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

* 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.