The Understory


July 31, 2012


Tropical rainforest trees are well-adapted to their environment and have mastered the problem of poor soils. Since the first six to eight inches (15-20 cm) of soil is a compost of decaying leaves, wood, and other organic matter, it is the richest source of nutrients on the ground. To tap this resource, canopy trees are shallow rooted, whereas most temperate tree roots extend more than 5 feet (1.5 m) deep. Many tropical species have roots that actually grow out of the ground to form a mat on the forest floor in order to more efficiently collect nutrients. These tiny roots form a network that, along with the mycorrhizae fungi, rapidly absorb nutrients.

The configuration of shallow roots and great height causes a great deal of instability for rainforest trees, especially with wet soils and strong winds of the upper canopy that can accompany tropical storms. To counter this, many tree species have extensive root systems that in some cases may run for over 325 feet (100 m). Other trees, especially tall emergent species, have evolved buttress roots—large, thin extensions of the trunk that begin some 20 feet from the ground. These structures are thought to also aid in water uptake and storage, increase surface area for gas exchange, and collect leaf litter for additional nutrition. Some trees, especially palms, have stilt roots for support.

Thus when colonists cut the forest and burned the vegetation, they were destroying the delicate rainforest system which allows vigorous growth on such poor soil. Burning the dead wood and vegetation released enough nutrients into the soil to allow crops to grow for several years, but without the mycorrhizae, and other soil organisms to fix nutrients, soils were rapidly leached by the harsh tropical sun or washed away by heavy rains. Essential minerals were not replaced by new decaying matter since there was no longer forest above to drop leaves and wood. Within a few years, the soil becomes nutrient deficient and can no longer support productive yields of conventional crops. Essentially, the colonists destroyed several links in the semi-closed nutrient system of the rainforest and had to abandon the site for another forested patch. While this seems similar to the "slash-and-burn" technique of native Indigenous peoples, the difference is in the scale and form of the cleared sites. By clearing large areas, the colonists created a major break in the rainforest nutrient cycling system; something which takes generations to recover. In the smaller patches cleared by traditional forest dwellers, forest can quickly recolonize after agriculture is abandoned, especially if the patch is left surrounded by forest. Within 20 years, relatively well-developed secondary forest can reclaim such an agricultural plot.

Not all rainforest soils are so poor; some rainforests grow on nutrient-rich floodplain and volcanic soils. Some of the best soils are found on steep slopes because minerals are released when the exhausted topsoils erode. Such rich soils are found in the Amazonian floodplains, Andean foothills, and volcanic areas of Southeast Asia (Java), Africa, Central America, and the Caribbean. However, without proper management, these soils as well can be rapidly leached of nutrients by heavy rains and the sun. For example, a hectare of tropical rainforest rarely loses more than 1 ton of soil annually. However, when the forest is leveled and planted with various crops, the erosion increases drastically. If the forest is replaced with dense vegetation like a coffee plantation, the hectare loses between 20 and 160 tons, whereas if it is replaced with field crops, the patch can lose more than 1,000 tons annually.'s Rhett Butler in front of a giant fig tree in Panama in 2011. Photo by Rhett A. Butler

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