Stratification refers to the five different layers that trees grow at in the tropical rainforests. These five layers are the emergent, canopy, understory, shrub, and herb.
The emergent layer generally reaches heights between 130 and 250 feet (40 and 76 meters) above the ground. The emergent layer trees grow high above the other layers. The top of the trees can spread an acre in width.
The canopy layer usually reaches heights between 65 and 130 feet (20 and 40 meters) above the ground. The canopy is very dense. Tops of trees touch each other and vines wrap around their branches. The canopy is important because it shields excessive winds and sunlight from the rest of the rainforest. Because the canopy filters out 98 percent of sunlight from the layers below, vegetation is sparse except when breaks in the canopy occur. Because the enormous trees in this layer and the emergent layer have shallow roots, they need thick vines and large bases, called triangle buttresses, to keep them sturdy in the thin soil.
The next layer is the understory. It is mainly composed of tree trunks and younger trees growing to reach the top.
The shrub layer, usually occurring only 20 to 30 feet (6 to 9 meters) above the ground, consists of thinly populated shrubs. These shrubs do not receive much light and, therefore, do not grow as densely.
The final layer is called the herb layer. Little grows here because of the lack of sunlight. Mainly small, soft stemmed plants like herbs grow here. The other layers protect them from the heat, driving winds, and rain.
The rainforest is so densely populated in different tree species that in the Malaysian forest 750 different tree species were counted on only a 25-acre (10-hectare) plot of land. This is more tree species than can be found in all of North America.
Plants usually receive more water during the rainy season than they need. One way plants have adapted to this is by forming leaves with a slick outer coating so rain slides off the leaf and does not weigh the plant down. Leaves have also developed drip tips. Drip tips help guide raindrops off of the plant.
Other amazing adaptations have been made by trees to protect themselves from plant eaters. These trees produce tannins and other chemicals on their leaves that make them hard to digest and, therefore, undesirable to eat. Once these chemicals reach the ground by way of rain it is poisonous to other plants. This keeps plants out of the trees personal space.
The cecropia tree protects itself from plant eaters by allowing armies of ants to feed on its sweet, liquid droplets. These ants are very protective of their wonderful food source and will attack insects and animals over a hundred times their size who come close enough.
Passion flower vines also protect themselves from pesky leaf-eating caterpillars. Sometimes they will mimic leaf patterns of nearby trees to hide from butterflies. If a butterfly lays its eggs on the tendril of the plant, the plant will shed its tendril in order to get rid of the eggs. Passion flower vines also form yellow dots on their tendrils so butterflies think that, that spot has already been taken.
Another nuisance to trees are plants called epiphytes. A few common epiphytes are ferns, lichens, bromeliads, and orchids. Epiphytes live on trees and other plants above the ground. They dangle their roots from many feet (meters) above the ground. They collect nutrients from raindrops with dust and other particles in them.
Sometimes these plants get too heavy for trees and can even break branches. When trees get tired of epiphytes, they can shed their bark to get rid of them. Other trees also secrete toxins that discourage epiphytes from living there. Some trees, like the cecropia, also form relationships with ants to keep their branches clear of epiphytes.
Vines also wrap around trees. Lianas, which grow woody tissues like trees, get a good hold on trees. Using their clasping roots, barbed thorns, or twisting tendrils, lianas can climb up any tree to reach the sun.