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 suitable for dryland agroforestry because most existing techniques for determining water uptake by roots cannot distinguish between absorption by tree and crop roots. We describe a method for measuring sap flow through lateral roots using constant temperature heat balance gauges, and the application of this method in a study of complementarity of water use in agroforestry systems containing Grevillea robusta A. Cunn. Sap flow gauges were attached to the trunks and roots of Grevillea with minimum disturbance to the soil. Thermal energy emanating from the soil adversely affected the accuracy of sap flow gauges attached to the roots, with the result that the uncorrected values were up to eightfold greater than the true water uptake determined gravimetrically. This overestimation was eliminated by using a calibration method in which nonconducting excised root segments, with sap flow gauges attached, were placed adjacent to the live roots. The power consumption and temperature differentials of the excised roots were used to correct for external sources and internal losses of heat within the paired live root. The fraction of the total sap flow through individual trees supplied by the lateral roots varied greatly between trees of similar canopy size. Excision of all lateral roots, except for one to which a heat balance gauge was attached, did not significantly increase sap flow through the intact root, suggesting that it was functioning at near maximum capacity.