The conventional lowland rainforest is normally described as having “layers”. This concept is now generally accepted as one of the fundamentals of rainforest ecological understanding. Although there are often exceptions and it seems to me a little too simplistic, in general, any patch of undisturbed lowland rainforest will usually display such layering, or “stratification” which will become more obvious upon close inspection. However, in disturbed forest, such as those that have undergone logging, this layering is often obfuscated due to the removal of the biggest trees.
These layers are typically described as being five in number, namely:
- The emergent layer comprising the tallest trees
- The main canopy layer comprising most of the tall trees
- The subcanopy comprising mainly small shade tolerant trees
- The sapling and understory layer just above the forest floor
- The forest floor
The emergent layer is composed of the tallest trees in the given area. These typically range from 35m to 65m in height, depending on the area. Sometimes, these emergent trees can grow to be even taller (70-80m), and the lowland dipterocarp rainforests of Malesia (Malaysia and Indonesia geographical regions) generally have the tallest rainforests in the entire world, taking into account the emergent layer. These emergent trees grow head and shoulders above the majority of the other trees, which make them appear like “islands” in the midst of a sea of tree crowns.
Growing taller than other trees confers an advantage – more sunlight and living space. But the downside to this is more exposure to the scorching sun, lightning, and strong winds. Being more exposed to the elements, it is not a very hospitable place for a lot of life forms. Climbers and epiphytes do make their way to the very top, but they are not as abundant as in the main canopy below. Emergent trees also provide convenient outposts for birds of prey to build their nests and scour the canopy below.
One aspect of the dipterocarp rainforest is the tendency of emergent trees to grow clustered close together rather than in isolation. This aspect was noticed by some foresters decades ago. These groupings go on to form “mini canopies” of their own, and are noticeable if pointed out from aerial photos. The benefits to the trees are obvious; better group protection from the elements of the sun, wind, and lightning.
Immediately below the emergent layer is the main canopy. The canopy is not separate from the emergent layer; the emergent layer is simply a continuation of it, as the lower branches of the tallest trees are often in contact with the leaves and branches of other trees in the main canopy, allowing vertical access by its denizens.
This canopy of the rainforest is one of the most complex and least well known habitats on planet earth. The rainforest canopy probably can be considered as a near-separate ecosystem of its own, as there are many inhabitants of the canopy that are so adapted to spend their entire lives up amongst the treetops that they would never survive down on the forest floor. The canopy normally sits 25-40 meters above the ground level, and is a veritable sea of leaves, with the bulk of the forest trees spreading their crowns at this level, and competing with each other for the sun.
Most of the vines, epiphytes, and climbers which hitch a ride up on the backs of the giant trees reach their destination here. They in turn help create a microclimate of their own up in the canopy, by providing more shade and shelter there, trapping rainwater and moisture, and fertilizing the nooks and crannies of the tree crowns with their fallen leaves and branches. It’s easy to see that the rainforest canopy turns into a very suitable habitat for its numerous denizens – many of which are small invertebrates that in turn provide food for other creatures higher up the food chain, such as birds or lizards.
The abundance of leaves and fruits in the canopy also provide food for monkeys, gibbons, birds, civets, shrews, and many other creatures that live up in the canopy; most of them have no need to descend to the forest floor at all and indeed, many never do all their lives. At the same time, there are also life forms that live down on the forest floor, but make regular trips up into the canopy to obtain food, such as certain types of termites, ants, and even centipedes, spiders, and snakes.
An experience not to be missed for the intrepid jungle traveler to the rainforests of South East Asia is to catch the characteristic dawn chorus of the gibbons and siamangs (the largest known gibbon species), which make the canopy their home. Gibbons are highly adapted to live amongst the trees, as can be inferred from their long gangly arms and the ease with which they swing amongst the branches, and their diet consisting almost wholly on the leaves and fruit produced by the trees. Their morning calls are a way for them to mark their territory using sound as the means; another adaptation to living so high up in the air.
