Thirdly, drying is timed to precede the first rain. Burning is important to produce the ash. It is the source of fertility for subsequent planting for a number of years.
Then the farmers may plant crops. Seeds, which derived from collection or previous harvests, are spread onto the land. Apart from weeding which is done twice during the growing season, there is nothing else the farmers could do. The crops, e.g. taro, sweet potatoes, cassava are grown naturally under the hot wet climate. Harvesting takes place as the need arises, so the entire crop is not removed at one time.
After that, the farmers abandon the site. After 2 or 3 years, the soil fertility is virtually depleted since the land is exposed to leaching and the nutrients are exhausted. At the same time, coarse grasses and other vegetation are beginning to take over. Instead of attempting to cope with these conditions, the natives field varies abandon the clearing and make a new one.
Therefore, shifting farmers make the best use of natural resources for their survivor.
The soil results from the high annual temperature and rainfall, which cause rapid chemical weathering of bedrock and create the optimum conditions for breaking down the luxuriant vegetation. Continuous leaves fallen within the forest give a thick litter layer but the underlying humus is thin due to the rapid decomposition of organic matter by intensive biota activity. There is a rapid recycling of nutrients but the release of bases prevents the soil from becoming too acids. The soils have a relatively low nutrients status, which is maintained only by rapid and continuous replacement from the lush vegetation. Heavy rainfall causes severe leaching, humus were removed and then silica, iron and aluminum left. It is unsuitable for the farmers to cultivate crops. Once the source of nutrients has been removed, the soil rapidly loses its fertility and local farmers, often slotting cultivators, have to move to clear new plots.
B. Impact of human activities to the natural environment
The impact of human's activities to the natural environment is very large, because it may influence in the atmosphere, the hydrosphere, the lithosphere and the biosphere.
Because of the population explosion, it intensifies all types of human activities in the tropical rainforest (TRF). Extended and large areas of TRF are used. It increases the scale and rate of deforestation in order to create the new land for the human beings. Therefore, the negative impact on the TRF ecosystem is found.
(i) Human impact on atmosphere.
Because of the burn and slash farming, human disturbs carbon and oxygen balance in the atmosphere. Originally, plants may release oxygen and absorb carbon dioxide. However, after the process of deforestation, it reduces the production of oxygen and marines oxygen depletion by burning. It decreases the carbon dioxide intake through photosynthesis. Moreover, it increases the production of carbon dioxide through burning. Because of the increasing the concentration of carbon dioxide (CO2) in the atmosphere, it cannot go out to the space, so that more heat energy is retained in the atmosphere. Finally, it may lead to increase the air temperature. The planet becomes warmer and warmer. Furthermore, the global warming leads to the melting of polar ice and rising the sea level is resulted. Finally, the submergence of low-lying areas occurs. On the other hand, the increasing of temperature and relative humidity of westerly from 30° N/S to 60° N/S intensify the temperate cyclones. It increases the frontal precipitation. By the research, the Elnino is intensified by the increasing CO2 content.
From the view of microclimate, because of the deforestation, the trees were removed. So, the decrease of shading leads to increase the range of temperature. And it decreases the transpiration on the surface because there are few trees. And it also decreases the evaporative cooling. The wind velocity is strong, because the ground becomes smooth. It increases the surface albedo. Most insolation is reflected back to the space. It reduces the possibility of convective activity. Finally, it reduces rainfall. These of all make the environment forwards a worse situation.
(ii) Human impact on hydrosphere.
Deforestation leads to the decreasing of interception and infiltration, because there is no frees to trap rainfall. It is easily to increase the amount of surface runoff and increase the storm runoff in rivers. The erosive power is enhanced by the running water. Because there are few trees, lesser roots of vegetation bind the soil particles. This makes to the increase of soil erosion. Moreover, sediment yields in rivers increase. The river is silted up. Finally, the river is risk to be flooded. This is the result of the raise of riverbed and reducing carrying capacity of the channel.
(iii) Human impact on lithosphere
Deforestation accelerates soil erosion. Deforestation increases the amount and velocity of runoff as a result of less infiltration and interception. Because of the lack of vegetation cover, the land is exposed to the direct impact of the intense rain droplets. Moreover, the soil particles are easily carried out by the running water. Finally, rill and gully erosion is happened. It is also exposed to direct sunlight. The soil dries up, and then also soil particles are blown by the wind.
(iv) Human impact on biosphere
Besides it increases the rate of the process of laterization, which causes poor drainage. Moreover, it reduces infiltration and hence soil moisture content also reduces. Lacking of vegetation bind up the soil. Burning trees lead to the death of woody species, which cannot tolerate. Adverse change in composition and structure of vegetation for example, the height, species, and density decrease. The structure of vegetation becomes from complicated to simple. The valuable species are lost through selective cutting (lumbering). Natural vegetation with rich species diversity and high stability is destroyed by the ago-ecosystem with poor species diversity and lower stability. Climatic climax community may be totally destroyed and turned into bare earth. The loss of vegetation leads to loss of animal life or extinction of some animal species.
Loss of soil fertility results from deforestation. Deforestation reduces the litter supply. Moreover, there is lack of vegetation to bind up the soil. The litter is removed by runoff. Fewer nutrients enter the soil. Nutrients in soil loss quickly because serious erosion is encouraged. On the other hand, nutrient no longer is uptake by vegetation immediately; hence it intensifies the process of leaching. The sesquioxides is deposited in the subsoil. The soil becomes hard.
In conclusion, the environment limits the man to use the tropical rainforest. At the same time, human activities disturb the natural environment.
