Ulin / Ironwood (Eusideroxylon zwageri): Growth, Allometry and Carbon Potential

Updated 2026-11-02 · 3 min read · By the akar.earth specialist team

Ulin (Borneo ironwood) carbon sequestration: very dense wood, slow growth, range, allometry, conservation status and its role in Indonesian restoration.

Ulin stores carbon in some of the densest wood in Southeast Asia, so a mature ironwood holds a lot of carbon for its size. But it grows very slowly, so in a reforestation project it is a long-term conservation and stock species, not a source of quick credits.

Identity and range

Ulin, or Borneo ironwood (Eusideroxylon zwageri), is in the laurel family, Lauraceae. Local names include ulin, kayu besi (iron wood) and bulian in Sumatra. It is the only species in its genus.

Ulin is native to Borneo, southern and eastern Sumatra, the Bangka-Belitung islands and parts of the southern Philippines. It grows in lowland rainforest, usually below about 500 m, on well-drained soils. In Indonesia most remaining natural stands are in Kalimantan, including East Kalimantan and Central Kalimantan, with smaller populations in Jambi and South Sumatra.

Growth habit and rotation

Ulin is a shade-tolerant canopy tree that can grow to 40 m or more. Seedlings establish under forest cover and grow very slowly. Its large seeds germinate slowly, sometimes over many months, and are eaten by animals.

There is no practical “rotation” for ulin. Natural-forest trees take a very long time to reach large sizes. Reported diameter growth rates are low and vary with light and site; we do not state a single figure. In planted trials, ulin responds to more light, but still grows far slower than pioneers or meranti.

Ulin heartwood is extremely durable in water and soil. It is used for piles, bridges, roofing shingles and boat building, and old ulin buildings last for generations.

Wood density and biomass estimation

Ulin is among the densest timbers in the region. Reported values are commonly around 0.8 to over 1.0 g/cm³. Be careful which measure you use: biomass models need basic density (oven-dry mass per green volume), which is lower than air-dry density at 12–15% moisture. Wood density also varies with site and tree age.

Use the Chave et al. 2014 model for above-ground biomass:

AGB (kg) = 0.0673 × (ρ × D² × H)^0.976

with ρ in g/cm³, D (DBH) in cm and H in m. We are not aware of a widely used ulin-specific equation; regional lowland forest equations from Kalimantan are an alternative for natural stands. See the allometric equations guide.

Biomass converts to carbon with the IPCC default 0.47 and to CO2 with 44/12.

Carbon role in ARR and agroforestry

Ulin plays a supporting role in carbon design:

• Enrichment planting in degraded natural forest, alongside meranti and other natives.
• Long-term stock. Over many decades, ulin’s dense wood adds high carbon per tree. Under VM0047 and other ARR methods, unharvested native trees add to durable removals.
• Conservation value. Planting a threatened native species supports biodiversity claims and community-valued heritage trees.

In ex-ante estimation, assume low early growth, or ulin will inflate forecasts.

Co-benefits and livelihoods

Ulin is culturally important in Kalimantan and Sumatra, where it is used in traditional longhouses and as a valuable heritage timber. Restoring it supports cultural values and future, legal, sustainable use. Its fruit feeds wildlife.

Risks and cautions

• Conservation status. Ulin is assessed as threatened on the IUCN Red List because of overharvesting and habitat loss. Harvest and trade are regulated in Indonesia; confirm current rules with the forestry authority.
• Illegal logging. High value creates pressure on remaining stands and on planted trees. Plan patrols and community agreements.
• Seed supply. Seeds are large, slow to germinate and heavily predated, which limits nursery output.
• Slow growth. Credits from ulin alone will be small for decades.
• Fire. Seedlings and saplings die in ground fires.

Planning notes for project developers

• Combine with faster natives. Plant ulin with meranti and pioneer species so early carbon comes from faster growers.
• Seed handling. Collect fresh seeds, protect them from rodents and allow long germination periods in nursery planning.
• Community agreements. Agree protection rules with local communities, who often value ulin highly.

Worked example

A mature planted ulin with DBH 30 cm, height 20 m and basic density 0.90 g/cm³:

1. ρ × D² × H = 0.90 × 900 × 20 = 16,200
2. AGB = 0.0673 × 16,200^0.976 ≈ 864 kg
3. Carbon = 864 × 0.47 ≈ 406 kg C
4. CO2e = 406 × 44/12 ≈ 1,489 kg CO2e (above-ground only)

The same size tree at sengon’s density would hold about a third of this. Test values in the tree CO2 calculator, and see the ARR hub for how slow natives fit project design.