AGRICULTURAL ISSUES

Agroforestry is an approach to land use based on the planned integration of trees with crop and/or livestock production systems (Fig. 1.8) (Young, 1989; Kang et al., 1999). It is an ancient practice, which has nevertheless benefited from methodical research and experimentation since the 1970s (ICRAF, 1997). Agroforestry can be productive, and more profitable and sustainable than other land-use systems (Kang et al., 1985; Nair, 1993), and it has the potential to offer rural house-holds a large variety of products for trade and household use. Researchers and farmers throughout sub-Saharan Africa have developed new agroforestry practices (Franzel et al., 2001). Agroforestry manufacturing systems provide a large number of products and benefits (Huang and Xu, 1999).

The net production of phytomass can be increased by the well-organized sharing of site resources among trees and other intercropping components, together with nitrogen fixation and micro-climate modification (Sharrow and Ismail, 2004). About 40 years ago, a plantation of A. senegal Figure 1.7 ‘Tear drops’ of gum arabic, the dried, gummy exudate obtained from acacia trees.

was reported to increase total nitrogen and organic carbon while having no effect on the texture, pH, available phosphorus or available potassium of a sand sheet soil. The higher nitrogen content in the topsoil may have been partially due to symbiotic fixation (Gerakis and Tsangarakis, 1970).

Agroforestry can also potentially advance the water-use efficiency of systems by minimizing the non-productive part of the available soil water (Ong et al., 2002). In other words, agroforestry systems can considerably increase rainfall utilization as compared to annual cropping systems (Ong et al., 2002). Plant growth is dependent on the availability of light, water and nutrients, and manipulation of tree density in agroforestry systems can therefore modify the biomass production of component species (Eastham et al., 1990). Benefits in terms of biomass and grain yields can be expected if there is complementary resource sharing by agricultural crops and trees (Cannell et al., 1996). Tree density has a strong effect on the distribution and depth of the stand’s roots (Boswell et al., 1975). Agroforestry systems have been recognized as a tool for rehabilitating already degraded lands (Bandolin and Fisher, 1991). No less important is the fact that trees improve crop productivity by reducing wind flow, thereby reducing water loss through transpiration (Zinkhan and Mercer, 1996). Much work has already been done on studying tree-crop interactions under various tree-spacing regimes to improve the productivity of agroforestry systems (Gupta et al., 1998).

The most important forest in the Sudan may be the gum arabic belt. The “belt” refers to a zone of approx. 520,000 km² that expands across Central Sudan between latitudes 10° and 14° N, accounting for one-fifth of the country’s total area (IIED and IES, 1990). The belt accommodates

~20% of Sudan’s population and 66% of its livestock. It acts as a natural barrier, protecting over 40% of the total area of Sudan from desert encroachment, and it represents the site for most of the agriculture and animal production. This includes irrigated, mechanized rain-fed, and traditional rain-fed agriculture and forestry (Ballal, 2002). Until recently, the traditional A. senegal-based Figure 1.8 Maize crops growing together with Faidherbia albida (Acacia albida Delile) and palms (http://en.wikipedia.org/wiki/Image:Faidherbia_albida.JPG, photo by Marco Schmidt).

Role and Sources of Exudate Gums ◾ 11 agroforestry system was considered one of the most successful forms of natural forest manage-ment in tropical drylands (Fries, 1990), and was regarded as sustainable in terms of its environ-mental, social and economic benefits (Ballal, 1991). Traditionally, the A. senegal tree is managed in temporal succession with agricultural crops such as sorghum, pearl millet, groundnut, sesame and karkadeh (Hibiscus sabdariffa L.). This agroforestry system allows a period of 10 to 15 years for reestablishment of the soil’s fertility after a short period of arable cultivation (Ballal, 2002).

The cycle thus consists of a relatively short period of cultivation followed by a relatively long fal-low period. The bush falfal-low sequence begins by clearing a 15- to 20-year-old gum garden for the cultivation of field crops. Trees are cut at 10 cm from the soil surface, and stumps are left to start vigorous coppice regrowth. The cleared area is cultivated for a period of 4 to 6 years, during which time the coppice shoot regrowth is removed to improve the establishment and growth of agricul-tural crops. However when the soil fertility declines, as reflected by low crop yield, crop growing ceases and the area is left fallow under A. senegal. The remaining trees are tapped for gum arabic until the age of 15 to 20 years, after which they are cleared again for crop cultivation. Therefore, the final tree stand is mainly the result of coppice regeneration, as well as some regeneration from seeds dispersed naturally or sometimes on purpose for enrichment planting (Ballal, 2002). The bush-fallow system of cultivation has proven to be a successful, sustainable farming system, par-ticularly on the marginal lands of Kordofan. A. senegal supports the local population’s livelihood, since its gum represents a major cash crop, and in addition, fuel wood is obtained from this tree for household use and for sale (Sharawi, 1986).

