Rainforest knowledge

Pls note that this is not a “tour package – it is general knowledge that applies to most packages we offer. On this page we point out some things you may want to look closer at once you are inside the tropical lowland rain forest.  What makes the tropical rainforest so special is that the things listed here occur all in the same place, at the same time. Rainforests are estimated to cover only 6% of the Earth’s surface, and still contain more than half of its species of plants and animals.

Rain is a dominating factor in the rain forest

Tropical rainforests appear in a belt around the equator. The rainfall is often more than 2500 mm per year. (I can compare with 550 mm in central Sweden.) Air moisture at ground level may be as high as 90%, even if there has been no rain for some days. The ground is always damp and you need to bring something to sit on when you take a rest. It is difficult to get clothes dry inside the forest. You should know that the intact rain forest never burns. Only when logging or other causes have opened up the canopy to let in sunlight will ground fire enter.

In the forest our perspiration, our body’s spontaneous response to physical effort and heat, makes us wet but does not cool efficiently. This is because the air is almost saturated with moisture, and there is hardly any wind at all at ground level, so the perspiration does not evaporate (which is where the cooling happens)! If you feel hot it is better to use a fan or to move on slowly, rather than stop or sit down.

Bring a raincoat or a plastic sheet in case of rain. But you cannot walk for very long like that – it just gets too hot and you get wet from inside the plastic. Better wait for the rain to pass, which it normally does rather soon – unless it is the rainy season.

We usually set up camps in clearings or on river banks to get a chance to dry our clothes etc.

Over the millennia rain and run-off have contributed to shaping the terrain inside the rain forest. From the outside you see a rather smooth canopy. But when you get inside you will find that the ground often consists of carved-out ravines and ridges that may be steep and slippery to climb. The guides will of course look for the best way, which often is a trail on a ridge top, opened by animals or hunters.

The persistent rains have leached the soils and made them rather poor in nutrients – which may be surprising when you see the thick vegetation. In fact, most of the nutrients are stored in the vegetation. If the forest is cut down erosion sets in immediately and the top soil is washed away. This is a reason why deforestation, or even modest selective logging in the tropics generally is a bigger threat to the environment than in temperate climates.


The leaves have many different forms, sizes and nervation patterns, but very often they have a marked, elongated tip. This may be a result of evolution, speeding up run-off from the leaves to prevent water stagnating on the leaf surface, where fungi and bacteria could thrive. In this cut-throat competition every sunray counts. The cleaner the leaf, the more efficient the photosynthesis. We do not have seasons here and leaves may last for four years before they are shed.

Species richness, or biodiversity

The rainforest is famous for its biodiversity – its unusual richness in species, of all orders. The tree leaves are like fingerprints and help in the identification of tree species. (Some of the trees are so tall that you need binoculars to determine which leaf comes from which tree!) The variation in size, shape, nerve pattern, form of the petiole, and texture etc is very wide and helps in identification. The rain forests of S E Asia are known to contain 400 tree species or more. An amusing manifestation of this is that if you stand beside one tree you can be almost 100% sure that the next tree beside you is of another species. The area that biologists call Sundaland, (which includes Malaysia, Sumatra and Borneo/ Kalimantan, is globally a biodiversity hotspot, and is home to some 25 000 flowering plants. If we only look at the important family Dipterocarpaceae (or “dipterocarps” where most of the commercially important timber trees are found) Sumatra alone has about 100 species. Most of those species grow very tall with big stem diameters and they dominate the forest structure. Seen from above – like in a drone photo, or if you fly over the forest – the crowns of these dominating trees look like cauliflower. This tells you at once that the forest has not been logged, or logged carefully. The dipterocarp timber is valued for straight grain and almost no knots. You will see those giants in the Gunung Leuser and  Betung Kerihun National Parks, where they are still protected. Aside from a multitude of tree species, the Leuser Ecosystem is said to shelter 100 species of mammals (including orangutan, tiger, elephant and rhino), 380 bird species and 95 species of reptiles.

