Many people have no idea what is the meaning of a primary or secondary tropical rainforest, and assume all jungles everywhere are the same. In fact, the term “primary”, “old growth”, or “virgin” is bandied around far too freely, and in most cases, inaccurately. Simply put, a primary forest is an untouched forest, most often within the context of logging activities, which should have never taken place in that forest area, whereas a secondary forest is a forest that has already been logged or disturbed in a significant way, by the activities of human beings. Here I will try to explain all this in as layman terms as possible.
First off, let’s look at the background context. There are very few forests left in Malaysia, where they can be considered “primary” or “virgin”. The vast majority of forest reserves you see in Malaysia as you travel around are already secondary forests, having been logged in the past. With the local timber supply long exhausted, many forest reserves are undergoing second or even third cycles of logging.
So how do you distinguish or tell apart primary forests from secondary? The easiest way is to observe the trees. A logged rainforest will have smaller trees and the there will be a poorly defined or unclear emergent layer, consisting of scattered leftover large trees; in heavily logged forests, even these emergent trees are missing, and the entire forest will look unmistakably “shredded” in appearance with only small trees standing (depending on the severity of the logging). Worse still, many forest reserves are now completely clear felled to make way for oil palm plantations.
Other obvious clues of disturbance are the dominance of secondary growth vegetation, such as bamboo, wild bananas, wild gingers, and the abundance of pioneer trees, and climbing vines that heavily enshroud the canopy of logged forest, as well as the ubiquitous resam fern (Dicranopteris linearis), which smothers the ground all along disturbed fringes, such as logging trails. Trees that regrew after logging took place many decades ago, may have curved/kinked trunks, as a result of trying to grow through dense thickets of climbers which always spring up due to increased sunlight penetrating the damaged canopy.
Generally speaking, mature rainforest trees have large, spreading crowns often spanning 20 meters or more in width, while immature rainforest trees have smallish cone shaped crowns that look slightly like Christmas trees, from afar. A preponderance of cone shaped or narrow crowns in the canopy layer is a strong indicator of past disturbance. When you consider how long it takes for most big rainforest trees to reach maturity (studies indicate it takes trees typically 60 years just to make the canopy), you begin to appreciate why untouched lowland rainforests are so valuable today; man cannot regrow what takes nature many hundreds or even thousands of years.
A good indicator that you are in primary lowland rainforest if you enter it is the frequency of large trunks everywhere. Typical trunk diameters at breast height (dbh) should be around 70-100+ cm. Here and there, giant trees stand like gothic pillars in the dim light of the undergrowth, some individuals reaching almost 2 meters dbh or more, often arrayed with huge buttresses. Secondary forest on the other hand, would have trees more than 70 cm in trunk dbh as the exception, not the rule; large trees are rare.
The undergrowth in primary jungle is also usually sparse. This is not a very reliable sign however, since old secondary forest where the last logging activity took place many decades ago, also has sparse undergrowth. However, another characteristic of secondary forest (from personal observation) is increased mosquitoes. It is a misconception to think that all “jungle” has plenty of mosquitoes. Logging destroys the stratified layers of the rainforest, thus “chasing” out mosquitoes that previously inhabited these micro habitats, and bringing them into contact with humans. Logging or severe disturbance likely alters the micro habitats within the jungle, making the environment more conducive for mosquitoes to multiply too, as their former predators are reduced in numbers.
A unique example of a “virgin” patch of rainforest that is also degraded at the same time, is the Bukit Nanas Forest Reserve, a tiny relic patch of lowland rainforest that still exists below the KL Tower. There are a few big trees still standing in this forest, but because its environment has been so compromised with all the development in KL, that there are relatively high numbers of mosquitoes there, although mosquitoes are mostly uncommon in untouched, virgin jungle, excluding swamplands.
Finally, people who study animals, from birds to insects, will also readily tell you that the diversity and composition of species is drastically different, if comparing undisturbed forest with disturbed forest. Certain species of birds or insects are very rare or non existent in secondary forests, and only found in undisturbed tracts.