Project Amazonia: Threats - Deforestation

Deforestation is a problem that affects the entire Amazon Rainforest ecosystem as a whole. Models of the future impact of deforestation based on current trends¹ predict dire consequences for the Amazon. To examine the effects of deforestation more closely, one 1990-1993 study replaced tropical forest and savannah with pasture in South America, north of 30S². The most prominent affects on the water ecosystem are as follows:

Deforestation causes increases in erosion and flooding.  The land of the Amazon Rainforest is naturally nutrient-deficient because most of the nutrients are stored within the aboveground biomass of the vegetation. Tree root systems hold the soil together to slow the rate of flooding and reduce erosion.  Trees themselves also absorb water during the rainy season.  When the trees are removed from the environment, the rainy season can have devastating effects.  Rains wash away the vital topsoil and what nutrients are left.  Increased deforestation therefore leads to decreased biodiversity and species richness.

Research has shown that deforestation of the Amazon basin will cause a decrease in evapotranspiration of 0.7 mm/day².  It will also result in a decrease of total runoff by 0.7 mm/day².  Surface runoff, however, will increase substantially, primarily as a result of decreased soil infiltration capacity and changes in the spatial distribution and intensity of rainfall³.  Temperature will increase 1-4°C because of a decrease in the energy required to evaporate water at the canopy and soil surface and a decrease in roughness².

A decrease in precipitation of 25% or 1.4 mm/day² will result from deforestation of the Amazon. In addition, rainfall decreased in almost every month from 1990-1993. However, reductions in rainfall do not occur uniformly across the Amazon region. At some locations, rainfall may decrease by up to 65%, whereas other locations (typically the mountainous regions of Peru and Ecuador) will experience increases in rainfall.  Furthermore, changes in precipitation are not confined to the Amazon River basin itself. For example, during the southern summer and autumn, there are large fluctuations in precipitation in eastern Brazil which seem to correlate with precipitation changes over deforestation areas³.  

These changes in the hydrologic cycle will be caused by: 

1)       Decreased surface roughness

2)       Increased surface albedo

3)       Changing soil properties

4)       Decreased rooting depths, and 

5)       Decreased infiltration rates².

Observations that the reduction in precipitation is larger than the reduction in evapotranspiration suggest that the length of the dry season will increase, leading to self-perpetuating deforestation¹.

Table 1: Model fields averaged over the simulation and over the Amazon Forest²

The method of slash and burn deforestation has a strong impact on the carbon cycle. The forests of the Amazon region act as a very large natural carbon sink.  Plants and soil hold about 460-575 billion metric tons of carbon. Each acre of tropical rainforest releases about 180 metric tons of carbon.  This carbon joins with oxygen and goes into the atmosphere as CO₂.  The equivalent to a football field of rain forest is destroyed every minute that passes.

The causes of deforestation are very complex. A competitive economy forces the need for money in poorer third world countries. Brazil sells logging concessions to raise money for projects, to pay international debt, or to develop industry. Brazil had has an international debt of $251 billion as of 2001⁴. 

Deforestation also is fueled by expansions in the amount of acreage used for agriculture and ranching. When the land is deforested by slash-and-burn, for example, the nutrients are mixed back into the soils. However, as the tree root structures die, the stability of the soil decreases and it becomes more susceptible to erosion. The land’s fertility decreases as nutrients and topsoil are washed away.

Increases in deforestation can result in increased flooding and therefore expansion of wetlands or floodplains. Water in wetlands then cuts off the oxygen supply to the soil. This results in anaerobic fermentation, which forms large amounts of methane, a greenhouse gas⁵ from the flooded forests.

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1: Henderson-Sellers, A.; Dickinson, R. E., Durbudge, T. B., Kennedy, P. J., McGuffie, K., and Pitman, A. J., 1993, Tropical Deforestation: Modeling local- to regional-scale climate change, Journal of Geophysical Research Vol 98 No. D4 P7289-7315.

    Shukla, et al. 1990.

2: Dickinson, Robert E. and Kennedy, Patrick, 1992, Impacts on regional climate of Amazon deforestation, Geophysical Research Letters 19 (19) P1947-1950. 

3: Lean, J. and Rowntree, P. R., 1993, A GCM simulation of the impact of Amazonian deforestation on climate using an improved canopy representation, Quarterly Journal of the Royal Meteorological Society 119 pp 509-530.

4: CIA Fact Book

5: Hauser, et al. 2002.