Tropical rainforests, found in parts of Central and South America, Africa and Southeast Asia, are evergreen broadleaf forests. Temperate rainforests in the Pacific Northwest and British Columbia, plus the coastal areas of Ireland, Scotland, southern Norway, parts of the Balkans, Japan and Korea are mostly covered with evergreen conifers and trees with needles.
Many trees in tropical rainforests grow from 100 to 240 feet tall, with umbrella-shaped canopies. The canopies of these tall trees reduce the light that falls to the forest floor.
The understory, or lower canopy, contains trees that are about 60 feet tall plus plants, shrubs and the trunks of the taller trees. There is little air movement in this humid zone that is in almost constant shade.
In temperate rainforests of western North America, the tallest trees are the stately redwoods, Douglas fir, spruce and hemlock, which lack the high, spreading canopies of the tropical broadleaf trees.
With rare exception, the floors of both tropical and temperate rainforests are completely shaded; few shrubs or small plants grow there. Worms and fungi consume the litter that falls to the ground.
Tropical Rainforest Plants
Plants that grow on branches high in the trees and extract moisture from the air are called epiphytes. They are found on leaves, branches and trunks.
Stranglers start out as epiphytes, then send their roots down tree trunks and into the soil.
Lianas are woody vines that grow up the trunks of trees. Asians make furniture from rattan vines. Philodendrons are lianas.
Bromeliads have thick, waxy leaves in the shape of a bowl to catch rainwater, often giving a home to mosquito larvae, salamanders, snails, beetles and frogs. Pineapples are bromeliads.
Trees in both tropical and temperate rainforests have shallow roots that can spread up to 30 feet wide.
Some tall trees in tropical rain forests have buttress roots grow as high as 15 feet above the tree base. The buttress roots spread out, supporting the tree and increasing the area from which it can obtain nutrients.
Tropical rainforest trees have smooth, thin bark because the trees don't need protection from water loss or freezing temperatures. The smooth bark also helps prevent plant parasites and epiphytes from finding a place to grow.
Tree bark in temperate rainforests is usually thick and crusty to protect the interior of the tree from freezing temperatures.
The pores, or stoma, in tropical broadleaves give off water in a process called transpiration, which is responsible for up to half of the precipitation in a rainforest. Many tropical broadleaves are glossy and grooved, with pointed drip tips to help them shed water. If they did not shed water quickly, the heavy, water-soaked branches might break.The leaves also need to dry quickly to prevent the growth of mold in the humid air.
The leaves on plants growing under a canopy are usually larger, to get as much of the rare sunlight as possible. The small, leathery, dark green leaves on the canopies of tall trees keep the loss of water at a minimum in the strong sunlight. On some plants, the leaves at the bottom are larger than those at the top. In order to receive maximum light, the stalks of some leaves turn to follow the sun.
Since temperate rainforests are farther north, where plants get less sun, evergreen needles and scales are dark green to help them absorb more sun. They also have thick exteriors and special resins that prevent them from freezing. Older needles can conduct photosynthesis in late winter.
Heterotrophs are plants that do not obtain nutrients through photosynthesis; these include saprophytes and parasites. Saprophytes obtain their nutrients from rotting material on the forest floor. Parasites tap into the stems or roots of photosynthetic plants. Rafflesia arnoldi, a parasite that lives off the roots of a liana, has a flower that is more than 3 feet wide, the largest in the world. Both tropical and temperate rainforests have saprophytes and parasites.