Other environmental concerns apply more to large-scale industrial monospe-cific plantations of exotic species. Concerns include their potential deleterious effects on soils (e.g., increases in soil acidity under pines) and decreases in water yield downstream (Cossalter and Pye-Smith 2003). Some plantation species, such as eucalyptus, are highly susceptible to fire because of the fuel that they can accumulate as litter on the ground under their canopies. A classic example from South Africa shows how dramatic declines in water yield actually occurred following planting of riverside areas with eucalypts, black wattle, and pines in the 1950s and 1960s, leading to the elimination of the plantations in the 1990s (Cossalter and Pye-Smith 2003). However, in other cases, eucalypt plantations have been shown to have beneficial effects on the ecosystem: for example, in the People's Republic of Congo, plantations of fast-growing eucalypts established on degraded savanna soils have been shown to have improved the soils by building up organic matter, and even encouraged the return of natural vegetation and wildlife to the site (Loumeto and Huttel 1997). In all cases, plantations have to be put into context with the rest of the landscape; the type of land use plantations are replacing (natural forests, pasture, agriculture); their main purpose (industrial, recovery of degraded land, etc.); and other benefits plantations provide (carbon sequestration, economic growth, and social functions such as greater income and employment) (Cairns and Meganck 1994; Evans 1999; Keenan et al. 1999; Sedjo 1999).
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