Bornean ironwood: the belian tree (Eusideroxylon zwageri)

The tree itself carries a lot of names. In Indonesia: ulin, belian, tebelian, tulian and onglen. In Sabah you will also hear tambulian. The Berawan of Sarawak call it terras. In the Philippines it is tambulian or sakian. In English it is Bornean ironwood or Borneo ironwood, and in older timber records billian; the French trade calls it bois de fer. In Jambi in Sumatra people tell several ironwood varieties apart by eye, and a 2015 study set those local names beside the trees' form and their DNA (Irawan et al. 2015). Sarawak gazetted belian as its official state tree on 20 March 1984 (Sarawak Gazette No. 1484). In Iban, a language of Sarawak, belian is a diamond, the stone and the suit of cards. I used to think that was because a diamond is pure carbon and very hard, like belian wood. In actual fact, that is a loanword from the Dutch briljant.

The scientific name is Eusideroxylon zwageri. Eusideroxylon is built from the Greek for iron and wood, with "true" on the front. The species is named after Zwager, a Dutch official, and the tree was described by Teijsmann and Binnendijk in 1863 (Kew, Plants of the World Online).

Two other Malay words are spelled belian and have nothing to do with the tree. One is beli with the ending -an, meaning a purchase, which is useful when you're shopping for field supplies. The other is an old word for a shaman, still sometimes used in Borneo, which goes back to an Austronesian root reconstructed as *balian, "shaman, shamaness, shamanistic ceremony" (Blust and Trussel, Austronesian Comparative Dictionary). Same spelling, different words: none of the three is related to the others.

That third sense is not dead in Borneo. Among Dayak communities in East Kalimantan, the Benuaq and the Luangan among them, a belian is a healing ceremony: an overnight performance led by a shaman to call back the spirit of someone who is ill. Herwig Zahorka catalogued the plants used across a four-night belian sentiu ritual (Zahorka 2007), and Isabell Herrmans has written a book on the practice (Herrmans 2015). So in Borneo the word belian can point at a tree or at a ceremony, depending on who you ask and where you are standing. The same root surfaces right across the region: a healer is a balian in Bali, a babaylan in the Philippines and a bobohizan among the Kadazan-Dusun of Sabah (Blust and Trussel, Austronesian Comparative Dictionary).

The same paper compared the ages of belian and dipterocarps. The estimated lifespans of dipterocarps at full size ranged from 60 to 570 years. The growth rate of belian was among the very slowest of them, and its lifespan was well beyond any. Kurokawa's group suggested the tree may keep fruiting for 800 years or more, and put the long life down to the density of the wood and the defensive chemistry inside it. Part of that chemistry has adopted the tree's name: eusiderin, a compound isolated from belian heartwood in 1960 (Hobbs and King 1960). It is a life history strategy: grow slowly, build wood that nothing can eat, and outlast everything around you.

Growth that slow has a consequence for anyone hoping to replace what was cut. Seedlings in the shade of a closed forest hardly ever die, but they hardly grow either. Opening the canopy around a five-year-old seedling roughly trebles its growth (Kiyono and Hastaniah 2000). Even so, a trunk 30 centimetres across is a matter of a century, which is why nobody has ever built a belian plantation at scale. Restoring a logged forest is slow for the same reason, and a synthesis of restoration outcomes across tropical Asia shows what actually works.

Why belian wood is so dense

Dense wood is stored carbon. Two trees of the same size can hold very different amounts of it, and belian is at the heavy end. That is why mapping carbon across Malaysian Borneo means measuring more than canopy height. Measured on planted trees in Sarawak, belian's basic density is about 0.86 grams per cubic centimetre, the same from the centre of the trunk to the bark (Aiso-Sanada et al. 2020); the US Forest Products Laboratory gives 0.86 to 0.92, which works out at about a tonne per cubic metre once the timber is air-dry (USDA Forest Products Laboratory). So belian is a sinker, not a floater. It is also hard enough that a team wanting to core belian trunks in Sabah for growth rings had to design a new drill first (Williams et al. 2015).

In the Global Wood Density Database, the reference set ecologists use to turn tree size into carbon, belian's records sit between 0.71 and 0.84. That puts it in the densest tenth of South-East Asian trees, and denser than the great majority of dipterocarps, whose median is around 0.61. But a few of them, the balau group of Shorea and several Hopea, are just as heavy, and they sink too. What sets belian apart is not that it is the densest wood in the forest. It is that it is very dense, very durable and very old at the same time.

