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© Roger Leguen / WWF
Wonder trees and plants on the world’s poorest soils
Within the Amazon rainforest itself, several types of forests are found: some are dense, jungle-like rainforests, others are open forests with palms and some are open forests with plenty of lianas1.
But they all share one common characteristic: abundant rainfall. Over the course of one year, a patch of rainforest will receive between 1,500 mm and 3,000 mm of rainfall2. This creates the typical tropical atmosphere of a rainforest with average temperature hovering around 24 °C or more3.

The paradox of rainforest soils

Tropical soils are notoriously thin and poor in nutrients. In some parts of the Amazon River Basin, white, sandy soils are found, which have evolved through erosion over hundreds of millions of years. And yet, although these soils have lost their mineral content and fertility, rich rainforests grow on them.

In rainforests, some of the highest trees on the planet shoot to the sky. Dead plants and animals quickly decompose and their organic matter is utilized by other organisms.
Efficient recycling of dead matter

Fungi and bacteria, small but vital actors of the rainforest food web - convert dead organic matter into compounds that become available to the roots of plants. In this task they are assisted by a range of other organisms that obligingly perform key functions in processing organic matter.

Life in the canopy

Treetops form a vast canopy characterized by high productivity: More sunlight is captured there by leaf area than in any other ecosystem in the world.

This light is converted by plants into energy matter through a process rainforests perform at a rate that is unrivalled by any other ecosystem: photosynthesis.

Underneath the vibrant canopy, light is scarce and because of that, growth is limited. In some places however, light does come through, such as in the forest gaps, which can be created by falling trees.

Green energy machines

Rainforests are the most productive ecosystems on Earth, using the energy they generate for self-maintenance, reproduction and new growth.

Coupled with a climate that is constant and warm, and with almost continued availability of water and light, there are few obstacles to sustaining this productivity throughout the year.

The exception to this is the occasionally severe climatic effects and the destructive interventions of people.

The rain routine

Temperature plays a significant role in the daily routine of the rainforest. As the temperature rises, plants lose water to the atmosphere through evaporation, forming clouds, and eventually rainfall, which is then soaked up by the plants again.

Rainforests are clearly dynamic, but sensitive ecosystems, subject to disturbing factors that may not be that obvious. For example, despite rainforests’ ‘wet’ appearance and intense humidity (around 100%), there is evidence in their soils that in the past one thousand years, they have been affected by fires, occurring during periods of prolonged dryness.4

Conversion of rainforests to pastureland

More recent and worrying trends have been the conversion of rainforests to pastureland. This type of impact affects the rainforest in different ways, depending on the size of the deforested land used for pastures and the amount of time the rainforest takes to recover.

Studies show that heavily grazed sites remain as grassland, with few trees able to grow again. The areas that do recover somewhat, never quite match the original rainforest’s biological richness and appearance.

1Pires and Prance, 1985, in Kricher, 1997
2Salati and Vose, 1984 in Kricher, 1997
3Myers, 1980 in Kricher, 1997
4Uhl et al, 1990 in Kricher, 1997