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The Tropical Rainforest

Dense shade in the Tropical Rain Forest in Sarawak. Image credit: Manop Kaewfoo The tropical rainforest is a forest type that captures huge attention from the public. It is undoubtedly one of the most fascinating interactions between climate, vegetation and organisms, and soil that exists in the world today. The rainforests which previously occupied up to 24 million square kilometres today occupy only some 10 million square kilometres on either side of the equator. The main areas of this immensely important rainforest are: the Amazon basin, Central Africa, mainly involving Zaire, and Southeast Asia.

The climatic conditions in which these forests have developed are characterised by average annual temperatures of about 27 C, average rainfall of about 200cm and permanently high humidity. These are the ideal conditions for lush plant growth.

Dense Tropical Rain Forest undergrowth in Pasoh, Malaysia. Image credit: Ian Baillie Such climatic conditions can support enormous amounts of vegetation, but in the tropics it is not only the amount of vegetation that is amazing but also the variety both above ground in terms of species, and also below ground in terms of organisms and their activities. A rainforest is like a biological powerstation running on and recycling the products of nature.

There are some commonly misunderstood points about tropical soils in books and museums. Tropical soils may often be described as being thin. This is certainly not so - did you know that tropical soils are among the deepest in the world! Many tropical soils have been under forest cover for millions of years. Over this period, and under high rainfall conditions, deep tropical soils have been formed from the underlying rock.

Brazilian Tropical Rain Forest at Jari. Image credit: Ian Baillie Tropical soils are often several metres deep, but the soils are often washed out, or strongly leached, with large amounts of nutrients and minerals being removed from the subsoils and considerable thickness of rock broken down to produce soil. Over many millions of years this leaching has left most of the soils lacking many of the fundamental nutrients needed by the above ground vegetation.

So how does such a lush vegetation exist if the soils are so depleted of nutrients. The answer lies in the very thin topsoils, made up mainly of decaying vegetal and animal remains. An amazing cycle exists between the huge body of vegetation above ground and this thin topsoil. The rainforest depends for its nutrients on the constant recycling of its enormous biomass.

The high canopy of the tropical train forest in Thailand. Image credit: Manop Kaewfoo Plant remains fall to the ground, are consumed and broken down by the huge range of soil organisms in and on the soil, converted by these back into nutrients which can then be used by the dense vegetation above. It is a constant cycle. The thin layer of topsoil is the engine house for the food supply for the tropical forest and, together with climate, is responsible for the maintenance of the huge biomass. This is surely one of the most incredible cycles in nature - and it works.

The tropical rainforest is often in the news for various reasons. There are major concerns about deforestation of it and the consequent damage to the soil. From a soil point of view, cutting down of the rainforest disturbs the natural soil-plant cycle and makes the soils extremely vulnerable to soil erosion and loss of this vital topsoil.

The topsoil also holds huge amounts of carbon which is now known to have a major potential influence on CO2 levels in the atmosphere and hence a major potential influence on climate change.