Methods to Estimate Sap Flow

A variety of methods is available to calculate mass flow of water in the transpiration stream by using heat as a tracer. Measurements can be taken both in herbaceous and woody plants and in any conductive organ, including roots. Depending on the method, measurements are taken in the part of the conductive organ where the sensors are located, or in the whole perimeter of the conductive organ. Some methods integrate the sap flow in the whole sapwood, while others give information to calculate sap flow at different depths below the cambium. Some methods are invasive, since the sensors are located within the sapwood; others are non invasive, being the sensors located outside, although in close contact, of the conductive organ. Some methods are suitable for stems of small diameters, while others can be used in large trees. Calibration is convenient in all cases, being compulsory for the invasive methods, since the insertion of the probes alters the xylem characteristics. Some methods measure continuously, while others measure discontinuously. Some cannot measure below a threshold of heat velocity, while others are useful for measuring very low sap flow values, even those occurring during reverse flow. The user, therefore, must be aware of the potential and limitations of the different available methods before choosing the most suitable for his/her purposes. Below we mention the existing methods for estimating sap flow from heat measurements, together with references to key papers where details on the fundamentals and performance of each method are given (see also review papers by Campbell 1991, Swanson 1994, Cermak 1995, Smith and Allen 1996, Kostner et al., 1998, Cermak and Nadezhdina 1998, and others cited in the “Key papers” Section).