A botanical excursion to Northern Sumatra

H. N. Ridley, then Director of the Singapore Botanic Gardens, published this account in 1923 of a brief botanical excursion to the Berastagi plateau and Sibayak Volcano in northern Sumatra undertaken in February 1921. The article’s overarching thesis is that the mountain flora of Sumatra, while closely allied to that of Java, is fundamentally distinct from the highland flora of the Malay Peninsula, and that isolated patches of Himalayan-type vegetation in both regions constitute evidence of a former continuous land connection now severed by the encroachment of dark wet rainforest.

Summary

Ridley’s narrative opens with practical details of the journey from Penang to Belawan and onward by motor car to the Karu plateau, where he was hosted at a bungalow belonging to the United Malay Association Rubber Company. The bulk of the article, however, is a taxonomic enumeration of the plants collected during the excursion and in supplementary collections made by Mohamed Nur (August 1921) and Mrs. Burkill (December 1921) at Sibolangit, Sibayak, and Berastagi. Ridley describes approximately eighty new species across a wide range of families, from Clematis sumatrana and Medinilla vulcanica to Calamus karuensis and Cyclophorus grandis, establishing the first substantial flora of this previously unexplored region.

The biogeographical discussion that frames the taxonomic list is the article’s most significant intellectual contribution. Ridley identifies a series of palaeartic genera—Ranunculus, Anemone, Viola, Mahonia, Sanicula, Neillia, Sambucus, Lonicera, Carlemannia, Hydrangea, Disporum, Juncus, Potamogeton, and Equisetum—that occur together at Berastagi in light open woods and are also found in the mountains of northern India and Java, yet are strikingly absent from the Malay Peninsula except in the small valley of Telom on the Perak–Pahang border. He argues forcefully against the then-common explanation of long-distance dispersal by birds or wind, noting that the plants in question lack any special dispersal adaptations and that multiple species from different orders occur together at both localities. His conclusion is that these floras represent relicts of a once-continuous land area bearing a single flora, subsequently fragmented by climatic change or mountain denudation, with the dark wet rainforest having over-shadowed the older Himalayo-Sumatran element everywhere except in isolated refugia.

Ridley also draws attention to marked quantitative differences between the Sumatra–Java and Malay Peninsula floras in particular genera: Saurauja is represented by about 40 species in the Sumatra–Java area versus only 5 in the Peninsula; Cyrtandra by over 50 versus 7; while Didymocarpus shows the reverse pattern with 55 Peninsular species against only 7 in Sumatra. The scarcity of palms at altitude in Sumatra (only one Pinanga and one Calamus observed in the woods) is noted as a further point of contrast.

Key Findings

• The excursion took place 4–18 February 1921; Ridley departed Penang on 4 February and returned on the 18th, with the main collecting concentrated at Berastagi (approximately 5,000 ft) and an ascent of Sibayak Volcano (reaching the crater at 10:30 a.m. after a 7 a.m. start) (pp. 46–48).
• Approximately 80 new species were described from the collections, including Medinilla vulcanica, Clethra pulcherrima, Eugenia sulphurata, Carlemannia sumatrana (the first record of this genus from the Malay islands), Calamus karuensis, and Cyclophorus grandis (pp. 47, 59–60, 66, 103, 112).
• The palaeartic (Himalayan) element at Berastagi includes Viola serpens, Sanicula europaea, Disporum multiflorum, Pouzolzia, Mahonia nepaulensis, Sambucus javanica, and Lonicera pulcherrima n. sp., all of which also occur at Telom in the Malay Peninsula, providing evidence of a former land connection (pp. 48–49).
• Saurauja is represented by ~40 species in Sumatra–Java and ~30 in Borneo but only 5 in the Malay Peninsula; Cyrtandra by >50, 40, and 7 respectively; Didymocarpus by 7 in Sumatra but 55 in the Peninsula (p. 50).
• The highest plant recorded on Sibayak was the fern Litobrochia incisa, forming stunted masses of under a foot tall on the bare volcanic rocks near the crater; the crater itself contained a small lake and large sulphur-encrusted fumaroles (pp. 47–48).
• Potamogeton oxyphyllus, a Japanese species, was collected from a solfatar in warm water in the Petain valley on Sibayak, also previously recorded by Robinson on Korinchi Peak (p. 106).

Conclusion

Ridley’s definitive historical takeaway is that the isolated occurrences of Himalayan-type flora at Telom in the Malay Peninsula and at Kunadiyaparawita in Ceylon cannot be explained by dispersal but must represent survivors of a once-widespread continuous flora that has been driven to extinction in the intervening areas by the later advance of dark wet rainforest. The Berastagi collections confirm that this Himalayo-Sumatran element was formerly much more extensive and that the present distribution is a product of biotic replacement rather than long-distance colonisation.

Context

• The primary archival basis consists of Ridley’s own field collections from February 1921, supplemented by specimens collected by Mohamed Nur (sent by I. H. Burkill from the Singapore Botanic Gardens) in August 1921 and by Mrs. Burkill in December 1921, all from the Sibolangit–Berastagi–Sibayak area (p. 50).
• The article represents the first substantial account of the flora of northern Sumatra and established the biogeographical framework—particularly the Telom–Berastagi connection—that would inform subsequent work on the disjunct distributions of palaeartic elements in Malesia.

References