Studies have shown that F. albida significantly changes the soil beneath the canopy and that the overall effect of these changes is increased soil fertility (Barnes and Fagg, 2003). The tree cover increases water infiltration and also has a beneficial effect on bulk density, structural stability and chemical and biochemical properties. It is a crucial nutrient source and also helps in cycling of nutrients. F. albida has been shown to improve some physical and chemical properties of soils under its canopy; it has been shown to values of total nitrogen and organic carbon while having no effect on soil texture, pH and available phosphorus (Zomer et al., 2009). The trees extensive root systems mine the surface layers of the soil beyond the reaches of its crown and in so doing redistribute the nutrients in the litter that then falls beneath its canopy. In a study of F. albida and its effects on Ethiopian Highland Vertisols (Kamara and Haque, 1992) found a significant inverse relationship between SOM, N, P, K concentration and distance from the tree. The F. albida did not seem to influence soil reaction (pH) and the exchangeable cations Na, Ca and Mg. The build-up of SOM, N, P and K under the tree canopies was attributed to the litter fall accumulation. They found N and P contents in the fresh leaves and twigs to be 3.85% N and 0.3% P for the leaves and 1.27% N and 0.2% P for twigs (Table 1) (Getahun et al., 2014).

 

 

Mature F. albida  trees supplied significant amounts of nitrogen, organic carbon and K to the soils under their canopies resulted in a clear fertility gradient for these nutrients. The N, OC and K levels were 42, 31 and 25% respectively higher under the canopies than outside (Umar et al., 2012). The benefit of F. albida is its nitrogen-fixing quality, which is the result of protein-rich foliage (pods) that fall from the tree in large quantities during dormancy in the early rainy season and enrich the soil with nitrogen, phosphorus, and exchangeable calcium. It sheds its leaves when ploughing begins and hardly competes for light and water during the growing season of the crop.

THE ROLE OF Faidherbia albida ON CROP YIELDS

Several trials have shown the positive effect of F. albida on crops. The species has a potential to improve the yield of intercropped plants for instance barley yield was significantly affected by distance from the center of F. albida trunk and by the interaction of distance and land use systems. As shown in Figure1 below significantly higher barley yields (p<0.05) were found at 1 m distance from the tree compared to yields at 25 and 50 m for land use systems F. albida only, and F. albida and livestock. In contrast, in the F. albida and Eucalyptus camaldulensis land use systems barley yields did not change significantly with distance from the tree although average yields were lowest under the tree. F. albida is renowned for the so-called 'albida effect', that is crops growing under F. albida trees have higher yields than crops growing away from the tree canopy .

 

 

Faidherbia albida FOR CONTROL OF WEEDS

The food production potential of the semi-arid region of West Africa, especially cereals, for example sorghum (Sorghum bicolour (L.) Moench), millet (Pennisetum glaucum (L.)) R. Br. and maize (Zea mays L.) (Kassa et al., 2010) has seriously been reduced due to the parasitic weed Strigahermonthica. Pearl millet is a major food crop grown in the semi-arid region of West Africa .

In areas where there is too little crop rotation, severe cases of Striga infestation are more noticeable, often resulting in total crop failure. Great yield losses can occur in cereal crops due to Striga infestation; for example, grain yield losses in sorghum mayreach up to 70% as a result of Striga infestation (Gworgwor et al., 2001). According to (Sauerborn, 1991), the cultivated areas actually infested by Striga in Africa are estimated at 21 million hectares. The overall loss in grain production amounts to 4.1 million tons. The loss of revenue from sorghum, pearl millet and maize due to the parasite infection could total 2.9 billion $ US.

One of the most successful and promising control measures is the adequate application of mineral nitrogenous fertilizers. One of the cheapest means of improving his soil fertility, which could effectively reduce or eliminate Strigainfestation, is the use of the F. albida tree  in an agroforestry practice onhis farm. F. albida is well spread all over the dried region of West Africa, and East and South Africa where there is a long dry season (Giffard, 1964), which equally favors Striga survival.

