Where are the Rocks in the Lower Amazon?

Rainforests are tremendously vegetated. Early European settlers in the tropics were convinced (and even assured by scientists at the time) that the lushness of the "jungle" was due to the rich soils, so they cut down large patches of forest to create croplands. The cleared land supported vigorous agricultural growth, but only for one to four years, when mysteriously, plant growth declined to a point where copious amounts of fertilizer were required for any growth. Settlers wondered why their crops perished and how such poor soil could support the luxuriant growth of tropical rainforest. The answer lies in the rapid nutrient cycling in the rainforest.

NUTRIENT CYCLING

The colonial settlers did not realize that they were dealing with an entirely different ecosystem from their temperate forests where most of the nutrients exist in the soil. In the rainforest, most of the carbon and essential nutrients are locked up in the living vegetation, dead wood, and decaying leaves. As organic material decays, it is recycled so quickly that few nutrients ever reach the soil, leaving it nearly sterile.

Dung-mimics The attractiveness of dung to small rainforest insects has led to the development of dung-mimics both amoung predators and prey. These animals, generally insects and spiders, sit motionless for hours at a time trying to look as dung-like as possible to avoid detection.

Decaying matter (dead wood and leaf litter) is processed so efficiently because of the abundance of decomposers including bacteria, fungi, and termites. These organisms take up nutrients, which are released as wastes when organisms die. Virtually all organic matter is rapidly processed, even fecal matter and perspiration. It is only a matter of minutes, in many rainforests, before dung is discovered and utilized by various insects. Excrement can be covered with brightly colored butterflies, beetles, and flies, while dung beetles feverishly roll portions of the waste into balls for use later as larval food. Insects are not only attracted to dung for the energy value, but often for the presence of nutrients like calcium salts. Human sweat is a treasure for several species of butterflies, which gather on the necks and hats of tourists, and for annoying sweat bees, which can cover seemingly every inch of exposed skin in some forests.

As vegetation dies, the nutrients are rapidly broken down and almost immediately returned to the system as they are taken up by living plants. Uptake of nutrients by plant roots is facilitated by a unique relationship between the roots and a fungi, mycorrhizae. The mycorrhizae attach to plant roots and are specialized to increase the efficiency of nutrient uptake nutrient from the soil. In return, plants provide the fungi with sugars and shelter among their roots. Studies have also shown that mycorrhizae can help a tree resist drought and disease.

TREE ROOT SYSTEMS

Tree roots, Brazil 1999

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, expecially 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.

REGROWTH AFTER CLEARING

Carpet of moss in Perinet, Madagascar 1997

When Europeans cleared the rainforest for agriculture, they probably assumed that the forests would regrow relatively quickly like the temperate forests of Europe and North America. But rainforest will not readily return on lands with agricultural monocultures that have been devoid of forest for several years and have highly degraded soils. Tropical soils rapidly become inhospitable to growth due to swift leaching of nutrients caused by heavy rains and intense sunlight. In addition, many tropical hardwood trees are dependent on certain animal species for pollination, seed dispersal, and seed processing. The seeds of many tropical rainforest species are large (since they germinate in the shade of the canopy and must have enough food reserves to grow in the low light conditions of the forest floor) and require animal dispersers (wind or other mechanical means often are not sufficient for dispersing seeds of this size). The loss of these dispersal species when forest is leveled, means tree seeds are unlikely to be dispersed into cleared areas. Therefore these important forest tree species will not return.

The seeds and seedlings of those tree species not limited by animal dispersal and pollination agents are often specifically adapted to the light and climate conditions of the shaded rainforest floor. These seeds usually will not germinate in the hot, arid conditions of clearings, and those that do sprout rapidly succumb to the sun and poor soils. Tropical rainforest plants are accustomed to the stability of the rainforest, where they are robust. When they and their seeds are confronted with the entirely different set of conditions presented by forest clearing, they do not fare well.Their seeds have little or no capacity to remain dormant since they have no need under normal forest conditions.

The dry air of the forest clearing also dessicates the leaf litter causing the mycorrhizae to die. The elimination of the symbiotic mycorrhizae reduces the capacity of trees to take up nutrients from the soil. This fungi is especially difficult to replace since each species of tree may have its own symbiotic species of mycorrhizae. Regeneration is further stunted by the rapid encroachment of tough grasses and shrubs after the clearing of forest.

The situation is different when a cleared area is left surrounded by forest and the soils have not be decimated. Under these conditions, forest may rapidly reclaim the barren patch as fast-growing, weedy pioneer plants like forest grasses, bananas (Heliconia), gingers, and vines move into the clearing. Pioneer tree species—which require such conditions of bright sunlight and lower humidity for growth—like cecropia (neotropics) and macaranga (Asia) quickly colonize forest gaps. After several years, the small number of pioneer plants and trees like kapok and cecropia are gradually replaced with diverse species more characteristic of older-growth primary forest. The necessary mycorrhizae fungi can recolonize from the surrounding forest and facilitate nutrient uptake. After 20 years the formerly cleared land may again support vigorous growth, although this forest is only a shadow of the original primary forest. The new secondary forest, is far less diverse, has a less developed canopy, fewer animals, and thicker ground vegetation. It is unknown how long it takes for secondary forest to return to the complexity of primary forest, but the estimates range from hundreds to thousands of years.


Review questions:

• Why does traditional agriculture often fail in rainforests?
• How are nutrients recycled in the rainforest environment?
• How are tropical forest soils different from soils in temperate forests?