Seeds and seedlings of the belian tree

Each fruit holds a single seed, between seven and fifteen centimetres long, with a ridged coat almost as hard as the timber (Soerianegara and Lemmens 1993). No other tree in Borneo drops a seed like it, so sometimes you find the seed on the ground and then look around for the tree. The IUCN assessment suggests the seeds were once moved by large mammals, some of them no longer in the forest, and that they also travel by water, which is why belian is so often found along river banks (IUCN 2025).

Removing a lot of adult trees has implications for the next generation. In a four-hectare plot of old-growth forest at Maliau Basin in Sabah, Lan Qie and colleagues found fewer young belian than the adults should have produced, clumped rather than spread, and heavily browsed. Seedlings were heavily browsed, with sambar deer the likely culprit, and the losses were density dependent: the more seedlings in a patch, the larger the share eaten (Qie et al. 2019). Qie and her colleagues concluded that keeping the species going will take conservation effort aimed at belian itself, not only at the forest around it.

How to recognise a belian tree

The tree can be recognised by its pitted, pocked, greyish-brown bark. The leaves are simple and elliptic, set in a spiral up the twig rather than in opposite pairs, and a new flush comes out red (Sarawak Forest Department).

So people build with it, and have for as long as there have been longhouses. In Sarawak and Kalimantan the main posts, beams and stilts of a Dayak longhouse are belian, and so are the roof shingles. A belian house post is expected to outlast the house. It goes into jetties, piers, wharves and bridges, and into boats; the Sarawak Forest Department lists longhouse structure, furniture and the posts that pepper vines climb. It has long been valued as coffin wood. In Danum Valley there are belian coffins lodged on ledges of a limestone cliff above the river, carved with animal heads and reputed to be at least 250 years old, still whole in the open air. For the Berawan of Sarawak the tree is a cultural keystone, used in burial rites and carved into posts (Franco et al. 2014).

What belian wood costs, and why

The timber has always been expensive, and the official numbers suggest around ten times the rate for other timbers. Sabah charges an export levy (it calls it a cess) of RM 100 per cubic metre on belian sawn timber against RM 10 for sawn timber of any other species, and the royalty on converted belian, meaning sawn or otherwise processed timber, runs at more than double the general rate (Sabah Forestry Department schedules). Those are state fees rather than market prices, but they are set against what the timber is worth, and belian is the only species the levy singles out by name. The deeper reason is supply: a century to reach thirty centimetres across, no plantation at scale, and legal felling restricted almost everywhere it grows. Most of what you can buy today is reclaimed rather than newly cut.

How to identify belian wood

Four things give a belian board away. Weight is the first, and the most reliable: at around a tonne per cubic metre air-dry it is heavier than nearly anything it could be confused with, and an offcut dropped in water sinks. Colour is the second. A fresh cut is yellowish, sometimes faintly olive, and it darkens on exposure to deep reddish brown and eventually close to black (Commercial Timbers). Old surfaces left outdoors can weather to grey while the wood underneath stays sound, which is the contrast in the photograph at the top of this section. Sapwood is the third: a narrow, distinctly paler band at the edge. Insect damage is the fourth, because of its absence. Termites and borers leave belian alone, so a hundred-year-old beam full of holes is something else. Freshly cut belian is also often described as faintly lemony, which would fit a family that includes camphor, cinnamon and bay.

Kaltimber are at Batuan in Gianyar, a few miles south of Ubud. The Balinese are known for their craftsmanship and for their woodworking in particular.

What Kaltimber do is bring that tradition together with a modern mill and a commitment to work only with reclaimed ulin. They take old bridges, boats, jetties and warehouses that have been abandoned and cut them carefully into large, sound pieces of timber that can be used again in buildings. Most of it becomes decking and parquet flooring to a very high standard. They also make furniture and woodware, much of which ends up in Bali's hotels, where there is a real push to make tourism more sustainable.

They are good at making sure nothing goes to waste. Boards are sorted and marked by size so that offcuts stay usable, smaller pieces become homewares, and the smallest offcuts of all are turned into beads. I wore a strand of belian beads at the World Economic Forum in Davos, next to a togetherband.

Everything they work with is reclaimed and they mill no new ulin at all. Their stock is certified under SVLK, Indonesia's timber legality system, and the homewares made from offcuts go out under their own i-Rewood name.