QUICKTIME Movie--Tropical Deforestation since 1975 in Rondonia, Brazil--amazing rate of destruction. From NASA.
Tropical rainforests cover 13% of the Earthís land surface, account for 50% of the worldís forests and the vegetation contains 40% of the worldís carbon(6). The gene pool is incredibly rich in the tropical rainforests; one-third of the worldís birds, amphibians, and mammals reside in them, many being endemic. The plant varites are more overwhelming than the animal, with 92% of the ferns, 75% of the mosses and 70% of the plants with cancer fighting agents residing in the rainforest. In a single acre, 80 trees species can be found(8). It is apparent, with these figures, that the total destruction of these forests will make an impact on the global system, but the extent of this impact is still unknown. What is known is the rate of destruction of these tropical forests. Every minute, global tropical rainforests are being destroyed at the rate of 150 acres per minute. In other words, every 16 minutes a plot the size of New York Cityís Central Park is destroyed(3).
Methods of Destruction
There are many methods to the destruction of these rainforests, and with them the respective time for secondary growth of the rainforest varies. There are six main methods. The first and oldest is the slash and burn agriculture. In this method the vegetation is cut and later it is burned to release the minerals into the soil. This is a common practice for farmers hoping to enrich the soil. Rain washes some of the soilís nutrients away each time and after awhile the soil can no longer support crops. The period for this to happen is between 3 to 20 years. After the land is abandoned, the rainforest will take approximately 50 years to grow back because the soil quality is so poor(1).
Another method that takes 50 years or less for the vegetation to grow back is selective logging. Only particular trees are chosen for logging and the ones around them are left standing. Although there is research indicating that when 3% of the trees were harvested, 49% were damaged, but the regrowth is nevertheless relatively quick because of the patches of trees to disperse seeds and protect the seedling from too much sun(1). Perennial shade agriculture is a method that also leaves large patches of rainforest intact. The reason is that farmers need the shade from the large canopy trees to grow plant crops like coffee and chocolate. Because so much rainforest is left intact, the regrowth of the forest is 20 years, the fastest from all of the destruction processes(1).
The last three processes of forest destruction take the longest for the forest to grow back. Clear-cut logging is when all of the trees are cut regardless of the species, leaving the ground bare. No tree trunks, vegetation or nutrients are left on the land and so scientists do not know how long it takes for the forest to grow back except that it is longer than 50 years as of now. Another very destructive process is intensive agriculture, such as banana plantations. Banana plantations take a lot of work to keep them going. Irrigation systems with underground pipes and ditches are built which quickly changes the water balance of the land. Also many types of agriculture, especially banana plantations, use pesticides. The pesticides wash into the soil and kill more than just the pests which, in turn weaken the health of the ecosystem(1).
The sixth destructive activity is cattle pastures, which takes more than 50 years for secondary growth to occur(1). Cattle grazing accounts for 72% of global deforestation. One reason for this huge amount is that the grass growing in the cleared forest is only good for an average of two years and then a new plot is destroyed to provide a new grazing area. The Costa Rican rainforests have shown the repercussions of cattle grazing when it estimated in 1983 that 83% of its rainforests had been destroyed, mainly for meat production(3).
There are two major forces that are responsible for extensive cattle grazing: the United States, indirectly, and directly, the tropic countriesí governments. A majority of the cattle being grazed on the newly deforested area provides the meat for many of the major American fast food restaurants. It is estimated that the making of a McDonaldís Quarter-Pounder destroys 55 square feet of rainforest. This plot of land contains on average 1 giant tree, 50 smaller trees, 20-30 different tree species, over 100 species of insects, as well as the birds, mammals, and reptiles(3). With statistics like this, one would think that the governments would be trying to conserve the rainforests, rather than implementing deforestation.
The Brazilian government was well known for supporting the implementation of cattle ranches. The reason for this support was that the government was trying to boost the economy and pay off its tremendous debts. Beginning as early as 1966, owners of large cattle ranches were given large long-term loans, tax credits to cover investments, tax write-offs, tax holidays, and 75% subsidies, while the average man was given small short-term loans(7). Unfortunately for the government, not only did they destroy their tropical rainforests, their cattle ranching efforts failed. The overall meat output averaged only 9% of what was projected and most of the ranches were used as tax shelters rather than boosting the economy. Needless to say, the government is no longer supporting new cattle ranches, but continues to subsidize existing ranches which now costs the governmentís treasury more than $2.5 billion in lost income(7).
Cattle ranches not only cause a deficit in the government's treasury, but they also cause a deficit of incoming short-wave radiation in the natural system by increasing albedo. Albedo is the reflection of the incoming short-wave radiation. The higher the albedo values the lower the amount of energy put into the system. Therefore, if there were a higher albedo, then the energy balance of the system would change because of the lower energy input(5).
Direct measurements of albedo are hard to come by, but there are estimations through relative humidity, temperature and precipitation. For places without snow, the albedo is found through relative humidity. The correlation is: the higher the relative humidity, the lower the albedo. Thus, tropical rainforests have the lowest albedo for vegetation, 14%. Thus, the deforested areas with pastures and crops have a higher albedo than the original tropical rainforest. In areas with snow, the albedo is dependent on the temperature, which is the determining factor if there are trees present. When there are no trees present, the albedo is 70% between 60N and 60S latitude, and 80% between the 60 degrees latitudes and their respective poles(5).