All the space is occupied, from top to bottom

Foresters talk about strata or layers in the forest: emerging trees (the cauliflower!), canopy trees, understorey trees, and forest floor with seedlings. This is somewhat arbitrary, as there are leaves and branches almost everywhere and the layers are not clearly defined. The upper canopy is a world of its own. This is of course where the tall trees have their flowers and seeds, and this is where the insects and birds will flock. In some tourist spots you will have a chance to go on a so-called canopy-walk. Do not miss it! You will be amazed. A landscape of another kind.

For the photographer the vegetation density is a challenge – since wherever you point your camera there will be a leaf or two on front of the lens. You will have to manoeuvre and may have to bend branches to the side to get a clear shot. So you need time. – Moreover, it is dark inside the rainforest. The difference between a clearing and the dense forest at ground level may be 6 f-stops on the camera, or from a shutter speed of 1/250 in the clearing to ¼ inside the forest. Below 1/30 your photo will likely be blurred and you miss in detail and realism. If you do not bring a light tripod,  you can look for a tree to lean against, or use a walking stick to hold the camera on.

Again, the darkness at ground level of course limits the photo synthesis – so leaf surfaces must be clean as mentioned. The darkness also limits the ground flora, and it is rare to find flowers on the ground.

All age classes and sizes are found

Unlike in a plantation, where all the trees are of the same age – and sometimes even the same clone – the natural forest contains trees of all ages, heights and sizes, including seedlings of the big trees.  The seedlings are adjusted to a life in the shade of their parents, and just stay there almost dormant until a gap is created by a storm or the death of an old tree. When light flows in the seedling is stimulated to start growing quite rapidly towards the heights to fill the gap and to capture more light.

Buttresses and plank roots

Big trees in the rain forest develop this kind of support, more so than in any other type of forest, and this is explained by the fact that the root systems seldom go deep. So the support needs to be broader. Also the soils usually offer a very limited nutrient supply on lower levels. More under Nutrient circulation.

Fungi, ants and termites

The atmosphere inside the forest, especially at ground level, is really like in a green-house – a high and stable temperature and high moisture. This is of course ideal for fungi, and you will find many kinds of mushrooms along the trail. This makes for a fast decomposition of all organic matter such as the litter on the forest floor, transforming it to nutrients that can be taken up by photosynthesising plants like the trees for their growth. To this is added the consumption by ants and termites that also help break down wood, bark and leaves.

Epiphytes, including orchids

Epiphytes, plants that grow on other plants and in particular on tree branches (without taking any nutrients from the host – as opposed to parasites) are found everywhere in the rain forest canopy. Epiphytes depend on rain for their survival – and rain water is normally available in sufficient volumes. Nutrients are captured from the bark of the host tree and from falling leaves that land on the plant and decompose there. The most renown among epiphytes are of course the orchids, but staghorn ferns are also famous – and they illustrate how a plant can form a bowl to capture both water and falling leaves. Other epiphytes are mosses and the decorative Lycopodium.

A special case is the Strangler Fig (Ficus) which will not develop without the support of another forest tree. When adult I produces enormous numbers of fruits. The fruits are eaten by birds and contain some substance that makes the bird excrement slimy. This enhances the probability that a fig seed will stick to a tree branch where it will germinate and develop as an epiphyte.  As the fig seedling develops it will grow long roots that hang down like strings and eventually reach the ground. When the roots are able to take up water and nutrients from the soil the fig will enter a new phase of its life and develop very quickly, forming additional aerial roots that soon develop into stems and also branches with leaves for its own photosynthesis. Eventually the fig stems will surround the tree on which the seed germinated and in that way strangle it, as the surrounded tree can no longer grow and expand. This is why a really old fig tree will appear “hollow” with an empty space where the original tree trunk once was.  – Fig trees can produce enormous amounts of fruits when they are mature. I that way they become landmarks and flocking spots for birds in the daytime and fruit-eating bats at night. (Some bat species are huge and a sight in themselves. Usually the sleep in limestone caves in the daytime, but sometimes you can see them sleeping in trees, hanging upside down. – Talking about fruiting – the dipterocarps produce nuts but only every five years or so, and then in very big quantities. They flower profusely and some flowers produce “perfume” that fills the forest.   

