The hill dipterocarp forest in Peninsular Malaysia is quite unique to it, in the sense that you don’t have such a similar formation elsewhere, on the island of Borneo for example, even though both are the centers for mixed dipterocarp biodiversity. The dipterocarp forest on elevations of 300 – 800 meters ASL in Peninsular Malaysia is often classified as hill dipterocarp forest, and there are certain characteristics of it that I’m going to point out.
The chief denoting feature of hill dipterocarp forest is the presence of stands of seraya trees. Seraya (Shorea curtisii) are large sized dipterocarp trees with silvery hued leaves on the surface. From afar, their silvery or grayish crowns stand out like billowing gray colored broccoli, amongst the other trees of the forest canopy.
Seraya trees grow as pure stands most frequently on hill ridges, as they are well adapted to the dry conditions there. They can grow as large trees, although usually not to any particularly great height or size. Typically, they reach 30-45 meters in height at maturity; old trees are almost always hollow.
Seraya trees also grow well on low coastal hills near sea level, thus giving the name “Coastal hill dipterocarp” for forests found on such terrain. A common feature of hill dipterocarp forest is the prolific undergrowth of bertam palm (Eugeisonna tristis). Bertam is commonly found occurring in conjunction with seraya stands in hill dipterocarp forest, and may present a challenge when navigating through such forest – due to their thorns.
Another fairly common sight in hill dipterocarp forest is the thorny palm tree – Nibong or Oncosperma horridum. This palm frequently sprouts up in areas where there is some disturbance in the forest canopy. Heavily logged areas have a lot of such palms growing, often as tall as the other trees. These palms have a trunk spiked with thorns, and can usually grow to between 20-30 meters tall.
In areas where the soil is poorer, or on small isolated hills, hill dipterocarp can also occur, at elevations of 150-200 meters ASL; this is noticeable in much of Taman Negara on the Pahang side, where the soil is mostly sedimentary rock, which is poorer than granite/igneous rock derived soil, and dries out faster. In such conditions, Shorea curtisii excels.
Aside from the prolific growth of bertam palms and seraya, hill dipterocarp forests share plenty of similarities with lowland dipterocarp forest, and some foresters do not recognize any strict demarcation between them, apart from the seraya stands themselves. The balau group of heavy hardwood meranti (mostly the Shorea group), is also more common in hill dipterocarp forest. These are much valued timber due to their durability and weight.
Certainly, in appearance, upon entering a hill dipterocarp forest, a layman would be hard pressed to notice any difference with a lowland dipterocarp forest. The interior looks the same (for the most part), except that it is often more open, drier, and windier. Leeches which are a dreaded bane in lowland areas are much more reduced in number in hill dipterocarp forest, only coming back with a vengeance in the upper hill dipterocarp and montane oak laurel forest formation located at higher altitudes. In dry weather, leeches are almost non existent when trekking though (virgin) hill dipterocarp forest. Once the forest is disturbed though, leeches usually become more abundant.
Faunal biodiversity is also rarer in hill dipterocarp forest, as is floral biodiversity. Animals are rarer due to the reduced amount of food they can obtain in this forest formation. However, much of the faunal and floristic diversity is the same as with that of the lowland ecotone further below. Of course, there are also many species which are specific to the lowlands and many which are only found in hilly terrain – but the differences are far less than the similarities.
Due to the good demand for dark red meranti timber (which seraya belongs to), almost all of the hill dipterocarp forests have been logged out by now, and the only major stands remaining are located mostly along the main mountain ranges of Peninsular Malaysia. The timber quality of hill dipterocarp is often considered to be of higher quality than lowland forest, but lowland dipterocarp has suffered a worse fate because most of the prime lowlands have, and are being converted into, oil palm or rubber plantations. That is why almost all the “forest cover” in Malaysia are those of the hills and mountains.
Therefore, as the lowland forests in Malaysia have been much reduced in extent, it is down to the hill dipterocarp forests to continue playing the role of harboring most of the rich biodiversity of Malaysia’s natural heritage, which have no where else to go, anymore.