Tree–crop interactions: manipulation of water use and root function

This paper describes recent research findings on tree–crop interactions in the semiarid tropics focusing on the potential of agroforestry systems to improve the efficiency with which land and water are currently used; the trade-offs between crop productivity and environmental function, and strategies to manipulate tree root function. There is strong evidence that agroforestry has potential for improving water use efficiency by reducing the unproductive components of the water balance, i.e. run-off, soil evaporation and drainage. Examples from India and Kenya show that simultaneous agroforestry systems could double rainfall utilisation compared to annual cropping systems, largely due to temporal complementarity. Where soil loss through erosion is a serious problem, contour hedgerows can provide a viable alternative to conventional soil conservation measures. However, even though soil losses can be dramatically reduced, whether beneficial effects on crops will develop is often unpredictable and usually insufficient to attract widespread adoption of contour hedges. Strategies to reduce the trade-offs between crop and tree interactions or environmental function include the use of high value trees or trees which provide direct benefits to farmers.Recent reviews on root research indicate that there appears to be limited scope for spatial differentiation in rooting between trees and crops (i.e. spatial complementarity) in water-limited environments, unless ground-water is accessible to tree roots. Instead, it is argued that it is more worthwhile to manage below-ground competition by shoot and root-pruning. Pruning of lateral roots could redirect root function and be a powerful tool for improving spatial complementarity, provided that there are adequate resources at depth. However, the downward displacement of functional tree roots following root-pruning must not be allowed to affect their safety net role in the interception of nutrients leaching from the zone of crop rooting and the long-term hydrological implica

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DOI:

https://doi.org/10.1016/S0378-3774(01)00163-9

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