Plants need special adaptations to live successfully in the rainforest. The soil contains few nutrients, and the nutrients it does contain are concentrated on the soil surface. The warm, humid environment encourages the growth of mould that can harm the plants, and the vegetation in the upper canopy levels is so thick that the forest floor is almost completely shaded. Trees and other plants have evolved special features that allow them to survive in these conditions.
In temperate forests, trees have thick, rough bark because the air is drier; the bark helps the trees retain moisture and keep from freezing. The air in rainforests is warm and highly humid, so keeping moisture in and preventing freezing isn't as important. As a result, rainforest trees tend to have smoother, thinner bark. This bark may also keep other plants from growing on rainforest trees; however, plants called epiphytes have adapted to be able to grow on rainforest trees.
Lianas are woody vines that climb up rainforest trees. These vines put roots into the ground and then wind their way up a tree in order to reach the sunlight in the higher levels of a rainforest. Once there, they might wrap themselves around neighbouring trees or other liana vines, creating a stability network for the shallowly rooted trees against strong winds. Lianas can grow up to 3,000 feet and include rattan palms and philodendrons.
The warm, humid atmosphere of the rainforest is perfect for the growth of fungi and bacteria. Rainforest plants have adapted to avoid collecting rainwater in their leaves and thus avoid the growth of these organisms by having drip-tips or spouts on the end of their leaves. Along with the waxy coating on the leaves of many rainforest plants, these drip-tips keep water from pooling in the leaves and thus keep mould from growing.
Rainforest soil is nutrient-poor, and the majority of the nutrients are available at the surface, so rainforest plants are often shallowly rooted. Trees often have large buttress roots--ridges that can rise as high as 30 feet before they become flush with the trunk. These give the trees additional stability. Some rainforest trees, such as mangroves, have prop or stilt roots, above-ground roots that grow much faster than the tree itself--up to 28 inches in a month--and help support the trees.
Epiphytes and Epiphylls
Plants that grow on trees are called epiphytes or air plants. These plants grow from seeds or spores left on a tree by birds or the wind. By growing on the trunk and branches of a tree, epiphytes receive more sunlight than they would if they grew on the ground, which is heavily shaded. Orchids, bromeliads and cacti are examples of epiphytes. Epiphylls are small plants such as mosses, lichens and liverworts that live on leaf surfaces.