People new to forests tend to see a forest as one green mass. People who have walked longer see it as layers, and once you can see the layers you understand why there is birdsong here and only the sound of leaves there.
A dense tropical forest is usually read as four to five layers: the emergent layer, the main canopy, the understorey, the shrub layer and the litter layer at ground level. The number of layers is not a hard rule; the older and wetter the forest the clearer the layers, while secondary or dry forest may have only two or three.
Five layers, read from the top down
- Emergent layer: a few individual trees rising well above the general canopy, taking the wind and the direct sun.
- Main canopy: the continuous layer of leaves that covers most of the sky, where most of the forest photosynthesis happens.
- Understorey: smaller-trunked trees that tolerate shade, standing and waiting for a gap to rise into.
- Shrub layer: low plants, young trees and climbers, from head height up to a few metres.
- Litter layer: fallen leaves, rotting branches, moss, fungi and shallow roots, where material is returned to the soil.
Light decides who lives where
Light passing through the canopy grows weaker and changes in character. Under a dense canopy only a small part of the original light still reaches the ground, and most of the time it arrives as flickering patches as the leaves move.
So plants living in the lower layers usually have large, thin, dark leaves to catch as much weak light as possible, while those in the canopy tend to have smaller, thicker leaves to take sun and drought. When a large tree falls, a column of light opens and the race between the young trees begins in exactly that spot.
Why this is worth knowing when you walk in a forest
Knowing the layers helps you choose where to stand. To hear birds, look for a forest edge or a gap where the canopy is broken. To avoid the midday sun, keep to where the canopy is continuous. To see moss, fungi and the traces of small animals, slow down at the litter layer.
Walking in forests in northern Vietnam, we have found that scent also changes with the layer: where the canopy is closed and damp, earth and decaying wood come through more clearly, and where it is open and windy the scent thins and leans towards fresh leaves. This is our own impression, not a measured rule.
A few questions you often send us
Do forests in Vietnam have all five layers
It depends on the place. Forests that are wet, old and little disturbed usually have the full set; plantations or recently recovering forest have not formed all the layers yet. The simple check is to stand still, look up, and count how many layers of leaves sit at different heights.
Which layer holds the most life
The answer depends on which group of organisms you mean, so it should not be made absolute. The canopy is often mentioned because leaves and flowers gather there, while the litter layer is the place of fungi, micro-organisms and small animals.
Why a dense forest has fewer shrubs than I expected
Because there is not enough light. Where the canopy is continuous, the layers below do not get enough light for shrubs to grow thickly, so many old-growth forests are in fact fairly easy to walk. The tangled forest that is hard to walk is usually secondary forest or a forest edge, where there is more light.