The effect of the decomposition collaboration among ants, termites, bacteria and fungi is rapid recycling. You will very rarely find a dead animal as you walk along, not even the horn or bone of a deer or the feather of a bird. All is rapidly taken care of! Except the plastic which as we now know is not decomposed, only fragmented. (Please collect bottles, wrappings etc that you bring, and leave with the guides when you are back at the starting point.)

Nutrient circulation

The dense, lush and tall forest gives the impression of fertility. But due to the rains and the high temperature the nutrients in the soil are leached incessantly (Chemical weathering), so that the mineral soil is in reality poor in nutrients.  A very refined and effective recycling system has evolved that allows the forest to live on and reach height and density that is not based on richness in nutrients but on the very efficient use and re-use of them. The nutrients in the litter (leaves, twigs, dead insects etc) are released by some fungus species through decomposition. Other fungus species capture the nutrients and help feed them into the tree roots in the top soil, a web of tree roots and fungus mycelium.  We can to an extent observe this with the naked eye by removing the layer of leaves and litter on the forest floor. Below we will see a dense but not very thick mat of roots. Some of the fine roots are whitish and look a bit swollen. 

That is because the root is joined by the mycelium of fungi in a symbiotic arrangement (called mykorrhiza), where the fungus helps absorbing the basic nutrients, and the tree provides the fungus with sugar from its photosynthesis Without this collaboration/symbiosis between fungi and trees the rainforest phenomenon would not exist . (Step by step our scientists discover that most plants need this collaboration with fungi for their growth . including the carrots in our vegetable gardens!)

Below this thin mat of roots and mycelium you very often only find greyish mineral soil, low in nutrients.  If the forest is cut down or burnt, this root mat is soon destroyed – and the nutrients washed away. – The fungus species  involved and acting here are adjusted to a life at the forest floor – and do not withstand heat or even light. A consequence of this is that clearing of the forest kills off the fungus life. It then becomes impossible to restore the rainforest without a lengthy transition phase where suitable trees belonging to the so called pioneer species are introduced to cover the ground, provide shade and keep the temperature low and stable.

Pioneer trees  are strictly speaking not part of the rainforest flora but still an indispensable appendix to it. They tolerate the intense light on open land,  their seeds are small and easily spread, their roots go deeper so that they can withstand dry periods, they grow much faster than the rainforest trees so that they provide a ground cover within 1-2 years, and their root systems do not depend on or collaborate with mykorrhiza fungi but seem rather to depend on bacteria, including such that fixate atmospheric nitrogen. Note that imported or “exotic” pioneer species form the mainstay of the fast-growing pulpwood plantations in the tropics, e g some species or clones of Eucalyptus.





To digress a bit: Mangrove is a highly specialised tropical forest ecosystem found along the sea shore. The mangrove forest is composed of species that are adapted to living in saltwater, and they do not depend on rain. The mangrove forest can be said to be nature’s own agent for land reclamation, protection against tsunami waves from the deep sea, and breeding of fish that when grown up move out into deep waters. The mangrove forest is next to impenetrable.


Peat is a kind of soil consisting of accumulated organic matter that is prevented from complete decomposition by stagnant water that reduces the access of oxygen. (We find this also in temperate and sub-arctic climes, such as in peat bogs.) The organic matter stored generally consists of leaves, twigs and stems of trees growing in areas with high ground water level (freswater swamps). During periods of extreme heat and drought the peat may dry out, especially in areas that have been drained by canals. If the peat catches fire it will be very difficult to extinguish, and the fires  may keep burning for weeks until the arrival of the rainy season.