Tropical regions with a high rainfall have a tree dominant vegetation. The trees are tall broad-leaved evergreens, in fact nearly all species are evergreen, as a deciduous plant would not be able to compete against species with an all year growing season. These forests are the most luxuriant form of vegetation, and have the fastest growth rate of any community. Some of the largest tree species on earth are found here.
The structure of the humid tropical forest is the same wherever it occurs; only the species differ. The basic requisite for this kind of vegetation is a hot wet climate. The mean monthly temperature does not drop below 25 C (77°F) and the relative can be over 80 per cent. This humidity increases under the tree canopy with extra water being transpired by the vegetation. The rainfall is always above 1,500mm (sgin) a year.
The tree-layer of the forest is usually made up of three different heights of tree. The basic canopy is about 30m (90ft) high, but there are also several taller species projecting from this layer, with some shorter ones growing below. This shortest group have narrow crowns to make the most of what little light penetrates this far. Below the trees is the shrub layer, the plants here are sparsely distributed as they only grow where light penetrates the canopy. The remaining layer that constitutes the ground cover is made up mainly of plants, such as ferns, that need little light.
Woody climbing plants called lianas are common, growing up the trunks and into the canopy to reach the sunlight. The trees also support a large number of epiphytes (plants attached to the trees and not rooted in the soil). Many different species have adopted this growth habit and they include ferns, orchids and bromeliads. These plants obtain their nutrients from decaying vegetable matter which they often trap in specially adapted cup-shaped leaves or matted spongy roots. It is common to find the lower parts of trees thickly covered with epiphytes.
The hot humid climate promotes the growth of this lush vegetation. It also ensures that any dead plant matter decays rapidly to add nutrients to the soil and thus enable it to support this targe amount of growth.Occasional giant trees thrust huge crowns of dense foliage into the airy light above the forest.
Narrow-crowned trees compete for growing space in the light that filters between the trunks of taller trees.
In the patchy gloom beneath the trees sparse shrubs form a lower, separate layer.
The damp soil of the forest floor supports a rich growth of ferns and other shade-loving plants.
The broad crowns of closely-packed trees form an almost unbroken ‘sea’ of foliage 30m (90ft) above the forest floor.