The Understory

LIGHT GAPS IN THE RAINFOREST

The majority of the world's remaining forests are not the classical rainforest with towering trees, an open interior, and virtually no ground growth. Instead, most rainforests have been impacted in their recent history by storms, fires, logging, and landslides and subsequently have scattered areas in various stages of regrowth.

A common event in the tropical rainforest is the fall of an emergent tree, usually during a tropical thunderstorm. In fact, it is estimated that tree turnover rates in some rainforests are every 80-135 years. When one of these giants—laden with lianas connected to neighboring trees—falls, it takes out a sizeable portion of the canopy. This hole in the canopy is known as a "light gap" because direct sunlight reaches the floor in contrast to the usual 1-5 percent under full canopy conditions. The opening of a light gap brings many changes to the section of rainforest.

The light gap is rapidly colonized by the same pioneer species that colonize clearings including trees like cecropia, balsa, macaranga, musanga, and bamboo, and shrubby plants like gingers, bananas, nightshades, climbing lianas, and rattan palms. These species are well-adapted for rapid growth, but not for long-term existence in the forest. Their often white wood and leaves with poor chemical protection are subject to infection and infestation by insects. Generally, these pioneers flower rapidly and produce numerous fruits, but are soon overtaken by the hardier, better adapted hardwood trees which fill in the gap in the canopy. Many forest tree species are dependent on light gaps to complete their life cycle.

As a result of the increased light and abundance of fruits produced by gap colonists, light gaps are areas of increased animal activity. Carnivorous animals follow the herbivorous animals that are attracted to the fruiting plants.

Review questions:

• What is a light gap and why is important to forest regeneration?

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