Australia: The Land Where Time Began

A biography of the Australian continent 

The effects of large scale clearing, as is occurring in Australia, and in particular the clearing of tropical rainforests, can be severe, even catastrophic. Clearing of vegetation on a large scale changes the clime, locally and globally. In areas of very dense vegetation, such as rainforests, the latent heat of vaporisation has a local cooling effect. Complete clearing of the forests of Amazonia would be expected to cause a temperature rise of between 2^o C and 5^o C. Changing vegetation cover from rainforest to grass, as is occurring in large areas of the tropics, where forests are being cleared for farming and grazing, lowers evapotranspiration by about 20 %, which leads to a large decrease in cloud cover. As a result, dry seasons are longer and there is the associated effects of fire and erosion. Wind speed increases as the surface roughness is decreased by going from forest to grass, with its accompanying effects of increased drying and removal of the dried soil as dust. Soil surface temperatures increase, which leads to more drying, and when conditions are so dry grass can't grow, as near the end of the dry season or in drought, updraughts rise from the heated soil decreasing the rainfall locally. When rain does fall the unprotected soil is exposed to erosion.

In Western Australia, rainfall patterns have changed  in better-watered agricultural regions that grade into arid zones. It has been claimed that there has been no 'decent' rains since 1974. This can be attributed to massive clearing that has locally altered rainfall patterns. (White,2003).

A process that is occurring in Australia, as well are much of the rest of the world, is the changing of arid areas to desert, as a result of the climate being changed by vegetation change. Some of the main effects of changing plant cover are the reduction of water infiltration, increased soil temperature and increased erosion. Rainfall is lower on degraded soil than on adjacent areas with good vegetation cover. Australia already has a problem with drought, the clearing of large areas of trees adds to the risk of more droughts, that are longer and more severe.

The wet desert of India is an example of what can go wrong when forest is cleared, in this case the rainfall wasn't generated locally. It is on the southern edge of a limestone plateau above Bangladesh, in the Indian state of Assam, the Cherrapunji region of Megahalaya, which is on the northern border with Bangladesh, about 4000 feet above sea level. It is believed to be the wettest place on Earth, with an average annual rainfall of 12.7 m (500 inches). The region of the East Khasi Hills was originally covered with an oak forest. This forest was eventually completely cleared for expanding agriculture. The rate of erosion that followed the clearing has been estimated to be about 300 tonnes per hectare per year. The result is that the soil is too thin to grow grass. The deepest the soil reaches in other parts if 30 cm. The water that used to percolate though deep soils to the limestone bedrock now goes straight to the limestone and is channelled over the edge of the plateau in waterfalls into Bangladesh. On the plateau, the rivers only run when it rains on them, as the water that used to move through the soil slowly goes straight to the limestone and immediately runs off the edge of the plateau causing damaging floods. Now that it has been realised that the only solution is to reforest the area there is no soil to plant the trees.

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003