The compensation heat pulse method (CHPM) is of limited value for measuring low rates of sap flow in woody plants. Recent application of the CHPM to woody roots has further illustrated some of the constraints of this technique. Here we present an improved heat pulse method, termed the heat ratio method (HRM), to measure low and reverse rates of sap flow in woody plants. The HRM has several important advantages over the CHPM, including improved measurement range and resolution, protocols to correct for physical and thermal errors in sensor deployment, and a simple linear function to describe wound effects. We describe the theory and methodological protocols of the HRM, provide wound correction coefficients, and validate the reliability and accuracy of the technique against gravimetric measurements of transpiration.
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.
Abstract. Grevillea (GreviUea robusta A. Cunn.; Proteaceae) is used in agroforestry in many areas of the highlands of East and Central Africa, and is reported to be mainly deep rooted, with few shallow roots and correspondingly low levels of competition with associated crops for water and nutrients. To examine the extent of below-ground complementarity in water use between grevillea and cowpea (Vigna unguiculata L.; Leguminosae), experiments were carried out at the International Centre for Research 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 order 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.
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