Secondary causes of tropical deforestation relate to the activities that have immediate negative effects on forests. The major secondary causes are the following:
• logging and logging roads
• cattle ranching
• cash crops, small-scale cultivation, and fuel wood
• mining and oil drilling
• large dams
• new roadways
These causes can be grouped in various ways. For instance, logging roads create much the same problem as public highways by removing trees, causing erosion, and fragmenting habitat, while cattle ranching resembles mining because it requires large tracts of cleared forest.
The upheaval in the Amazon Basin provides an example of how human activities kill a forest over time. In the first phase, logging operations remove the best timber from a region, after which timber companies sell the land to cattle ranchers for their animals to graze, beginning the second phase of the land’s use. Ranchers may leave a few trees standing for shade, but after the land has been overgrazed, the ranches move to other places and families buy the land at discounted prices. These families cultivate small gardens and perhaps cut down more trees for cash crops or fuel and hunt the native animals. Eventually, the small farms deplete the nutrients from the soil so that it supports little new plant growth. The farmers move on to cleared land they can cultivate or they remove more forest. Meanwhile, other parts of the forest disappear as mining operations and oil drilling companies burn the already damaged patches of forest because burning is easier and quicker than cutting and hauling out the logs. The succession of human activities in the Amazon Basin described above is unsustainable. After a few decades, maybe less, the forested land turns into a bleak landscape that cannot support substantial human, animal, or plant life. The results of such actions are detailed in the sidebar “Chico Mendes—Activist for the Brazilian Forest.”
Ranchers and large farms have learned to reduce soil degradation by clearing the forest in a method called slash and burn. Slash and burn is a process of cutting down large tracts of forest, letting the downed trees dry, then burning them in place to release nutrients into the soil. Soils in tropical forests tend to be nutrient-poor due to the dense vegetation they support. Slash-and-burn methods fortify the soil for grazing or agriculture, but eventually the added nutrients also diminish and the ranches and farms move to another part of the forest to begin the process again. This constant using up of land and moving on to healthier sites is called shifting cultivation. Abandoned land that has been treated this way can again support a healthy mixture of growth through ecological succession in the succeeding decades. By the time the vegetation has returned, however, the shifting cultivation may also return as it progresses through a region.
Source of Information : Green Technology Conservation Protecting Our Plant Resources