Agroforestry systems are used in a variety of different ecological and economic conditions. For example, in fertile regions agroforestry systems may be very productive, yielding high quantities of crops such as cacao and rubber (Fig. 6.13). However, their greatest potential to increase productivity and sustainability of production systems is in degraded areas, in regions with soils of low fertility, or in semi-arid regions. Also, agroforestry systems have a potential to increase sustainability of agriculture and income to small farmers who lack adequate infrastructure or technical resources.
Agroforestry systems are especially important in regions where commercial fertilizers are expensive or unavailable, because of their ability to recover, recycle, or efficiently utilize nutrients. This ability is often linked to mechanisms associated with woody or perennial species that recycle nutrients mainly through litter fall and decomposition. While agroforestry systems can be profitable if established immediately after forest clearing, they often require a number of years to become profitable when established on degraded lands. For this reason, capital-limited farmers on poor soils may require subsidies to enable the establishment of agroforestry systems (Montagnini et al. 2000).
Fig. 6.13. Rubber trees intercropped with corn in a taungya system in Dak Lak province, Vietnam. (Photo: Hoang Dinh)
In contrast to regions with relatively fertile soils, where plantation forestry or intensive agriculture may be a preferred productive system, agroforestry may be a better alternative in regions with more nutrient-poor soils, or with poorer socio-economic conditions. For example, a taungya system may be a more acceptable system to farmers than plantation forestry where there is an immediate need to produce food or cash. Intensive agriculture such as coffee monoculture may be more economically profitable, but it requires high inputs which many small farmers cannot afford. However, coffee can be a productive alternative when mixed with shade trees which can contribute to nutrient recycling in a less intensive system with lower inputs.
Agroforestry systems also contribute to the conservation of biodiversity and carbon storage (Dixon 1995; Young 1997; Beer et al. 2003; Montagnini and Nair 2004). Agroforestry systems can play an important role in the conservation of biodiversity within deforested, fragmented landscapes by providing habitats and resources for plant and animal species, maintaining landscape connectivity (and thereby facilitating movement of animals, seeds and pollen), making the landscape less harsh for forest-dwelling species by reducing the frequency and intensity of fires, potentially decreasing edge effects on remaining forest fragments, and providing buffer zones to protected areas (Beer et al. 2003). Agroforestry systems cannot provide the same niches as the original forests and should never be promoted as a conservation tool at the expense of natural forest. However, they do offer an important tool for conservation and should be considered in landscape-wide conservation efforts to both protect and connect remaining forest fragments and promote the maintenance of on-farm tree cover in areas surrounding protected areas.
Agroforestry is important as a carbon sequestration strategy because of carbon storage potential in its multiple plant species and soil, as well as its applicability in agricultural lands and in reforestation. Proper design and management of agroforestry practices can make them effective carbon sinks. As in other land-use systems, the extent of C sequestered will depend on the amounts of C in standing biomass, recalcitrant C remaining in the soil, and C sequestered in wood products. Average carbon storage by agroforestry practices has been estimated to be 9, 21, and 50 Mg C ha-1 in semiarid, subhumid, and humid tropical regions (Schroeder 1994; Dixon 1995). Agroforestry can also have an indirect effect on C sequestration when it helps decrease pressure on natural forests, the largest sink of terrestrial C. Another indirect avenue of C sequestration is through the use of agroforestry technologies for soil conservation, which could enhance C storage in trees and soils (Young 1997). Agroforestry systems with perennial crops may be important carbon sinks, while intensively managed agroforestry systems with annual crops are more similar to conventional agriculture (Young 1997; Beer et al. 2003; Montagnini and Nair 2004).
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