Rainforest canopy research is still a relatively new science, considering there is a lot we still don’t know about it, and every time scientists fog the canopy (especially if they vary their locations), many new species of invertebrates are discovered, and it takes a long time to identify and sort out the species list from the tons of dead life forms that come falling down. Truly, the rainforest canopy is an Eden in the Sky!
Below the canopy is the mid level subcanopy. Comprising mainly small juvenile trees and those adapted to live in shade, bare branchless trunks of the big trees, and climbing lianas on their way up, this layer normally lies between 5 – 20 meters above the ground, but just below the first branches of the main canopy trees. If you were to use rope climbing equipment to ascend into the canopy but stop halfway and look around, you probably would not notice much action going on, but stay still and quiet; you might get lucky and spot one of the many flying acrobats of the South East Asian rainforests.
The dipterocarp rainforests of Sundaland are famous for flying creatures such as the red giant flying squirrel (Petaurista petaurista), Malayan colugo (Galeopterus variegates), or the Paradise tree snake (Chrysopelea paradise). Such adaptations to “fly” or rather to glide from tree to tree are most commonly seen in the rainforests of South East Asia, and can be considered quite peculiar to this region. Living high above the ground can be tough, as each tree is like a pseudo island; the ability to fly from one tree to another increases access to food and other resources when they turn scarce.
From above ground level to several meters above it, sits the understory or shrub layer, composed mostly of young saplings slowly reaching for the light. On level terrain, the understory is often dense, but on hillslopes, it is often sparse enough to walk around quite freely. In the dim light conditions, growth is very slow, and the saplings bide their time until a break in the canopy caused by a tree fall or perhaps a few major branches of the large trees breaking off, subsequently allows more light in, giving them a growth spurt.
This layer is also the abode of other shade tolerant understory flora, such as the palms, ferns, and even the occasional cycad. The palm flora of Malesia is exceedingly rich, with about 880 species recorded in that region alone. Malaysia has about half of that number, with 443 palm species recorded so far. Many of the palm species are very rare, and are only found in a few locations and nowhere else.
The ground layer is of course, terra firma. Apart from the canopy, this is the richest zone in terms of biodiversity. Covered in dense fallen leaves, broken branches, fallen trunks, and seedlings, you can expect to find many thousands of ants, termites, worms, larvae, and various types of bugs and creepy crawlies within every square meter of soil.
The floor of the rainforest is the richest ecosystem on earth. All the familiar “big game” wildlife like elephants, rhinos, tapirs, gaur, tigers, leopards, sun bear, wild boar, wild dogs, etc roam the forest floor, but it takes a lot of luck to encounter most of them. In recent years, camera trapping has proven far more successful than any other method in documenting these elusive inhabitants of the South East Asian rainforest.
The rainforest floor is also the abode of many kinds of herbs and medicinal plants, many of which are very rare, and only found in a few scattered locales. One thing they have in common is that all are very shade tolerant; it is estimated that some parts of the rainforest floor receives as little as 2% of the sun’s rays, most of which would have been intercepted by the many layers above.
Trees are mortal, and with so many trees in the rainforest, treefalls are common. Once a tree (or a group of trees) dies and falls down, it tears down all this “layering “creating a gap, and allows sunlight to pour through, affecting everything in the surrounding vicinity. Meanwhile, the smaller trees are always growing upwards all the time, and when they get an opportunity afforded by a canopy tree falling, their growth rate shoots up. This equilibrium of growth, life, and death illustrates the dynamic nature of rainforest ecology.
Therefore, the concept of layering in the rainforest has its limitations. It does not take into account the dynamic nature of the forest, which is ever changing, and mainly only helps in describing the vertical distribution of the jungle flora and fauna, showing the “mini habitats” found from ground level up to the treetops. In many ways, the layers of the rainforest reflect human society as well. Although human beings compete with each other, at the same time, humans need each other as well – very much like the trees in the rainforest. A lone standing jungle tree will perish sooner or later, due to exposure to the harsh elements. Likewise, humans are essentially social animals, and need to live in societies and communities, even while competing with each other for resources. This is a fundamental, universal paradox!