© 1996 Heron Publishing—Victoria, Canada
Sap flow measurements of lateral tree roots in agroforestry systems
J. E. Lott (1, 2), A. A. H. Khan (1), C. K. Ong (1) and C. R. Black (2)
1. International Centre for Research in Agroforestry (ICRAF), Nairobi, Kenya / 2. Department of Physiology and Environmental Science, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD,
U.K. / Received October 25, 1995
Summary
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.
Keywords:
Grevillea robusta, heat balance sap flow gauge, water use.
