USING SAP FLOW GAUGES TO QUANTIFY WATER-UPTAKE BY TREE ROOTS FROM BENEATH THE CROP ROOTING ZONE IN AGROFORESTRY SYSTEMS

Grevillea (Grevillea robusta A. Cunn.; Proteaceae) is used in agrofore stry in many areas of the highlands of East and Central Africa, and is reported to be mainly deep rooted, with few shallow roots and corresp ondingly low levels of competition with associated crops for water and nutrients. To examine the extent of below-ground complementarity in w ater use between grevillea and cowpea (Vigna unguiculata L.; Leguminos ae), experiments were carried out at the International Centre for Rese arch in Agroforestry (ICRAF) Field Centre at Machakos, Kenya. Sap flux was measured using heat balance gauges attached to the stems of young grevillea trees (10-18 months old), both before and after excavating the crop rooting zone (upper 60 cm of soil) around the stem base, in o rder to establish the capacity of the grevillea to extract water from below this zone. After excavation, the trees maintained sap fluxes of up to 85% of the unexcavated values, suggesting a high degree of below -ground complementarity.