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III. TRF in the humid tropics and its conversion


A. Forest conversion
B. Effects of forest conversion


Despite several reports on the rates of tropical deforestation (Myers, 1989; NRC, 1993a), there are no reliable estimates of the extent of TRF. Available estimates (Table 6) vary widely, often by a factor of 2 to 4. Large differences in the statistics on TRF are due to many factors:

- There are differences in the criteria used to define TRF (e.g., closed forest, open forest, wooded land, primary forest, secondary forest).

Table 6 Estimates of the area of TRF

(106 ha)

Region

Lanly (1982)

Postel (1984)

WRI (1992 93)

FAO (1990)

Tropical America 1679.5 1212 839.9 753
Tropical Africa 2189.4 1312 600.1 242
Tropical Asia 944.9 445 274.9 288
Pacific area   190    
World total 4813.8 3159 1714.9 1283

It is important to standardize the terminology and clearly define what constitutes a TRF and what does not.

- The methodology used to assess the area is not reliable. Most often, the results of aerial surveys are not checked with the ground truth.

- Most data are 10 to 20 years old, and statistics are often obsolete. Because of the high rates of deforestation and obsolete statistics, it is difficult to assess the exact area under TRF at any given time.

The extent of the original area under TRF is estimated at 1.5 billion ha, of which about 600 million ha have been converted to other land uses (Ehrlich and Wilson, 1991; NRC, 1993a). World Bank (1991) estimated the area under TRF at I billion ha, with an additional 0.5 billion ha of deciduous forest.

A. Forest conversion

Similar to the area under forest cover, the actual rates of TRF conversion are also difficult to obtain. In addition, estimates of the rate of TRF conversion vary widely, depending on what is considered forest and what is conversion. FAO (1992) estimated that the rate of deforestation is 17 million ha/yr. The rate of deforestation increased from 0.6% in the 1980s to 0.8% in the 1990s. The deforestation rate is high est in Asia (1.2%/yr), followed by Latin America (0.9%,/yr) and Africa (0.8%,/yr) (Table 7). The regional rate of deforestation is in the order of West Africa (2.1%/yr)>Central America and Mexico (1.8%/yr)>south east Asia (1.6%/yr)>Africa (1.2%,/yr) (WRI, 1992 93).

Table 7 Estimates of the rate of deforestation of TRF

(%)

Region O'Keefe and Kristofferson (1984) WRI (1992-93) FAO (1990)
Africa - 0 52 -0.8 - 1.7
Latin America - 0.63 - 0.9 - 0.9
Asia - 0.6 - 1.2 - 1.4
Total - 0.58 - 0.9 - 1.2

B. Effects of forest conversion


1. Local effects
2. Regional and global effects


There are several major concerns about deforestation of TRF. These concerns are related to local, regional, and global effects (Fig. 6). Local effects are the most drastic and are related to changes in soil properties, vegetation, and micro-climate. Regional effects are related to hydrological characteristics and changes in meso-climate. Global effects are due to changes in global cycles of C and N and water vapor and may be related to global warming or the greenhouse effect.

1. Local effects

The principal local effects of deforestation are related to changes in microclimate and soil properties.

(i) Micro-climate: Deforestation of TRF leads to drastic changes in microclimate (Lal and Cummings, 1979), as outlined in Fig. 6. In general, deforestation eliminates the buffering effect of vegetation cover and accentuates the extremes. Fluctuations in micro-climatic parameters are greatly enhanced (e.g., relative humidity, maximum and minimum temperatures for soil and air). Deforestation decreases rainfall effectiveness and increases aridization of the climate. Forest removal increases the magnitude and intensity of net radiation reaching the soil surface. Ghuman and Lal (1987) observed that in south central Nigeria, on average, 10.5 and ll.5 MJ/m2/day of insolation were received on a cleared site compared to 0.4 and 0.3 MJ/m2/day in the forest during the dry seasons of 1984 and 1985, respectively. There was no appreciable difference in solar radiation received under forest during the rainy (May) and dry (December) seasons (Table 8). Vegetation removal also increases wind velocity (Table 8).

Deforestation decreases the maximum relative humidity, especially during mid-day. There is also a corresponding increase in air temperature and evaporation rate. Perhaps the most drastic effect of deforestation is on soil temperature. The maximum soil temperature at I to 5 cm depth can be 5 to 20C higher on cleared land on a sunny day compared with land under TRF cover. Because of high soil evaporation, the soil moisture content of the surface layer is also lower in cleared than in forested soil (Fig. 7).

