helping to control pests, diseases, and weeds (Collins and Qualset, 1999) in about two thirds of the agroforests tested (Schroth et al., 2000). Agroforestry is thus capable of rehabilitating degraded farmland. Agroforestry systems support biodiversity conservation in human-dominated landscapes in the tropics (Schroth and Harvey, 2007), through reducing the conversion of primary habitat and providing protective ecological synergies; providing secondary habitat; and by offering a more benign matrix for "islands" of primary habitat in the agricultural landscape, especially by buffering forest edges and creating biological corridors which provide maintenance of meta-population structure (Perfecto and Armbrecht, 2003). Scaling up successful agroforestry approaches requires both improving livelihood and biodiversity impacts at the plot scale, and strategic placement within a landscape mosaic to provide ecosystem services (e.g., watershed protection, wildlife habitat connectivity).

Agroforestry strategies and techniques have been developed for the rehabilitation of degraded agroecosystems and the reduction of poverty particularly in Africa.

Agroforestry has evolved from an agronomic practice for the provision of environmental services, especially soil fertility amelioration, to a means of enhancing agroecological function through the development of an agroecological succession involving indigenous trees producing marketable products (Leakey, 1996). In this way it now integrates environmental and social services with improved economic outputs (Leakey, 2001ab). At the community level, agroforestry can positively affect food security andthe livelihoods of small-scale farmers. It can also reverse environmental degradation by providing simple biological approaches to soil fertility management (Young, 1997; Sanchez, 2002); generating income from tree crops (Degrande et al., 2006); minimizing risk by diversifying farming systems (Leakey, 1999b) and; restoring agroecosystem services (Sanchez and Leakey, 1997). Consequently, agroforestry has been recognized as an especially appropriate alternative development strategy for Africa (Leakey, 2001 ab), where the Green Revolution has had only modest success (Evenson and Gollin, 2003).

Agroforestry can mitigate anthropogenic trace gas emissions through better soil fertility and land management, and through carbon sequestration.

The integration of trees in cropping systems can improve soil organic matter, nutrient cycling and the efficient use of water, reduce erosion and store carbon due to improved plant growth. Early assessments of national and global terrestrial CO sinks reveal two primary benefits of agroforestry systems: direct near-term C storage (decades to centuries) in trees and soils, and, potential to offset immediate greenhouse gas emissions associated with deforestation and shifting agriculture. Within the tropical latitudes, it is estimated

that one hectare of sustainable agroforestry can potentially offset 5 to 20 ha of deforestation. On a global scale, agroforestry systems could potentially be established on 585 to 1275×106 ha of technically suitable land, and these systems could store 12 to 228 (median 95) tonnes C ha-1 under current climate and soil conditions (Dixon, 1995). Landscapescale management holds significant potential for reducing off-site consequences of agriculture (Tilman et al., 2002), leading to integrated natural resources management (Sayer and Campbell, 2001) (see 3.2.2.2.4).

Mixed farming systems, such as those involving cereal/legume mixtures can increase productivity and sustainability of intensive systems.

African savanna has a short growing season (4-5 months) with annual precipitation of 300-1300 mm. In these areas farmers typically grow maize, millet, sorghum, soybean, groundnut, and cowpea, often integrated with livestock production. Traditionally, the sustainability of intensive cereal- based systems in Asia was due to the presence of green manuring practices for soil fertility management and retention of below-ground biodiversity. However, increasing land prices and wage rates had made this option economically unviable at least in the short term and the use of green manures has declined substantially (Ali, 1998). Now shortduration grain legume varieties are available that can be incorporated in the cereal-based intensive systems (Ali et al., 1997). These grain legumes have enhanced farmers' income in the short term and improved cropping system productivity and sustainability in the long-term (Ali and Narciso, 1996). Mixed cropping also has the benefit of reducing pest infestations and diseases.

3.2.2.1.8 Watershed management
Watersheds are often mosaics that integrate many different land uses; when denuded they are very vulnerable to degradation, with severe downstream consequences in terms of flooding, landslides, siltation and reduced water quality (CA, 2007). Additionally, surface water tends to pass through deforested watersheds more quickly leaving towns and villages more susceptible to water shortages. Water storage schemes to supply urban populations and industrial complexes, or for irrigation schemes, can be wasteful and create conflicts between different water users.

Environmental sustainability of water resources is greatest when people work with natural systems and processes, rather than against them.

The most successful watershed management schemes involve participation of local communities. For example, there are traditional user-managed, water catchment and management projects in many parts of the world (e.g., in southern India, the mountainous regions of the Andes, Nepal, and upland South East Asia), which are more sustainable than those imposed by hierarchical water authorities. Schemes