As an outcome of the development of a vegetable oil industry in North Kordofan in the 1940s, which strengthened the production of groundnut and sesame, there was a favorable response in terms of prices and productivity to oil seeds. However, this development occurred at the expense of the gum orchards, and the traditional rotational fallow-cultivation cycle was dramatically short-ened or completely abandoned (Awouda, 1973). Consequently, the negative impact on soil and water has been considerable, to the extent that commercial agriculture is also beginning to face some problems (Ballal, 2002). Indications of system imbalance were noted decades ago and today, the area is experiencing a serious decline in fertility, as well as soil erosion and desertification.

Moreover, sustainable management of the gum gardens is threatened because of severe droughts and indiscriminate clearing of A. senegal stands for firewood and charcoal production as a short-term source of income (Elfadl et al., 1998). This has resulted in more degraded land. Accordingly, the removal of A. senegal trees and a general deterioration of the stands have resulted in a reduction in gum arabic production, by 30 to 70% between 1973 and 1984 (Bayoumi, 1996). The spread of desert-like conditions has also resulted from both physical conditions and misuse of resources (Ahlcrona, 1983; Suliman and Drag, 1983). A study of the recovery of biomass productivity in North Kordofan concluded that land degradation and the ecological imbalance associated with drought cycles and mismanagement could be reversed, if rational management practices were applied in accordance with water availability from rainfall (Yagoub et al., 1993).

A. senegal is the most important component of traditional dryland agroforestry systems in the Sudan. The spatial arrangement of trees and the type of agricultural crop used influence the interaction between them (Raddad et al., 2006). The influence of different A. senegal agroforestry systems on soil water and crop yields in clay soils of the Blue Nile region in Sudan was studied (Raddade and Luukkanen, 2007). Trees were grown at 5 x 5 m or 10 x 10 m spacing, either alone or in a mixture with sorghum or sesame. Results demonstrated no significant variation in the soil water content under different agroforestry systems. Intercropping also resulted in a higher land equivalent ratio. No significant variation was found in the yields of sorghum or sesame when these crops were grown with or without trees. At an early stage of agroforestry system management,

A. senegal has no detrimental effect on agricultural crop yield. However, the pattern of resource capture by trees and crops can change as the system matures. There was little competition between trees and crops for water, suggesting that in A. senegal agroforestry systems with 4-year-old trees, the clay soil has enough water to support crop growth over a whole growing season up to matura-tion and harvest (Raddade and Luukkanen, 2007). In conclusion, policy has a potential role in influencing the poverty and land-degradation problems facing Africa. Both ‘good’ and ‘bad’ poli-cies can affect the economic incentives determining poor rural households’ decisions to conserve or degrade their land (Barbier, 2000).

Many plant gum exudates are known worldwide. Four of them (arabic, ghatti, karaya and tra-gacanth) are of importance to the food industry (Glicksman, 1969). Many other gums, which are listed and described in Chapters 3 and 4, are known and used in their local areas of availability.

Sometimes these gums can serve as substitutes for others, especially if they have similar properties.

In the search for gum arabic substitutes, natural gum exudations in seven South American species of Prosopis and the productivity of induced gum exudation were evaluated (Vilela and Ravetta, 2005). Prosopis is a genus of about 45 species of leguminous spiny trees and shrubs, located in subtropical and tropical regions of the Americas, Africa and southwest Asia. They often thrive in dry soil and are resistant to drought, sometimes developing extremely deep root systems. Their wood is usually hard, dense and durable. Their fruits (pods) may contain large amounts of sugar.

Natural exudates were found in three species: P. flexuosa, P. chilensis and P. nigra. In the latter two, exudates were dark, liquid and bitter, while in P. flexuosa, up to 1.6 kg per tree of amber-clear gum was harvested (Vilela and Ravetta, 2005). High-productive trees were old, with very little vegeta-tive growth, and were growing on sandy soils. To induce gum exudation, trees were wounded, and these wounds exuded plentifully for 7 months. Exudation increased during late summer and fall, after the fruits had ripened (Vilela and Ravetta, 2005).