Sap-flux density measurement methods: working principles and applicability

Abstract

Sap-flow measurements have become increasingly important in plant science. Since the early experiments with dyes, many methods have been developed. Most of these are based on the application of heat in the sapwood which is transported by the moving sap. By measuring changes in the temperature field around the heater, sap flow can be derived. Although these methods all have the same basis, their working principles vary widely. A first distinction can be made between those measuring the sap-flow rate (g h–1) such as the stem heat balance and trunk sector heat balance method and those measuring sap-flux density (cm3 cm–2 h–1). Within the latter, the thermal dissipation and heat field deformation methods are based on continuous heating, whereas the compensation heat pulse velocity, Tmax,heat ratio, calibrated average gradient and Sapflow+ methods are based on the application of heat pulses. Each of these methods has its advantages and limitations. Although the sap-flow rate methods have been adequately described in previous reviews, recent developments in sap-flux density methods prompted a synthesis of the existing but scattered literature. This paper reviews sap-flux density methods to enable users to make a well founded choice, whether for practical applications or fundamental research questions, and to encourage further improvement in sap-flux density measurement techniques

How Reliable Are Heat Pulse Velocity Methods for Estimating Tree Transpiration?

“…Another popular sap flow method, the thermal dissipation or Granier method, has also been demonstrated to underestimate transpiration with as much, or more, error than HPV methods [9,24,43]. Rather, this review is a caution to scientists to not rely on theoretical calculations of transpiration from HPV or any sap flow method.…”

“…Four of the seven theoretical methods have been extensively tested and include the compensation heat pulse method (CHPM, [10]), heat ratio method (HRM, [11,12]), T-max method [13], and Sapflow+ method [14]. The methods have been reviewed extensively elsewhere [9,12,[15][16][17][18]. Briefly, following a heat pulse, the CHPM measures the time for temperatures to be equal in sensors installed upstream and downstream to the heater probe.…”

“…HPV sensors are popular for sap flow measurements due to relatively low costs, low power requirements, and low maintenance [9]. A HPV sensor consists of a heater probe and one or more temperature probes inserted radially into the xylem.…”

“…In xylem, thermal convection is the sap movement and thermal diffusivity is the 'natural' movement of heat through cell walls, intercellular space, and fluids. For the CHPM, thermal diffusivity is compensated via the probe configuration whereas thermal diffusivity must be explicitly estimated for the T-max, HRM, and Sapflow+ methods [9].…”

“…There are various types of sap flow sensors, based on theoretical and empirical methods, but all of them rely on measuring temperature changes in the xylem following the application of heat [7,9]. Sap flow measurements are mostly conducted with sensors based on thermal dissipation, heat balance, or heat pulse velocity (HPV) methods [7].…”