A forest with stories
Walk into a tropical rainforest and you will notice that conditions change as you look upward. The ground is dim and still; the treetops are bright and breezy. Ecologists often divide this gradient into layers, a bit like the stories of a building. The idea is a simplification, because the real forest is a continuous tangle with no flat boundaries, and different authors count four or five layers. Even so, the model is useful for explaining why different organisms live where they do.
The forest floor
The lowest layer is typically dark. Often only a tiny fraction of the sunlight that reaches the treetops penetrates to the ground, so few plants can grow there besides shade-tolerant seedlings, ferns and herbs. The air is still, humid and warm, with little temperature swing.
What the floor lacks in plants it makes up for in decomposition. Fallen leaves, branches and dead animals break down quickly in the warm, damp conditions, and the nutrients are taken up again by shallow roots and fungi, a process described in the article on why rainforests have nutrient poor soils. The residents include ants, termites, beetles, millipedes, snails and fungi, along with larger ground-dwellers such as forest-floor birds, deer, peccaries, rodents and, in some regions, big cats and large primates. Seeds and seedlings wait here, sometimes for years, for a gap to open above them.
The understory
Above the floor lies a layer of saplings, shrubs, palms and young trees growing in the shade of the taller canopy. Light is still limited, though occasional sunflecks get through. Leaves in this layer are often large and dark green, an adaptation for capturing the weak light available. Understory plants such as these often have fruit and flowers that attract animals.
Many frogs, lizards, snakes and insects live at this level, along with birds that forage in the middle of the forest. Humidity is high, and temperatures are more stable than higher up.
The canopy
The canopy is the main roof of the forest: a more or less continuous layer formed by the interlocking crowns of mature trees, commonly at a height somewhere in the range of 25 to 45 meters, though that varies. It is the most active layer in terms of biology. Most of the forest’s leaves, flowers and fruit are here, because this is where sunlight is strongest.
Conditions are harsher than below. Sun, wind and temperature changes are greater, and humidity can drop on dry afternoons. Many animals specialize in this layer. Monkeys, sloths, tree-dwelling snakes, many birds, bats, and enormous numbers of insects spend much or all of their lives in the canopy. Some never touch the ground. The branches also support dense growth of climbing vines and canopy epiphytes, which add habitat, such as small pools of water held in leaf rosettes.
Because the canopy is hard to reach, it was long poorly studied. Tools such as cranes, walkways and rope climbing methods have made it more accessible.
Emergent trees
A small number of very tall trees rise above the canopy, sometimes by 10 meters or more, with wide crowns that stand fully in the sun. These are called emergents. They face the strongest sunlight, wind and heat, and often have small leaves or other features that limit water loss. In many forests they produce seeds that are carried by wind, since they stand high above the canopy.
Emergents provide perches and nesting sites for large birds such as eagles and macaws in various regions. Because they are so exposed, they are also vulnerable to lightning, storms and drought, and can be conspicuous targets for loggers.
Why layers matter
Dividing animals by layer helps explain how so many species can live in a limited space: different species use different levels, or different parts of the same level. Many birds, for example, have a preferred height. Moving across layers is also part of the forest’s life, since fruit-eating animals carry seeds from the canopy to the ground, and nutrients move downward as leaves and droppings fall.
Gaps and change
The layers are not fixed. When a big tree falls, it creates one of many canopy gaps, and light floods in. Seedlings waiting in the understory race upward, and for a time the vertical structure is rearranged. Over years, the gap closes, and the layers re-form. In a disturbed or regrowing forest, the layers are generally less distinct than in an old stand, and the tallest emergents may be missing altogether. This is one of the visible differences between older forest and young regrowth.