Fig. 6 Local, regional, and global effects of conversion of TRF

(ii) Soil Properties: Deforestation also has a drastic impact on the physical, chemical, and biological properties of soil. The magnitude of deforestation-induced alterations in soil properties depends on antecedent soil conditions, tree density and species of the forest and under story vegetation, and methods of lend clearing used. Experiments on land-clearing methods throughout the tropics have shown that deforestation results in degradation of soil structure, decrease in porosity of the surface layer, increase in soil compaction, and decrease in infiltration rate (Lal and Cummings, 1979; Ghuman et al., 1991; Lal, 1992). Although increase in soil bulk density may be only 10% to 20%, it is the reduction in volume fraction of the macropores that may cause a drastic reduction in infiltration rate (Fig. 8). Densification of the soil, as evidenced by increase in bulk density and decrease in infiltration rate, is greater with mechanized than with manual methods of deforestation(Lal, 1981, 1992; Hulugalle et al., 1984). Soil wetness at the time of land clearing plays an important role in clearing-caused alterations in soil physical properties. The higher the soil wetness at the time of land clearing, the more deleterious is the effect on soil physical properties (Ghuman and Lal, 1992). Soil physical properties and infiltration rate are also affected by burning (Plate 8). Intense burning. as is the case in the windrow zone, may increase soil aggregation and improve infiltration rate (Ghuman et al., 1991). Biomass removal by a mechanized system without burning may cause drastic adverse effects on soil physical properties (Lal and Cummings, 1979).

Table 8 Effects of deforestation on solar radiation and wind speed recorded at Okomu, south central Nigeria

Parameter

Under forest

Cleared

Solar radiation (MJ/m2/day)
December 1984 0.40 0.06 10.40 0.95
December 1985 0.27 0. 05 11.58 0.51
Wind velocity 1986 (m/s) 0.03 0.02 0.55 0.17
Daytime relative humidity (%)
23 May 1984 70.0 50.0
17 July 1984 70.0 65.0
Maximum air temperature (C) 31C 36C
Maximum soil temperature (C) at 1 cm depth
Sunny day 27.0 37.0
Cloudy day 24.5 28.0
Pan evaporation (mm/day) in dry season 0.53 3.93

(Ghuman and Lal 1987)

Fig. 7a Generalized effects of deforestation of TRF on micro-climate

Fig. 7b Generalized effects of deforestation of TRF on micro-climate

Fig. 7c Generalized effects of deforestation of TRF on micro-climate

Fig. 8a Generalized effects of mechanized deforestation on soil physical properties

Fig. 8b Generalized effects of mechanized deforestation on soil physical properties

Fig. 8c Generalized effects of mechanized deforestation on soil physical properties

In addition, deforestation affects the chemical properties of soil (Table 9), and the magnitude of changes in soil chemical properties following land clearing depends on the nature of vegetation, antecedent soil conditions, and methods of land clearing. Biomass burning is a major factor responsible for changes in soil chemical properties. Shifting cultivation does not drastically alter nutrient cycling (Nye and Greenland, 1960; Andriesse and Schelhaus, 1987), but clear cutting does. Burning releases plant nutrients immobilized in the biomass. However, ash contains mostly cations (i.e., Ca, Mg, K) and several elements (i.e., N. S) are volatilized. Burning also releases a considerable amount of carbon and nitrogenous compounds into the atmosphere (Crutzen and Andreae, 1990). The magnitude of changes in soil properties also depends on the quantity of biomass being burnt and soil temperature during burning. Ghuman and Lal (1989) observed that clearing and collecting biomass from a high forest in south central Nigeria involved a biomass of about 500 kg/m2, which formed a 30- to 55-mm-thick layer of ash on the soil surface after burning (Plate 9). During burning, the maximum soil temperatures rose to 218C at 1 cm depth, 150C at 5 cm depth, 104C at 10 cm depth, and 70C at 20 cm depth. Because of drastic changes in soil color, from yellow-red to black interspersed with grayish-white, the maximum soil temperatures remained higher than the unburned non-windrow zone even 24 months after burning. High soil temperatures are likely to cause severe losses of N by volatilization.

