they occupy fertile land, as they are grown along boundaries, pathways, and across contours to curb soil erosion. Like many agroforestry and natural resource management practices, the adoption of fodder shrubs requires knowledge that is not always available to the farmer, such as raising seedlings, pruning trees and feeding leaves to livestock. Five factors contribute to the adoption of fodder shrubs: large NGOs promoting them, farmer-to-farmer dissemination, private seed vendors, trained extension agents and the integration of fodder shrubs into bigger projects. AKST is driven by the need to understand how these knowledgeintensive practices can be more efficient and effective (Franzel
and Wambugu, 2007).

Technologies for replenishing soil fertility often increase labor requirements and require careful management. Some options are to withdraw land from agricultural production for various periods of time, which could prove costly to the farmer. The returns on investments vary and are related to market opportunities and farm prices for crops (Kante, 2001).

3.4.1.4 Tree domestication
Herbaceous and woody species of trees are now being domesticated to meet the needs of local people for traditional foods, medicines and other day-to-day products (see Global Chapter 3). Agroforestry tree domestication involves developing useful tree products for farmers. It leads to diversification of small-scale farms in the humid, subhumid and drylands of SSA. It also provides farmers the opportunity to generate a cash income. It can only be successful, however, if developed with appropriate market opportunities.

Over the past 50 years, rural populations in Africa have increasingly planted trees for the tree products and services farms require, as opposed to relying on natural forests. This trend underscores the fact that forests in Africa are the most depleted of any tropical region, with approximately one-third of the original forest area remaining (Sayer et al., 1992). The deforestation rate is 1.7% annually. Africa is the least forested tropical continent with only 21.4% forest cover as a percent of land area in 2004 (FAO, 2007), in comparison to South America, which has 47.7% of its land in forests. On-farm tree planting varies from country to country in almost direct negative correlation to forest cover percentage and per capita forest cover. The range for forest cover percentage and per capita forest cover varies from 1% tree cover in Niger to 85% in Gabon; per capita forest cover varies from 0.1 ha in Ethiopia to 18.2 ha in Gabon. Local populations have always been familiar with the services and functions provided by trees including soil improvement, biodiversity habitat, stored energy, reduced soil erosion, shade, windbreaks and boundary markers.

A landmark meeting of the International Union of Forest Research Organizations in 1992 revealed how far Africa lagged behind in the area of tree domestication relative to Asia and the Pacific (Leakey and Newton, 1994). This information triggered a large increase in the amount of tree
research being carried out in Africa (see Leakey et al., 2005). Participatory tree domestication, an approach that involves farmers, market traders and consumers in activities such as species prioritization, trait selection, germplasm collec-

tion and strain development (Simons and Leakey, 2004), has been well received. These initiatives are now beginning to show positive impact in terms of increased tree planting (Franzel and Scherr, 2002) and increased product quality (Tchoundjeu et al., 2006).

Fruit trees are important for nutrition in SSA, where nutrition levels are the lowest in the world, and have been the target of domestication efforts. In Zimbabwe, trees such as Diospyros mespiliformis, Azanza garkeana and Strychnos cocculoides are important for household nutrition, and plantings have remained constant in proportion to climax woodland and cleared agricultural land. These trees bear fruit seasonally. In southern Africa, customary conservation practices range from seasonal restrictions on gathering medicinal plants to the widespread social conventions that prevent the felling of fruit-bearing trees such as wild medlar (Vangueria infausta) and magic gwarra (Euclea divinorum). These species are important in the maintenance of biodiversity on communal lands.

Post–World War II global concerns for timber supply drove the establishment of industrial plantations for sawnwood and paper in Africa and elsewhere. Pan-tropically suitable timber species, such as Pinus spp., Eucalyptus spp., and Acacia spp. were preferred (Barnes and Simons, 1994). These plantations were established predominantly on remote land or land of low suitability for agriculture, and they were primarily geared for export. In SSA, the most significant tree product required was fuelwood.

Recognizing an impending shortage of fuelwood and animal fodder, development-oriented forestry programs emerged in the 1970s and 1980s. These programs used multipurpose trees and relied heavily on exotic species, some of which have since become invasive (e.g., Acacia, Prosopis). Most planting efforts focused on Australian acacias, Central American dry-zone hardwoods, casuarinas, sennas and neem (Azadirachta).

Two trends have been noted in SSA: the number of trees in forests is declining and the number of people on farms is increasing (Tiffen et al., 1994; Place, 1995; Place et al., 2001; Kindt et al., 2004). Chapter 3 of the Global Assessment describes the relationship between farmers and depleting forests: after the forests are removed, tree populations increase as farmers integrate trees into their farming systems (Michon and de Foresta, 1999; Place and Otsuka, 2000; Schreckenberg et al., 2002; Kindt et al., 2004). This counterintuitive relationship, observed in West Africa (Holmgren et al., 1994), East Africa (Kindt et al., 2004; Boffa et al., 2005) and the Sahel (Polgreen, 2007), seems due, in part, to the availability of labor, domestic demand for traditional forest products and for marketable cash crops, and to risk aversion (Shepherd and Brown, 1998). In Cameroon, for example, tree density is inversely related to farm area, ranging from 0.7-6.0 ha (Degrande et al., 2006) and similarly, a given area of land has a greater abundance and diversity of trees when it is composed of small farms (Kindt et al., 2004). Recent studies show that there may be a rich diversity of trees in some locations, and a dirth in others (Kindt et al., 2004; Lengkeek et al., 2005). Some of the lower rates of diversity have been attributed to bottlenecks