Rainforest Diversity


July 31, 2012

The soils of a rainforest affect the diversity of the forest. Although nearly 70 percent of tropical rainforest exists on poor acidic soils, it retains its fertility in a large part thanks to nutrient recycling and other processes. However, in some areas, soils are so poor that only a limited number of tree species can grow (though these forests are still highly diverse by temperate standards). One example is the so-called "white-sand" or "blackwater" forests that grow on rocky, sandy soils. Some of these forests grow on nothing but rocks and the roots of other trees. Trees that grow under these conditions tend to be species with tannins in their leaves, which in turn, turn local rivers into "blackwater" rivers. The bitter tannins in their leaves limit insect populations, thus reducing the number of animals the forest can support (insects serve as a major food source for larger animals in most rainforests). These "blackwater" forests are self-perpetuating, since the "blackwater" rivers that result from the decay of their leaves only make the soils more acidic and prevent other tree species from growing on the already nutrient-lacking soils.

Forest tree diversity, and hence total diversity, may also be reduced in forests with soggy soils like those of the igapò or "swamp forest." The limited number of tree species like Cecropia and palms that can tolerate these wet soil conditions means that these few trees species tend to dominate these areas. Subsequently only the animals that feed on their fruits, leaves, and seeds are abundant in these areas.

New research suggests rich Amazon soils

High-diversity forests are often found on nutrient rich—sometimes volcanic— soils that are well-drained. These forests are frequently found in areas protected from major disturbances like strong wind and regular flooding.

Rainforest on Barro Colorado Island, Panama. Photo by Rhett A. Butler

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