SAP FLOW MEASUREMENTS OF LATERAL TREE ROOTS IN AGROFORESTRY SYSTEMS

Successful extension of agroforestry to areas of the semi-arid tropics where deep reserves of water exist requires that the tree species be complementary to the associated crops in their use of water within the crop rooting zone. However, it is difficult to identify trees suitabl e for dryland agroforestry because most existing techniques for determ ining water uptake by roots cannot distinguish between absorption by t ree and crop roots. We describe a method for measuring sap flow throug h lateral roots using constant temperature heat balance gauges, and th e application of this method in a study of complementarity of water us e in agroforestry systems containing Grevillea robusta A. Cunn. Sap fl ow gauges were attached to the trunks and roots of Grevillea with mini mum disturbance to the soil. Thermal energy emanating from the soil ad versely affected the accuracy of sap flow gauges attached to the roots , with the result that the uncorrected values were up to eightfold gre ater than the true water uptake determined gravimetrically. This overe stimation was eliminated by using a calibration method in which noncon ducting excised root segments, with sap flow gauges attached, were pla ced adjacent to the live roots. The power consumption and temperature differentials of the excised roots were used to correct for external s ources and internal losses of heat within the paired live root. The fr action of the total sap flow through individual trees supplied by the lateral roots varied greatly between trees of similar canopy size. Exc ision of all lateral roots, except for one to which a heat balance gau ge was attached, did not significantly increase sap flow through the i ntact root, suggesting that it was functioning at near maximum capacit y.