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Deforestation and the emission of radiatively active gases

Deforestation strongly affects the dynamics of soil organic matter. Experiments conducted in Africa (Greenland and Nye 1959; Nye and Greenland 1960; Lal 1976; Juo and Lal 1977; Aina 1979; Lal et al. 1980; Ghuman and Lal 1991,1992) show rapid decline in soil organic matter content following deforestation and cultivation. The magnitude of carbon decline in the top 5 cm depth can be as much as 50 per cent in 12 months and 60 per cent in 18 months. The organic carbon (C) content of the top 30 cm depth declines by about 50 per cent within 10 years of deforestation and intensive cultivation. Examples of the carbon loss from soils of the humid tropics within 10 years of deforestation and intensive cultivation are shown in tables 9.10-9.12. The rate of C loss may be as much as 1.13 mg/ha/yr from soil managed by conservation tillage and agro-forestry to 5.60 mg/ha/yr for soils managed with a plough-based conventional tillage system. That being the case, newly cleared land in the humid tropics may release between 98.7 billion kg C/yr and to 218.8 billion kg C/yr, with a mean emission rate of about 154.3 billion kg C/yr.

Table 9.10 Loss of organic curbon with continuous and intensive cultivation with no-till and agro-forestry in 10 years following deforestation

Source: Lal (1991).

Table 9.11 Loss of organic carbon with continuous and intensive cultivation using plough-based mechanized systems in 10 years following deforestation

Source: Lal (1 991).

The loss of organic C from soils under shifting cultivation is less than that from soils under intensive cultivation. Nye and Greenland (1960) observed that the loss of organic carbon in 100 years may be 20 per cent for a soil with 12-year fallow cycle to 45 per cent for a soil with 4-year fallow cycle. The annual loss of C due to shifting cultivation may be as much as 0.27 mg/ha. If shifting cultivation is prac tised on about 25 million ha, the total loss of C due to shifting cultivation is estimated at 6.25 billion kg C/yr. In addition to C, biomass burning also causes release of several other greenhouse gases, e.g. CO₂, CO, CH₄, and NO_x.

Table 9.12 Changes in soil organic carbon (SOC) content of the surface 0-5 an layer of two soils in southern Nigeria

a. The data from Ibadan are from Watershed 1.
b. The data from Okomu are from the manually cleared plots; data recalculated from Ghuman and Lal (1991).
c. The average (%/yr) is calculated for each year on the basis of the original SOC content.

Deforestation and hydrological balance

Deforestation of TRF can drastically alter the components of the hydrological cycle:

P = I + R + DS + D + ò Ed_t,

where P is precipitation, I is infiltration, R is surface runoff, DS is soilwater storage, D is deep drainage, E is evapotranspiration, and t is time. Deforestation decreases I and DS and increases R and D components. In general, deforestation may also increase E. The change in E, however, may also depend on the land use.

Several experiments have demonstrated the effects of clear-cutting on the increase in total water yield. The impact of deforestation on the hydrological balance of a 44 ha watershed was studied at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. Prior to partial deforestation in 1978 and complete defor estation in 1979, measurements of surface and subsurface flow were made under the forest cover from 1974 to 1977. Under the forest cover, the interflow was 0.4 per cent to 1.4 per cent and total flow 0.8 per cent to 2.7 per cent of the total rainfall. Partial clearing in 1978 increased interflow to 1.2 per cent and increased total flow to 6.6 per cent of the total rainfall (table 9.13).

Table 9.13 Effects of partial clearing in 1978 on total water discharge from Watershed 1

Note: The partial clearing was of 3.1 ha out of 44.3 ha.

Table 9.14 Hydrological components on an annual basis for Watershed 1, 1979-1986

a. Evapotranspiration includes soil water storage and groundwater recharge.
b. Subsurface flow is underestimated during wet years because it is computed as a part of surface flow during the storm runoff.

The entire watershed was cleared in 1979 and cultivated to food crops. The data in table 9.14 show that the total water yield ranged from 4.6% to 11.3% of the rainfall received. Because of the bimodal distribution of the rainfall, the hydrologic balance was computed separately for each growing season. The hydrologic balance showed that total water yield ranged from 1.4% to 11.8% for the first season (table 9.15) and from 0.8% to 18.1% for the second season (table 9.16). The intermittent stream, with a trace of flow after heavy rain and no flow during the dry season, became a perennial stream that recorded a measurable flow throughout the dry season (table 9.17).

Table 9.15 Hydrological components for the first growing season (March-July), 1979-1987

a. See notes to table 9.14.
b. See notes to table 9.14.

Table 9.16 Hydrological components for the second growing season (AugustNovember), 1979-1986

a. See notes to table 9.14.
b. See notes to table 9.14.

Table 9.17 Hydrological components for the dry season (December-February) for Watershed 1, 1979-1987

Note: The data for December are taken from the previous year.

An increase in the magnitude of interflow and its continuous discharge throughout the dry season may be attributed to the replacement of deep-rooted perennials with high water requirements with shallow-rooted annuals with relatively fewer water requirements. Further, annuals were not grown during the dry season.

