticaceae) are well represented (pp. 17–18, 19–30).
A significant portion of the article is devoted to discussing the “rock-plant” element—species that grow on or near boulders and outcrops in lowland forest regardless of rock type—and distinguishing these from a true calciphilous element. Henderson estimates that some 80 to 100 of the listed species belong to the rock-plant category and are probably indifferent to the nature of the substrate, while the remainder represent a genuine limestone affinity (p. 17). He also notes that the limestone flora shows a strong northern bias: of the 64 or 65 species confined to limestone in the Peninsula but distributed outside it, 50 are known only from Siam, Burma, Indo-China, or India, and most of these do not extend far south of Langkawi, Perlis, and Kedah, the limestone of Perak and Pahang acting as a barrier to further penetration (pp. 18–19).
Key Findings
- Of the total 745 species recorded, approximately 195 (about 26%) are known in the Peninsula only from limestone; of these, about 130 are endemic to and confined to limestone, representing over 17% of the grand total (p. 17).
- The limestone flora includes eight genera restricted to it locally: Pistachia, Stenothyrus, Dichiloboea, Lepidanthus, Gongylosperma, Buxus, Asparagus, and Hapaline; of these, only Stenothyrus, Gongylosperma, and Hapaline appear to be confined to limestone even outside the Peninsula (p. 19).
- Seven species of Impatiens are known from limestone, all confined to it; the most striking is I. mirabilis on the northern limestone, whose enormously swollen brittle stems are wedged into rock cracks (p. 20).
- Pandanus irregularis Ridl. is common on the wetter Kelantan and Pahang hills and is confined to limestone; Pandanus is exceedingly rare on the north-western limestone (p. 82).
- Cycas siamensis is apparently confined to limestone in the Peninsula, being commonest on the north-western limestone and very scarce or absent in Kelantan (p. 87).
- No species of Nepenthaceae has been found on limestone, consistent with the family’s preference for acid and poor soils; other absent families include Bignoniaceae, Polygonaceae, Onagraceae, and Ericaceae (p. 18).
Conclusion
Henderson concludes that the limestone flora of the Malay Peninsula constitutes a recognizable and distinct lowland assemblage, differentiated from the surrounding vegetation by a combination of the substrate’s greater geological age, more extreme temperature conditions, the great variety of microhabitats it offers, and its occurrence in relatively isolated outcrops. The differences are quantitative rather than qualitative—no large taxonomic groups are peculiar to limestone—but the high proportion of species restricted to or strongly associated with calcareous rock (over 26% of the total flora) indicates a definite calciphilous element that warrants further investigation, particularly regarding the physiological adaptability of these plants to non-calcareous soils.
Context
- The primary archival source is the Herbarium of the Singapore Botanic Gardens, which Henderson searched exhaustively for definite limestone records; he also scrutinized all prior publications on the local flora, including Ridley’s Flora of the Malay Peninsula and King’s Materials for a Flora of the Malay Peninsula (p. 13).
- This article represents the first comprehensive synthesis of the Peninsular limestone flora and established the baseline checklist against which all subsequent work on this habitat has been measured.