SOCIO ECONOMIC USE  Faidherbia albida

The functions of F. albida differ widely from one region to another within its vast natural distribution area, which covers the whole of semi-arid Africa, north and south of the equator. There was positive relationship between F. albida tree density and bee hives per farm household. Because of its reverse phenology in keeping its leaves and flowers during the long dry season and shedding them during short wet season (Hadgu et al., 2009), bees in the study areas were dependent on F. albida flowers as their main source of forage as most of other plants are dry during the long dry season. Therefore for bee-keepers, it has the advantage of producing flowers at the end of the rains while most of the sahelian species flower just before or during the rains. It therefore becomes the main source of pollen and nectar at this time. In other respects, the seeds, gum, bark, and wood are utilized for many purposes; food, traditional medicine, construction, furniture, canoes, and other domestic uses making F. albida the "miracle" tree of the Sahel (Figure 2) (Kessler , 1990). The interest in tree species as a food source for domestic animals lies in the fact that they offer green fodder rich in protein in a period when animal feed is scarce; (Hadgu et al., 2009).

 

 

Faidherbia albida PROPAGATION AND TREE MANAGEMENT

The hard coated F. albida seeds store well under dry conditions and are usually extracted by pounding the pods in a mortar. Pretreatment is often needed for rapid uniform germination. Various methods used include; mechanical scarification, dipping in concentrated sulphuric acid for 5-15 min and dipping in boiling water. Early seed collection is recommended to avoid heavy infestation by bruchid beetles. Seed storage behavior is orthodox; there is no loss after one year in hermetic storage at 4°C; viability maintained for several years in hermitic storage at 10°C with 6-10% mc. When treated with insecticides and kept in simple closed containers, seed can be stored for several years. 

Seedlings and direct sowing at site may be used for propagation of F. albida. Pruning in the second year to about half the tree height may be needed to control low branching. Repeated pruning during periods of average biomass production stimulates leaf production. It can be pruned twice a year. Resulting re-growth is especially vigorous in the  first  year  but  decreases  as  exploitation

continues; trees show stress at the end of the sixth year. Regular lopping (once every 3 - 4 years) removing 0.4 - 0.5 m³ of foliage (or 35% of the total volume) at the start of the growing season is recommended (Maundu and Tengnas, 2005). However, care should be taken as improper methods of lopping have been observed to cause wounds, predisposing the tree to attack by pathogens. This tree responds well to coppicing.

Natural stands of F. albida in Ethiopia are generally managed in two ways, depending on the type of farming system and area where it grows. In the coffee growing region of Hararghe (eastern Ethiopia), particularly in the Gelemso area, farmers usually maintain and protect F. albida in their coffee plantations for shade. Pods of the trees are collected and fed to livestock. On the other hand, mature trees found in cereal-based farming systems (southeastern Shewa region and the Fedis area of Hararghe) are regularly pollarded at intervals of 3-4 years. Whole canopies are cut right back to the trunk, leaving the trees completely bare. Farmers use the branches for fuelwood and for fencing their compounds and barns .

 

 

 

F. albida can simultaneously store significant quantities of carbon in aboveground biomass and increase crop yields. The unusual phenology and the ability to fix nitrogen makes it excellent in agroforestry systems. The mulch created by falling leaf litter and the canopy shade at planting time favors crop production beneath its canopy. It has positive effect on crops yield. Litter drop combined with high microbiological activity in the soil (especially during the rainy season) apparently constitute its main soil improving effect. The deep-rooting capabilities of the trees play a particularly important role in enriching surface soil horizons by drawing up mineral elements from lower horizons. Its reverse phenology of contributes much to crop fertilization, since its litter fall occurs in the rainy season. Other beneficial effects of F. albida, such as nodulation and the attraction it holds for cattle resulting in manuring of the site are important as well.

F. albida is considered as keystone species by local farmers as they derive benefits such as livestock fodder, bee forage, fuel wood and income through sale of wood products which contributes to the improvement of their livelihood. Besides, it is used as a vegetative soil cover during the long dry season, and serves as shade for livestock. This agroforestry tree species are not only an effective tool for smallholders to increase cereal yields and short-term food security, the long-term soil fertility and available soil moisture increases hold promise for making cropping systems more resilient under increasing climate variability. In addition, these species provide multiple benefits, developing carbon stocks that may be converted into offsets, providing on-farm fuelwood, as well as fodder, food and medicinal products.

 

The author has not declared any conflict of interests.