Measurement of sap flow in plant stems

Transpiration rates for whole plants, individual branches or tillers can be determined by techniques which measure the rate at which sap ascends stems. All of these methods use heat as a tracer for sap movement, but they are fundamentally different in their operating principles. Two methods commonly employed, the stein heat balance and trunk sector heat balance methods, use the heat balance principle; the stem is heated electrically and the heat balance is solved for the amount of heat taken up by the moving sap stream, which is then used to calculate the mass flow of sap in the stem. In the heat-pulse method, rather than using continuous heating, short pulses of heat are applied and the mass flow of sap is determined from the velocity of the heat pulses moving along the stem. In addition, rates of sap flow can be determined empirically, using the thermal dissipation technique, from the temperature of sapwood near a continuously-powered heater implanted in the stem. Users must understand the theory underlying each of these methods, so that they can select the method most appropriate to their application and take precautions against potential sources of error. When attempting to estimate transpiration by stands of vegetation from measurements of sap flow in individual plants, users must also select an appropriate sampling strategy and scaling method.