One study that utilized a GISS General Circulation Model measured the difference in albedo of an area of tropical forest and then simulated the albedo for the area after deforestation and a replacement of grassland used for cattle grazing. The result was that the surface albedo was increased significantly(4). Other simulations of tropical rainforests replaced by pastures have been conducted in Amazonia and they too, have the outcome of an increased albedo(9).
Other differences caused by the tropical rainforests being replaced by pastures are that the pastures have lower surface roughness length, higher stomatal resistance, a shallower and sparser root system, and a lower available storage capacity for soil moisture than tropical forests. The land surface and overlying air is warmer because of the reduced evapotranspiration and precipitation. All of these factors have led to a longer dry season, which over time will change the climate completely and make it impossible for forests to reestablish themselves(9). The drier, deforested system will support dry forests and savannas(2).
The loss of vegetation from deforestation creates major changes in the hydrological system. The vegetative cover in the rainforests takes the impact of the raindrops and allows the water to reach the ground at a slower velocity, disrupting the soil less. The vegetationís roots anchor the topsoil and increase the percolation of rainfall. The forest cover is most important in the tropical rainforests because of the high rates of rainfall that are characteristic of tropical storms. With vegetation, the tropical rainforests have a low runoff rate of 25%, but after clearing, an average of 75% of the precipitation results in runoff. This runoff dramatically decreases infiltration that causes a decrease in the available soil-moisture and aquifer recharge(2).
Another factor leading to the decreased moisture in the tropical region after deforestation is increased evaporation coupled with the loss of transpiration. Two climatological methods have shown that 50% of the total precipitation, an unusually high amount, is recycled through the process of evapotranspiration in a short amount of time(2). Thus, without the vegetation, the region becomes much drier, supporting the theory that a climatic result of a longer dry season that will make the region uninhabitable for rainforests to reestablish themselves(9).
Green House Gases
Looking at the global scale of change instead of the previous regional scale, the major repercussion of deforestation is the release of green house gasses. Carbon is the main gas released because some much of the worldís carbon is held and cycled in the tropical rainforest system. It is known that an undisturbed tropical rainforest is balanced, and part of this balance is cycling 3% of the atmospheric carbon, where there is no net gain of carbon or oxygen. The vegetation of a tropical rainforest contains 20-100 times more carbon than the agricultural lands that replace them. Tropical forests can hold as much as 600 metric tons of carbon per hectare when cropland can only hold about 5 tons. Therefore, when the deforestation occurs, the excess carbon is released into the atmosphere(6).
People have been blaming the rise in carbon in the atmosphere on global warming, which means that the destruction of rainforests is a large contributor to global warming. In 1990, tropical deforestation was contributing 35% of the worldwide emissions of carbon(6). In addition to carbon, deforestation also contributes to the more radiative green house gases methane and nitrous oxide(6).
Deforestation produces 38-42% of the global amount of methane, although only a small amount is released into the atmosphere compared to the amount of carbon dioxide during deforestation. The ratio of methane to carbon dioxide emitted in fires associated with deforestation is around 1%. Methane is generally two orders lower than carbon dioxide. The reason for its high production from deforestation is that it is indirectly produced by the cattle ranches and rice cultivation that replaces the tropical forests(6).
Between 25 and 30% of the global nitrous oxide is also produced by deforestation. Only small quantities are produced during the deforestation fires; most of the release occurs in the months that follow the fire. The fires change the chemical form of nitrogen in the soils that consequently favors nitrification, which has the byproduct of nitrous oxide. Later, the pastures that replace the tropical forest will also be emitting nitrous oxide, especially fertilized soil(6).
The rainforests are a large, diverse, important part of our planetís ecosystem and are being destroyed at the rate of 150 acres per minute. The destruction of the rainforests will have global repercussions, but we just donít know to what effect. We do know that there are massive amounts of green house gases released from the vegetation and soil due to deforestation. We predict that the earthís temperature and albedo will increase as well as the evaporation of moisture in the system decreases due to vegetation loss and decreased transpiration and precipitation. There are two ways to look at this situation. First, we can sit here and let deforestation occur and deal with repercussions as they occur. The second alternative is to learn as much about the rainforest as we can. Armed with the knowledge, we can help stop the destruction of the rainforest before we irreversibly alter the global climate system and the quality of life for all who live on this planet.
Tropical Rainforest Deforestation Sources
1. BSRSI Rainforest Report Card.(1999) http://www.bsrsi.msu.edu/rfrc/status.html
2. Cook, Allison (1990). Global Effects of Tropical Deforestation. Environmental Conservation, Vol. 17, No. 3, Autumn. The Foundation for Environmental Conservation. Switzerland.
3. Fast Food and Rainforests (1999). http://www.jlhs.nhusd.k12.ca.us/Classes/Social_Science/econ/cost_of_opport.../fast_food_%26_rainforest.
4. Henderson-Sellers, A. and V. Gornitz. 1984 Possible Climatic Impacts of Land Cover Transformations, with Particular Emphasis on Tropical Deforestation. Climatic Change 6:231-255.
5. Henning, Dieter (1989) Atlas of the Surface Heat Balance of the Continents. Gebruder Borntraeger, Berlin, Germany.
6. Houghton, Richard A. (1990) The Global Effects of Tropical Deforestation. Environmental Science and Technology, Vol. 24, No. 4, 414-21.
7. Kricher, John (1999). A Neotropical Companion. Princeton University Press. Princeton, New Jersey
8. National Resources Defense Council. (1998). Forests at NRDC: Tropical Rainforest Facts. http://www.nrdc.org/triv/rainf.html
9. Skukla, J (1990). Amazon Deforestation and Climate Change. Science Vol. 247 Jan-Mar.
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