In every rainforest, there are always some trees growing while others are decaying or falling.

A classic forest growth cycle begins with a gap phase, passes through a building phase, and then reaches a third, mature, phase as colonizing trees reach maturity.

No entire forest undergoes such a clearcut cycle. At any one time, the forest is a mosaic of these different phases of the growth cycle.

The coarseness of the mosaic depends on what causes the gaps. Some parts of the humid tropics-such as the Caribbean and the northern Philippines-are prone to cyclones that knock down great swathes of forest.

In seismically active and mountainous New Guinea, landslides are frequent and gaps are huge. In regions such as Malaya and Amazonia this type of extensive catastrophic damage does not occur. Instead, gaps usually develop as individual trees decay of old age or a small group is blown over by the wind that accompanies a local thunderstorm.

The trees that colonize big gaps have seedlings that are well adapted for growth in bright sunlight, high temperatures and relatively low afternoon humidity.

Such species compete strongly with each other for light, and typically grow fast or die if overtopped by other trees. They are often called pioneers because they must colonize big gaps and cannot perpetuate themselves in their own shade. These trees could be considered forest nomads, as they are always seeking a new home in which to survive.

In a small gap, seedlings or saplings already present will flourish. They can tolerate low light and require the higher relative humidity and lower temperature of the forest interior. The conditions of a big gap would kill them. They differ from pioneer trees because they are able to perpetuate themselves in situ, and so do not need cataclysmic disturbance. The structure of a forest formed by these species is more complex and is less likely to have groups of the same-sized trees.

Many pioneer trees are small and short-lived. They are soon replaced by shade-bearing species.

Others live many decades and reach large size. These are of great importance to forestry. They have characteristics that make them easy to grow in plantations, and their pale soft wood makes them commercially valuable. Balsa, native to South America, is probably the best-known rainforest pioneer tree.

Tropic rainforests have a layered structure that reflects the forest growth cycle.

The deep, narrow crowns of the building phase forest form a different vertical pattern from the broad crowns of the mature phase.

A forest of pioneers recolonizing a big gap tends to be single-layered and even-aged. The big trees have no small individuals waiting to replace them.

A forest formed in a series of small gaps has a different vertical and population structure, with big trees as well as smaller, younger ones.