Conservation of and competition for water and nutrients in semi-arid agroforestry

Conservation of natural resources and competition between system components in semi-arid agroforestry is reviewed. The possibility to fill under-utilized niches in the landscape with agroforestry is discussed. Examples of processes and mechanisms resulting in conservation or competition on plant level are given. Examples of the resulting outcome of conservation vs. competition are given in seven semi-arid agroforestry systems (savannah, silvopastoral systems, parkland, tree windbreaks, alley farming, contour hedgerows and agroforestry combined with water harvesting). It is concluded that nutrients tend to be conserved in semi-arid agroforestry. Water competition is often reported but there is also scope for water conservation linked to agroforestry. Intensification is needed in the semi-arid tropics because of population pressure, but risk for competition increases as system intensity increases. Ten long-term alley farming - a very resource-use intensive system - experiments were reviewed. It was found that 54% of the 28 "best bet" combinations in these studies resulted in crop yields equal to or higher than sole crop control. These results show that underutilized niches exist and could be developed to increase productivity. Two main options for minimizing competition are: 1) use of more appropriate tree species and 'design' and 2) better tree management. Elements as regards design include broadening the species choice and thereby tree traits (quality of tree litter, resistance to repeated pruning, etc.); to reduce tree-crop, interface (increase distance between tree rows and reduce degree of system intensity); practice biomass transfer followed by mulching and, finally; agroforestry combined with water conservation. Tree management options include timely coppicing and root pruning.