Deforestation occurs when forests are destroyed, but are not replaced. There is an immediate effect on the flora and fauna, on the soil of forested lands, and a loss of some valuable resources such as sources of medicines.The wider effects include global warming, through the release of carbon dioxide, and flooding.
One definition of deforestation is the loss of forests when trees are extensively cut and are not replaced through the planting of new trees.
The primary cause is land-clearing for agriculture and logging. Rainforest logging causes problems because rain-forest soils are poor and only sustain fertility by a cycle of leaf and tree's fall and renewal. Eliminating the trees breaks the cycle and leaves the soil too poor for growth.
The result is a quick economic gain from having logs to sell. However, there is long term loss, as once the trees are felled, the land is of little economic use.
Another problem is habitat loss and the consequent loss of biodiversity. Whole species can be eliminated. This matters, as the ecosystem is a complex web whose stability is sustained by biodiversity. The loss of any species is therefore, bad. Some plant species might be the source of useful antibiotics and other beneficial medicinal chemicals. So, losing these plants deprives humans of the ability to discover these chemicals. For example, chemicals from the now-extinct Madagascan periwinkle plant improved the survival rates of children suffering from leukaemia, from 20% to 80%.
Forests are the lungs of the planet. Trees draw carbon dioxide from the air. Much carbon dioxide is stored in the wood of a tree. If the tree is cut down, it cannot store this carbon dioxide. If the wood is burned, this gas is released.
Carbon dioxide is a greenhouse gas that stores heat. As it rises in the atmosphere, more heat is trapped and the planet warms up. This can cause icecap melting and a rise in the sea level. This is not an argument against cutting trees, it is an argument for sustainable forestry whereby what is felled, is replaced.
The effect of deforestation includes flooding. For example, floods in Bangladesh have been worsened because deforestation has occurred in the Himalayas. Tree roots sustain water and release it gradually. If the tree roots are lost, rainwater runs off mountain slopes all at once. Massive amounts of water then, pour off the Himalayas too quickly for the river system and the flood defences to cope with.
Had there not been the deforestation of the mangrove swamps on the shores of Thailand, the tsunami's intensity could have been checked. Human lives could also have been saved.