Monday, July 09, 2007

How does deforestation result in soil infertility?

8 Jul 07, 17:37

rizwan: does cutting of trees and deforestation result in soil infertility? How?

This needs a bit of longish answer. If you are having some natural sciences background, it would be easy to understand. But the following stuff that I found on the Net is very easy to comprehend. Take a look:

Rainforest microbes are extremely efficient at breaking down and recycling waste organic matter - the leaf litter and layers of detritus on the ground. When there is deforestation, almost no nutrients reach the forest soil and it is consequently poor. Removal of the trees allows the soil to dry out and the little humus that exists to deteriorate. This causes the rainforest microbes to die and the soil becomes largely inert, biologically. The degraded soil is also prone to erosion by wind and when land floods, it can be washed away.

The root systems of the trees are confined to the topmost layers of the soil. This is not unexpected, since the deeper layers of the soil do not contain appreciable quantities of nutrients. Because of the resulting poor soil conditions, sustained farming after forest clearance is difficult and people regularly move on, looking for more fertile soil. The types of crops grown take the few remaining nutrients that are present from the ground, without putting anything back. Typically farmers move on after just three years and each time they move, they "slash and burn" virgin forest, destroying everything that grows.

If total deforestation of an area of rainforest occurs and the topmost layers of nutrient-deficient soil are removed by erosion, the virgin forest can once again be regenerated by a new primary succession. However, this is not possible if the underlying rock (such as weathered sandstone and alluvial sands) is low in nutrients. In this case only a much simpler ecosystem develops, such as a heath-population or a sparse savannah. Once destroyed, the rainforest is gone for ever.

Typically in an area like the Amazon basin, which is home to the world's largest single rainforest, only around 4% of land is suitable for agriculture of any type. 75% of the land is so poor that cultivators are unlikely to get more than a single crop from the soil, before it is completely exhausted.

Settlers tend to cut the largest trees, thinking that the soil is the most fertile there, but the rainforest natives know that the places where the trees have thin trunks often have the best soil. The soil infertility has caused the large trees to develop a highly efficient system of nutrient extraction. Tree roots can extend up to 100 meters along the ground from the tree trunk and form a root mat 30 centimeters (a foot) or more thick. This mat can capture over 99% of the nutrients that fall on it.

On the one hand rainforest soil is permanently damaged by water erosion. Deforested areas are exposed to rain and flooding, both of which washes away top-soils. Tree roots tend to hold soils together, act to provide physical reservoirs for water by making barriers and slow the rate of flow of moving water, thus reducing its eroding effects. In Nepal and India, for example, deforestation in the foothills of the Himalayas has led to catastrophic flooding of the river Ganges. Bangladesh has been in receipt of the results, with well known effects. Forests act as giant sponges, absorbing enormous quantities of water in the rainy or monsoon seasons. Normally this water is released in a controlled way over the following year, providing those downstream with a steady and sustained flow. Remove the rainforest sponge and the waters simply rush downstream and flood, often taking soils, humus, and the micro-fauna with them.

Conversely, soil can be baked by the sun. One consequence is that the important mycorrhizal fungi are destroyed by dehydration. The fungi live in a symbiotic relationship with trees, and every rainforest tree species may have its own, very specialized, fungal species associated with it. These unique fungi enable the tree to absorb more minerals from the soil than it would otherwise be able to, in exchange for energy. These fungi are similar to those associated with mushrooms that are commonly found in temperate forests. In both instances, most of the fungus is in tiny filaments that surround the tree's roots. When the mycorrhizal fungi are not present, the trees cannot grow. In deforested areas, fungi will not grow in the warmer and drier soil that results when the forest canopy is removed. The degraded soil is taken over by coarse grasses and other hardy species.

Mining activity not only destroys trees with clearings and roads for the mines, it also pollutes rivers and water tables with heavy metal toxins that are almost impossible to remove. Rainforest areas downstream of mines can be affected for hundreds of miles. These toxins - often mercury-based compounds - not only kill animals and plants, they can affect the microorganisms as well.

The microorganisms are at the bottom of the food chain. Their survival is essential for the survival of all the other species. Sad as it may be to lose something as beautiful as one of the jungle cats to extinction, through human abuse of the rainforest, the forest itself and most fauna would survive nevertheless. Loss of rainforest microbes, however, would have a severe effect throughout the entire rainforest ecology. The rainforest is dependent upon the actions of microbes, to sustain the base-level of food supply. Microbes dispose of dead matter by decaying and rotting it. They provide nutrients both from their by-products and from themselves, for other usually larger and more complex life-forms - they are food.

Many species of animal and plant could consequently suffer extinction, if microbial populations were destroyed or degraded. Although microbes are numerous and there are many species, they can be sensitive to the smallest of environmental change - changes in water acidity, levels of sunlight, toxins, etc. They can be quite fragile. However, they and their diversity are key to the survival of the forest.

The microbial world of the rainforest, though lacking the appeal of the more exotic larger creatures, should be regarded as just as important as any other rainforest population. Therefore efforts should be directed towards understanding and preserving their habitat and the way they interact with the forest environment.