Table 9 Possible effects of land clearing and biomass burning on soil chemical and nutritional properties

Soil property Biomass burning Biomass removal
 

Windrow zone

Non-windrow zone

In situ burning

 
Soil organic carbon Increase* Decrease Increase Decrease
  20-30% (10-20%,) 5-10% (5-10%)
Soil pH Increased Decrease Increase Decrease
  1-2 unit (0.2-0.5 unit) 0.5-1 unit (0.2-0.5 unit)
Total acidity Decrease Increase Decrease Increase
  (50-60%) 5-10% (10-20%,) 10-20%
Exchangeable cations Increase Decrease Decrease Decrease
  8-10 times (10-20%) (50-100%) (10-20%)
Base saturation Increase Decrease Increase Decrease
  50-60% (5-10%) 10-20% (5-10%)
Al and Mn concentrations Decrease Increase Decrease Increase
  (10 20%) 5-10% (5-10%) 5-10%
Available P Increase Decrease Increase Decrease
  4 10 times (5 10%) 1 2 times (5-10%)
Total N Decrease Decrease Decrease Decrease
  (5 10%) (2 5%) (2 5%) (5 10%)

* The increase in soil carbon following burning may be due partly to addition of ash and unburnt biomass.

There are also changes in the biological properties of soil. Burning may change soil fauna population and species composition (Plate 10). Intensive burning especially in the windrows, usually increases soil temperature to above 100C down to a 10 cm depth. Soil temperatures in excess of 60C may persist for 30 hours or more. The burnt soil is essentially sterilized to a 10 or 20 cm depth (Raison, 1979; Ghuman and Lal, 1989). The sterilization may cause drastic, albeit short-term changes in soil biological properties (e.g. biomass carbon; activity and species composition of macro-fauna, including earthworms and termites).

(iii) Hydrological Properties and Processes: Deforestation leads to drastic changes in components of the hydrological cycle and of the energy balance, the two being intimately linked through evaporation and evapotranspiration. The hydrological balance can be expressed by the simplified equation

P = R + l +D S+ E dt

where P is precipitation, R is surface runoff, I is interflow or deep seepage, AS is change in soil water storage, E is evapotranspiration, t is time, and dt implies derivative with respect to t. Deforestation increases the amount of precipitation directly reaching the soil surface increases losses due to surface runoff and soil evaporation, decreases total evapotranspiration because of the removal of deep-rooted vegetation, decreases soil water storage in the root zone because of alterations in soil structure and increased losses due to runoff and evapotranspiration, and increases seepage and interflow components. The data in Table 10 from western Nigeria indicate that deforestation caused a drastic increase in runoff and interflow. An increase in runoff rate and amount increases soil erosion and interflow. An increase in interflow is attributed to the absence of deep-rooted perennials. Deforestation may also increase leaching losses of minerals and dissolved organic carbon.

2. Regional and global effects

The regional and global effects of deforestation are difficult to quantify. Deforestation of large watersheds increases stream discharge due to increase in runoff and interflow (Hibbert, 1967a; Pereira, 1973: Lal, 1992). Some researchers argue that large-scale deforestation may change the rainfall amount and distribution pattern over the region (Salati and Vose, 1985). On a regional scale, deforestation also influences the transport of water and nutrients out of the ecosystem.

Table 10 Deforestation effects on components of the hydrological cycle in western Nigeria

Component Pre-clearing 1975 Partial clearing 1978 Post-clearing* 1979
Rainfall (mm/yr) 1453.3 758.8 1431.9
Runoff (mm/yr) 6.1 42.7 94.6
Runoff (% of rainfall) 0.4 5.4 6.6
Interflow (mm/yr) 39.2 9.4 10.6
Interflow (% of rainfall) 2.7 1.2 0.7
Total water yield (mm/yr) 45.3 58.9 105.2
Total water yield (% of rainfall) 3.1 7.5 7.3

* Post-clearing by tree pusher root rake combination.

(Lal 1992)

A major global effect of deforestation is related to global carbon balance. Deforestation of TRF is considered to contribute substantially toward emission of about 1.6 pg-C/yr, or 30% of the total carbon emission into the atmosphere (Houghton, 1990a, b; Houghton and Skole, 1990; Post et al., 1990; Schlesinger, 1991). It is difficult to estimate how much of the total carbon released by deforestation of TRF comes from soil compared with that from the biomass. Burning may affect global carbon balance directly and indirectly. Directly, it releases carbon from the biomass during combustion. Indirectly, it accentuates carbon release from soils from which vegetation has been burnt. Soil exposed by burning undergoes drastic changes in soil properties that can possibly enhance the rate of carbon mineralization. It is estimated that loss of carbon from soil by shifting cultivation practiced over some 25 million ha annually may be as much as 6.25 tg/yr (Lal, 1993b). These effects of deforestation and related activity on global carbon balance are extremely important and need to be quantified.


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