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Deforestation of tropical and sub-tropical forests causes enormous envinonmental problems downstream. Image credit: Ian Baillie Soils support forests in many parts of the world. The major areas of forest are in the tropics and sub-tropics, the best known examples of which are the Amazon Basin and the Central African forests, and in the taiga region of northern Europe and Asia. There are also forests in temperate areas of the world but many of these were felled centuries ago to provide wood for homes and for ship-building. Currently, there is concern that many of the tropical forests are also being felled. Here, the fragile soil-forest relationship is being broken, and the soils themselves are being damaged irreparably. In many European countries and also China where there was a long history of deforestation, the tide seems to have changed. Significant areas are being reforested, albeit mainly with conifers, and there is much more integration of forestry and farming than there has been previously.

Soils build up a unique relationship with the forest trees that they support. In most cases where the trees are the natural vegetation forests have been in place for thousands of years. During this time a cycle has developed in which the soil supplies nutrients to the growing vegetation. When the vegetation dies leaves and other vegetable matter are returned to the soil surface. In the soil they are broken down by the numerous organisms and the nutrients become available again for transport back to the plants. This cycle continues as long as the forest is intact and enables the forests to develop and be maintained year after year. It is one of the wonders of nature. The forest protects the soil from the direct impact of tropical rainstorms and the litter layer absorbs much of the water, from which it can gradually infiltrate into to the soil to be used by plant roots

The organic matter layer in tropical soils is thin and fragile. The soils themselves, though deep, are not very fertile because leaching over many centuries has left them lacking nutrients and with few reserves of nutrients to tap in the deeper parts of the soil. Many of the soils have become quite acid. As one moves away from the tropics to more temperate regions and to the colder regions the organic layers become thicker and more robust. The soils are often much younger in temperate and colder regions because many are formed in deposits left by the most recent ice age in the recent. They are less leached and contain more nutrients and are a less fragile entity than tropical soils. Currently there are major concerns about the amount of deforestation that is taking place, particularly in the tropics and sub-tropics and the damage that is being caused to the soils, to the ecological balance and to the economies of the areas.