The challenge of HIV/AIDS: Where does agroforestry fit in? - World ...

116World Agroforestry into the FutureTable 2.Soil organic carbon (SOC) increase over the duration of the fallow phase in a few tropical soils with different tree species in thesub-humid tropics.SOC increaseCountryFallow duration(years)Soil typeFallow speciesSampling depth(cm)Total(Mg ha –1 )Annual(Mg ha –1 yr –1 )Togo 5 Ferric Acrisol (sandy) Acacia auriculiformisAlbizzia lebbeckAzadirachta indicaCassia siamea0–10 3.41–12.46 0.68–2.49Kenya 1.5 Arenosol (sandy) Crotalaria grahamianaC. paulina0–20 1.69–2.15 1.13–1.43Kenya 1.5 Ferralsol (clayey) Crotalaria grahamianaC. paulinaTephrosia vogelii0–20 2.58–3.74 1.72–2.49Source: Albrecht and Kandji (2003).variability, and that is where agroforestrycan be a relevant practice.Successful and well-managed integration oftrees on farms and in agricultural landscapesoften results in diversified and sustainablecrop production, in addition to providing awide range of environmental benefits suchas erosion control and watershed services.In western Kenya, the World AgroforestryCentre, together with various partners, hastested the potential of improved fallowsystems for controlling soil erosion, usingfast-growing shrubs such as Crotalaria spp.and Tephrosia spp. These species showedgreat promise in reducing soil losses (Boyeand Albrecht 2005). At the same time a significantimprovement in soil water storagehas been observed in the improved fallowsystems (Figure 3). We now understand thatclimate change may translate into reducedtotal rainfall or increased occurrence of dryspells during rainy seasons in many semiaridregions. Therefore, optimizing the useof increasingly scarce rainwater throughagroforestry practices such as improved fallowcould be one way of effectively improvingthe capacity of farmers to adapt to drierand more variable conditions.Under many of the different farmer practicesin Africa, crops will still fail completelyor yield very little in drought years. Resultsfrom improved fallow trials were used tomodel these various systems. The modelsuggested that it would be possible to producean acceptable amount of food in lowrainfall years if practices such as improvedfallows were pursued (Table 3). As expected,maize production was higher afterimproved fallow than in a continuous croppingsystem in good rainfall years (typically962–1017 mm of rain). A similar trend wasobserved in low rainfall years (< 600 mm).Most interestingly, the model predictedthat maize yield in a low rainfall year aftera Sesbania spp. fallow period was evenhigher than maize yield in the continuouscropping system in a good rainfall year.If we define rainfall use efficiency (RUE) asthe amount of maize (in kg) produced witheach mm of rainwater, then, apparently,the maize crop after improved fallow madebetter use of the available water than thecontinuous crop, especially when rainfallwas low (Table 3). In low-rainfall years, wateravailability to crops is paramount andseems to be the dividing factor betweenabsolute crop failure and reasonable foodproduction. Buffering agricultural cropsagainst water deficiencies is, therefore, animportant function agroforestry would haveto play in the adaptation battle.There are other mechanisms such as improvedmicroclimate and reduced evapotranspirationthrough which agroforestrypractices may improve the adaptive capacityof farmers. In the African drylands,where climate variability is commonplace,farmers have learned to appreciate the roleof trees in buffering against production