Kerengas Forests
Description: Kerengas is an Iban term meaning a place where rice can't be grown. Kerengas forests occur where the soil is very sandy and nutrient-poor. Because of the sandy soil, kerengas forest usually drains quickly, so it is often dry as well. Kerengas is typically found in areas where the underlying rock is sandstone (such as Bako National Park), and along exposed ridges on mountain slopes (it can be difficult to distinguish from "ridge-top forest", with which it shares many characterisitcs).
The poor quality of soils and harsh conditions mean that kerengas forest is generally much smaller and less diverse than mixed dipterocarp rainforest. Trees in kerengas forest (even dipterocarps) generally do not grow to be more than 30m tall, and the vegetation is usually much scrubbier - with lots of small trees growing closely together, and a more dense understory than in mixed dipterocarp rainforest. Kerengas forest has been described as "pole forest" because of the large number of small trees.
Because of the harsh conditions, certain species of tree grow better in kerengas forest than others - typically hardier tree species such as casuarinas. However, a number of dipterocarps are commonly found in kerengas forests, and in the more sheltered and damper areas (such as creek beds), palms proliferate as well. 
The poor nutrient levels in kerengas forest also means that some plants have to supplement their "diet". Pitcher plants (Nepenthes sp) are common in kerengas forest. These amazing plants have special pitcher-shaped leaves which are full of digestive fluids. The pitchers are designed to trap and digest insects, providing nutrients to the plant which it cannot get from the poor soils. These pitcher plants come in a huge variety of shapes and sizes, probably most easily seen at Bako National Park. (Shown here is an aerial pitcher of a Nepenthes rafflesiana.) There are other carniverous plants as well, including the sticky sundews, which catch their prey like flypaper.
Some other plants in kerengas forest have adapted to live with insects, rather than devouring them. These include the so-called "ant plants", which provide shelter and food for ants in especially developed growths, chambers or leaves. In exchange, the ants protect the plant from other insects, and also provide nutrients through their waste products. (This picture shows a vine called Dischidia raflesiana, which is wrapped around the branch of a small tree. The big pyramidal yellow growths are specially modified hollow leaves, in which ants live.)
As with other vegetation types, there is rarely a clear deliniation between kerengas and other forest types, which tend to gradually blend into each other around the edges. Different sub-categories of kerengas forest have also been identified.
In Bako National Park there is a particular kind of kerengas which is even more fragile and impoverished than kerengas forest found elsewhere in Sarawak. It is commonly known as fire padang (padang means "field" in Malay), and is thought to occur in areas which had been burned at some stage. (Water runs off or through the sandstone of Bako very quickly, leaving it very dry and susceptible to fire.) In fire padang, the vegetation is very scrubby and there are very few trees at all. These areas are very dry and nutrient poor, and resemble scrubby impoverished Australian bushland more than the lush tropical rainforest which you expect to find in Borneo.
Because of the tough scrubby vegetation (and absence of fruit trees), mammals and birds are not common in kerengas forest. However, a few skinks scurry about, and you will notice the odd bird or squirrel jumping about in the branches of bigger trees in some areas.
Best places to see: Bako National Park has probably the most accessible and obvious kerengas forest (and fire padang), and is also probably the best place to see nepenthes (pitcher plants). However, patches of kerengas forest can also be seen in many of the other national parks, including Kubah National Park & Matang, Lambir Hills National Park and Gunung Santubong.