Lowland dipterocarp forests

The building blocks of a dipterocarp forest

Dipterocarp forests carpet the Borneo lowlands, forming a green expanse composed of a high number of plant species: as many as 240 different tree species can grow within 1 ha.¹

Here, the Dipterocarpaceae (dipterocarps) and Euphorbiaceae (spurges) are by far the most dominant tree families, comprising approximately 22% and 12% of all trees, respectively.² Comparatively, the tropical forests of Africa and America are dominated by trees of the legume family. ³

A prominent tree of the lowland forests of Borneo is the Borneo ironwood (Eusyderoxylon zwageri), also known as belian. Much sought after by the timber industry, this tree’s durable and dense wood does not need to be treated, and is in high demand for bridges, roof tiles and pillars for houses. Other notable species include the giant nyatoh (Palaquium species)⁴  and kenari⁵  (Canarium odontophyllum) trees.

Strangler figs (Ficus species) are conspicuous elements of tropical forests. These plants typically wind themselves around their host tree, eventually covering and killing it.

The size of their trunk can be truly awesome: the total circumference of a single mature strangler fig with all its stems can be more than 10 m. These fig trees produce fruits frequently and are a major food source for many animals.

The many faces of lowland dipterocarp forests

Dipterocarp forests are far from being uniform across Borneo. Depending on altitude, the diversity of plants they comprise varies and hence so does their overall appearance.

In areas below 150 m, lowland dipterocarp forests often contain high numbers of trees of the legume family (although less than in America and Africa) and massive strangler fig plants. Where coarse, sandy soil is available on moderate slopes, many aromatic kapur trees (Dryobalanops species) are found, which were once exploited for camphor.⁶

As we move on up beyond 150 m, the dipterocarp forests in some parts of Borneo contain a greater diversity of plants than the extreme lowlands, in addition to large timber trees and wild fruit trees.

Above 500 m, it is possible to encounter one of the 17 species of Rafflesia,⁷ which produces the largest flower in the world (1 m diameter). At about 1,000 m, coniferous trees (pine relatives) start appearing, and the dipterocarp forest gives way to montane forests.

Wildlife that swings, waddles, scurries or flies

The threatened orangutans of Borneo (Pongo pygmaeus species) are extremely reliant on dipterocarp forests, which represent a home and fridge for their daily needs.

Other large mammals of these forests include a Sumatran rhinoceros subspecies (Dicerorhinus sumatrensis harrissoni) and the Borneo pygmy elephant (Elephas maximus borneensis).

Down on the ground and along the branches, some of the region's smaller mammals scurry about - including the mountain treeshrew (Tupaia montana) , the Bornean black-banded squirrel (Callosciurus orestes) and Whitehead's pygmy squirrel (Exilisciurus whiteheadi).

The dipterocarp forests are also the hunting ground of the leopard cat (Cynocephalus variegatus), while proboscis monkey (Nasalis larvatus) and the Bornean tarsier (Tarsius bancamus) are also found in these forests.

Threats to dipterocarp forests

Dipterocarp trees are generally tall and of low wood density. As a result, the species is in great commercial demand and dipterocarp forests are being cut at unprecedented rates.

In Borneo, satellite images show that about 56% - more than 29,000 km² - of the protected lowland forests in Kalimantan (Indonesian Borneo) were cut down between 1985 and 2001.