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Eusideroxylon zwageri Teijsm. & Binnend.

Eusideroxylon zwageri Teijsm. & Binnend.





Vernacular Names

Malaysia Belian (Sarawak, Sabah), tambulian (Sabah), im muk (Cantonese, Sabah).
English Ironwood, billian.
Indonesia Belian (General), onglen (Sumatra), tulian, tebelian (Kalimantan).
Philippines Tambulian, sakian, biliran (Sulu).
Brunei Belian.
French Bilian, bois de fer.

Geographical Distributions

Eusideroxylon zwageri is monotypic and occurs in eastern and southern Sumatra, Bangka, Belitung, Borneo and the Sulu archipelago and Palawan (the Philippines).


E. zwageri is an evergreen tree that can grow up to 40(-50) m tall. The bole is straight, branchless for up to 20 m tall but usually less, sometimes slightly fluted at the base and measuring up to 150(-220) cm in diametre. There are many buttresses, which are small, rounded and giving the base an elephant-foot like appearance. In the moist places, the base is often characteristically set with a mattress of slender rootlets. The bark surface is red or grey-brown with thin cracks and debarking in small scab-like subquadrangular pieces which are turned up at the lower side. The exudate is absent. The crown is dense, globular; with smooth twigs, slightly angular and tomentellous.

The leaves are arranged spirally, simple, entire, leathery, elliptical to ovate, measuring 14-18 cm x 5-11 cm, rounded-subcordate at the base and obtuse to shortly acuminate at the apex. The upper surface is hairless while the lower surface is hairy on the larger veins. The petiole is 6-15 mm long. The stipules are absent.

The inflorescence is an axillary, paniculate, dense, drooping, measures 10-20 cm long and with densely short-hairy. The flowers are bisexual and actinomorphic, which are on a pedicel measures 3-11 mm long. The perianth tube is shallow. There are 6 tepals in 2 whorls, which are imbricate, caducous, measures 3-3.5 mm long, greenish, yellow or purplish and with puberulous outside. The stamens are without glands, in 4 whorls where in the outer is with 2 staminodial and petaloid whorls, measure 1.5 mm long, yellowish with a purple tip and ciliate. Those of the third whorl are fertile, thick and with red minute or white anthers with a pink hue. There are 4 anther cells that are in 1 horizontal row where the 2 central anthers are extrorse while 2 lateral anthers are sublatrorse. The stamens of the inner whorl are staminodial, subulate and small. The ovary is superior, sessile, unilocular, with a single ovule and tapered into the awl-shaped style. The stigma is small and discoid.

The fruit is drupaceous, on a thick pedicel, 1 or 2 in each panicle, completely included in and with adnate to the accrescent perianth tube, ellipsoid to ovoid or globular, measuring 7-16 cm x 5-9 cm, glossy black at maturity and contains a single seed.

The seed is very large, very hard seed-coat, furrowed, brittle and with a pale bony. The embryo is very small. Seedling is with hypogeal germination. The cotyledons are often partly fused, succulent but tough and long persistent. The internodes are sparsely adpressed pubescent. All leaves are arranged spirally, conduplicate or induplicate when young and provided with a lateral branch in each axil.

Ecology / Cultivation

E. zwageri is a constituent of primary or old secondary tropical rainforest. It thrives in a climate with an average annual rainfall of 2500-4000 mm. It prefers well-drained soils in valleys or on hillsides or even low ridges when soil moisture is sufficient. It is found from sea level up to 500(-625) m altitudes. E. zwageri is generally occurs on sandy soils of Tertiary origin, on clay-loam soils or on sandy silt-loam soils, but large specimens have also been found on limestone. E. zwageri occurs scattered or gregarious and often as the dominant canopy species. It sometimes forms almost pure stands. In Sumatra, the 'ironwood forest' is recognised as a distinct forest type characterised by exceptionally low species diversity. E. zwageri occurs also in mixed dipterocarp forests and has been found associated with Koompassia, Shorea and Intsia species.

Line Drawing / Photograph



  1. Plant Resources of South-East Asia No. 5(1): Timber trees: Major commercial timbers.

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