What are the four layers of a rainforest?

From the top down: the emergent layer, where a few giant trees stand clear of everything else at around 60m; the canopy, a closed roof of crowns at 30 to 40m where most of the life is; the understorey, smaller trees of 15 to 25m in shade all day; and the forest floor.

How much sunlight reaches the forest floor?

About 1 to 2% of it. Published measurements of closed tropical forest put the light at the floor at typically 1 to 2 per cent of what arrives at the top, which is why the ground is so dark. In a treefall gap it is a different story: the light there can be over 70%, and the forest races to fill the space.

Which layer takes most of the light?

The canopy, and it is not close. Drag the meter from above the canopy to just underneath it and the reading falls from 100% to about 12% in eleven metres. Everything below that is sharing what is left, which is why the understorey and the floor look so similar on the meter.

Why is the forest floor so easy to walk through?

Because it is dark. The thick, tangled jungle of films is what grows at the EDGE of a forest, or in a clearing, where the light gets in. Under a closed canopy there is not enough light for much to grow at ground level, so the floor is mostly leaf litter, roots and fallen wood.

Why do rainforest trees have those huge roots at the bottom?

They are called buttress roots. Rainforest soil is surprisingly thin and most of the goodness is in the top few centimetres, so a tree 60m tall cannot hold itself up with deep roots. It grows flanges out from the base of the trunk instead, which is why the giants on this board have them and the smaller trees do not.

Why are understorey leaves so big?

To catch what light is left. A plant living on a few per cent of full sunlight needs as much leaf area as it can manage, so understorey plants grow a small number of very large leaves. It is the same problem the climbers solve a different way, by using a tree instead of growing a trunk.

Layers of the Rainforest

Geography · Year 3–Year 6

Drag a light meter down a rainforest and watch the sunlight run out.

About the rainforest layers

A tropical rainforest cut through from the top of the tallest trees to the ground, with a light meter you drag down through it. The meter reads how much of the sunlight is left at that height, and the shaft of light beside it gets thinner and fainter as you go.

The interesting part is not that the forest floor is dark. It is WHERE the light goes. Above the canopy the meter reads 100%, and eleven metres later, at the underside of the canopy, it is down to about 12%: one thin band takes nearly all of it, and the twenty-seven metres of forest below take hardly any because there is hardly any left. Drag it slowly and a class can see exactly where it falls off a cliff.

Hide the reading and ask the class what they think is left at the bottom before anybody drags it there. Most will say something like a half. The answer is about 2%.

The picture is to scale in height. A metre is the same number of units everywhere on the board, so the emergents really are drawn at about 60m against a canopy at 30 to 40m and understorey trees at 15 to 25m. A photograph of a rainforest cannot show that, because there is nowhere to stand to take it.

The reading is a published equation rather than a number we chose. Fauset and colleagues measured vertical light profiles in intact tropical forest and fitted an exponential fall-off with depth below the canopy top; this board uses their equation, a coefficient between the two they report for intact plots, and the 1 to 2% at the floor that their measurements and their text both give. Real forests vary enormously: in a treefall gap the floor can be over 70%, which is why a gap fills up so fast.

The forest floor is dark AND open, which is the opposite of what most children expect of a jungle. At one or two per cent of the light almost nothing grows down there, so the ground is leaf litter and roots rather than impassable undergrowth. The plants that do manage it have enormous leaves, and the climbers give up on trunks altogether and use somebody else's.

The other way to live up in the light is to start there. Epiphytes are plants that grow ON the branches of the big trees, high in the canopy: ferns, orchids and bromeliads that never touch the ground at all and take their water straight out of the air and the rain. They are too small to draw honestly at the size this board draws a 60m tree, so they are here in words instead.

Biomes is the other half of this: that tool marks the seven biomes across the whole world and says what grows in each, and this one is inside one of them, in section, where the subject is light rather than place.

How to use it on your whiteboard

1Drag the light meter up and down the forest, or use Higher and Lower for one metre at a time.
2Hide the reading and ask the class what they think reaches the floor, then press Go to the floor.
3Drag slowly through the canopy and watch where the number collapses.
4Press Layers to name all four at once.

Questions

What are the four layers of a rainforest?: From the top down: the emergent layer, where a few giant trees stand clear of everything else at around 60m; the canopy, a closed roof of crowns at 30 to 40m where most of the life is; the understorey, smaller trees of 15 to 25m in shade all day; and the forest floor.
How much sunlight reaches the forest floor?: About 1 to 2% of it. Published measurements of closed tropical forest put the light at the floor at typically 1 to 2 per cent of what arrives at the top, which is why the ground is so dark. In a treefall gap it is a different story: the light there can be over 70%, and the forest races to fill the space.
Which layer takes most of the light?: The canopy, and it is not close. Drag the meter from above the canopy to just underneath it and the reading falls from 100% to about 12% in eleven metres. Everything below that is sharing what is left, which is why the understorey and the floor look so similar on the meter.
Why is the forest floor so easy to walk through?: Because it is dark. The thick, tangled jungle of films is what grows at the EDGE of a forest, or in a clearing, where the light gets in. Under a closed canopy there is not enough light for much to grow at ground level, so the floor is mostly leaf litter, roots and fallen wood.
Why do rainforest trees have those huge roots at the bottom?: They are called buttress roots. Rainforest soil is surprisingly thin and most of the goodness is in the top few centimetres, so a tree 60m tall cannot hold itself up with deep roots. It grows flanges out from the base of the trunk instead, which is why the giants on this board have them and the smaller trees do not.
Why are understorey leaves so big?: To catch what light is left. A plant living on a few per cent of full sunlight needs as much leaf area as it can manage, so understorey plants grow a small number of very large leaves. It is the same problem the climbers solve a different way, by using a tree instead of growing a trunk.