market potential. However, some are also sources of edible oils which are needed for cooking and livestock feed but are deficient in many tropical countries (FAO, 2003b). In West Africa, edible oils are extracted from the fruits/kernels of Allanblackia spp. (Tchoundjeu et al., 2006), Irvingia gabonensis (Leakey, 1999a), Dacryodes edulis (Kapseu et al., 2002), Vitellaria paradoxa (Boffa et al., 1996) and many other agroforestry species (Leakey, 1999a). Unilever is investing in a new edible oil industry in West Africa, using Allanblackia kernel oil (Attipoe et al., 2006). Many agroforestry trees are also good sources of animal fodder, especially in the dry season when pasture is unavailable, and can be grown as hedges, which can be regularly harvested or even grazed by livestock. Opportunities for cattle cake exist from by-products of species producing edible fruits and nuts (e.g., Dacryodes edulis, Canarium indicum, Barringtonia procera, etc.). The nuts of Croton megalocarpus are good poultry feed (Thijssen, 2006). In Brazil, new agricultural commodities from agroforestry systems are being used in the manufacture of innovative products for the automobile industry (Panik, 1998).

Twenty-five years of agroforestry research have developed techniques and strategies to assist farmers to reverse soil nitrogen depletion without the application of fertilizers.

Leguminous trees fix atmospheric nitrogen through symbiotic associations with soil microorganisms in root nodules (Sprent and Sprent, 1990; Sprent, 2001). The soil improving benefits of this process can be captured in ways that both improve crop yield and are easily adopted by resource-poor farmers (Buresh and Cooper, 1999), conferring major food security benefits to these farming households. Some techniques, such as alley-cropping/hedgerow intercropping are of limited adoptability because of the labor demands, while others such as short-term improved fallows are both effective and adoptable (Franzel, 1999; Kwesiga et al., 1999). Short-term improved fallows in Africa involving species such as Sesbania sesban, Gliricidia sepium, and Tephrosia vogelii, accumulate 100 to 200 kg N ha -1 in 6-24 months and to raise maize yields from about 0.5 to 4-6 tonnes ha-1 (Cooper et al., 1996). An external source of phosphorus is needed for active N fixation in many P-deficient tropical soils.

Tree/crop interactions are complex but can be managed for positive outcomes.

There are many different types of competitive interactions between trees and crops in mixed farming systems, which can be evaluated on the basis of the Land Equivalent Ratio. After 25 years of intensive study the complex physiological and ecological impacts of tree/crop interactions are now well understood (Ong and Huxley, 1996; Huxley, 1999; van Noordwijk et al., 2004); there is much evidence of the overall productivity (biomass) of agroforestry systems being

greater than annual cropping systems, due to the capture of more light and water, and improved soil fertility (Ong and Huxley, 1996). Ultimately, however, it is the economic and social outcomes of beneficial interactions that usually determine the adoption of agroforestry systems (Franzel and Scherr, 2002). The numerous examples of agroforestry adoption indicate that farmers, especially small-scale farmers, recognize that the benefits are real.

Vegetated riparian buffer strips are planted for bioremediation of herbicide and nitrate pollution.

Vegetated buffer strips have been shown to retain >50% of sediment within the first few meters (Young et al., 1980; Dillaha et al., 1989; Magette et al., 1989; Mickelson et al., 2003). The planting of trees in strategically important parts of the catchment to maximize water capture and minimize runoff is one of the generally recognized ways of conserving water resources (Schultz et al., 1995, 2000; Louette, 2000; Lin et al., 2003, 2005). In the corn belt of the US, agroforestry strips (trees planted in grass strips) on the contour in a corn/soybean rotation had decreased loss of total P by 17% and loss of nitrate N by 37% after three years (Udawatta et al., 2004). This minimization of nutrient loss is one of the most important environmental services performed by agroforestry trees (van Noordwijk et al., 2004). Among several possible management practices, a tree-shrub-grass buffer placed either in upland fields (Louette, 2000) or in riparian areas (Schultz et al., 1995, 2000) is recognized as a cost effective approach to alleviating non-point sources of agricultural pollutants transported from cropland. Herbicide retention by buffers can also be substantial (Lowrance et al., 1997; Arora et al., 2003).

Enhanced agroecological function is promoted by agroforestry.

Agroecological function is dependent on the maintenance of biological diversity above and below ground, especially the keystone species at each trophic level. The ways in which biodiversity stimulates the mechanisms and ecological processes associated with enhanced agroecological function are poorly understood in any crop (Collins and Qualset, 1999); nevertheless, based on numerous studies, the principles are well recognized (Altieri, 1989; Gliessman, 1998) and are based on those of natural ecosystems (Ewel, 1999). Through the integration of trees in farming systems, agroforestry encourages and hastens the development of an agroecological succession (Leakey, 1996; Schroth et al., 2004), which creates niches for colonization by a wide range of other organisms, above and below-ground, in field systems (Ewel, 1999; Leakey, 1999b; Schroth et al., 2004; Schroth and Harvey, 2007). Integrating trees encourages and enhances agroecological function, providing enhanced sustainability as a result of active life cycles, food chains, nutrient cycling, pollination, etc., at all trophic levels, and