Sustainable use of the TRF ecosystem

Criteria for sustainable land use

The tropical rain-forest ecosystem must be used, improved, and restored. Continuous depletion of these resources has economic and ecologic ramifications at local, regional, and global scales. Sustainable use of soil and water resources in the TRF ecosystem should take the following into consideration:

· the nutrient capital of the soil resources should be enhanced by applications of chemical and organic fertilizers;

· the management systems adopted must optimize energy flux as well as energy use efficiency - energy efficiency alone is not adequate in view of increasing population pressure;

· Iosses of nutrients and water out of the ecosystem should be minimized;

· nutrient recycling mechanisms from subsoil to surface horizons must be an integral aspect of the land-use system;

· land degraded by past mismanagement must be restored by afforestation with ecologically adapted and quick-growing species.

Land capability assessment

Land capability assessment is necessary for the rational utilization of forest resources. Sustainable use of TRF resources necessitates a detailed and accurate inventory of the soil, water, vegetation, and climatic characteristics of the region. These inventories/surveys should be conducted at reconnaissance scales (1: 50,000 to 1 :1,000,000) and detailed scales (1 :10,000 to 1:50,000). The land resources should then be classified according to their potential capability as follows (FAO 1982):

Forest land

There are several types of forest land:

(a) Natural forested land should be preserved as natural forest and left alone. It has limitations of topography, shallow/stony soils, poor water regime, etc. Some examples are marginal steep lands, forests in the vicinity of regions with short supplies of firewood, inaccessible areas, small islands, and regions with other sociopolitical connotations.

(b) Production forests are suitable for managed logging of timber and other forest products.

(c) Planted or man-made forests are fertile, prime lands and are suitable for tree plantations, e.g. Gmelina, teak, Cassia.

(d) Protected forests are forest reserves protected in order to preserve the natural biodiversity.

Arable land

Arable land is prime agricultural land and is suitable for supporting continuous and intensive agriculture for food-crop and livestock production. Such land should be developed and managed according to ecologically compatible methods of deforestation and land development. When deforestation for arable land use is inevitable, land development should be carefully planned and implemented according to scientific guidelines.

Guidelines for land use in the TRF ecosystem

The development of TRF for alternative land uses has become a global issue. For some countries, the question is no longer whether to remove tropical forest for alternative land uses; the important consideration is how much to remove and by what method so that ecological concerns are adequately addressed. It is the ill-planned and improper management of TRF that has created severe ecological, economic, and socio-political problems. The sequence of steps needed to achieve a rational use of the TRF ecosystem is outlined below:

1. Iand capability assessment;
2. choice of proper land use (e.g. arable land, protected forest, manmade forest);
3. use of proper methods and time of deforestation (e.g. manual, chainsaw, shearblade);
4. adoption of soil conservation measures (e.g. cover crop, mulch farming, vegetative hedges);
5. use of science-based agronomic techniques of soil and crop management (e.g. balanced fertilizer use, proper crop rotation and cropping sequences, appropriate tillage methods, and integrated pest management).

Best management practices for sustainable agriculture

Some soils supporting the TRF ecosystem can be converted to intensive arable land use with sustained production provided that:

(a) expectations of agronomic yields are not too high,

(b) the soil and crop management systems adopted ensure the replenishment of plant nutrients harvested in crops and the maintenance of biophysical resources,

(c) the soils are taken out of production and put to restorative land use long before the degradative processes are set in motion.

Some research-proven agronomic practices based on these guidelines are listed in table 9.18. Just as use of prime agricultural land is essential not only for food-crop production but also for establishing pasture and forest plantations, so is the use of chemical and organic fertilizers for enhancing soil fertility. Most soils of the TRF ecosystem are of low inherent fertility. Enhancing soil fertility, therefore, is crucial to sustained agricultural productivity.

Imperata control

Land misuse and severe soil degradation encourage encroachment by Imperata cylindrica and other noxious weeds. It is important to maintain soil fertility at a high level to curtail encroachment by lmperata and to reclaim already infested lands. Reclamation of Imperata-infested land requires a combination of mechanical, chemical, and biological measures. Soil inversion, to uproot rhizomes and expose them to high temperatures during the dry season, followed by the use of systemic herbicides and sowing an aggressively growing cover crop, is essential to eradicate the noxious weed. Biological methods of Imperata control, slow as they may be, are often effective on a long-term basis. Preventing encroachment by adoption of the best management practices (BMPs) outlined in table 9.18 should be the best overall strategy.

Table 9.18 Best management practices for sustainable land use in TRF ecosystems

Restoring degraded forest lands

Restoration of degraded lands in TRF ecosystems is a high priority if the rate of new deforestation is to be reduced. The choice of land restorative measures to be adopted depends on the type of degradation, the processes involved, and antecedent soil properties and vegetation. Knowing the critical/threshold levels of soil properties, beyond which the soil's life support processes are severely jeopardized, is crucial in this endeavour. Land restorative techniques for soils degraded by different processes are outlined in table 9.19.

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