Revisiting the Heat Field Deformation (HFD) method for measuring sap flow

Abstract

The Heat Field Deformation (HFD) technique is a thermodynamic method for measuring sap flow. Based on continuous heating the HFD method allows for high time resolution measurements which are highly important when studying plant responses to abrupt environmental changes. This work provides a succinct review of previously described features of the HFD methodology. Analyzing symmetrical and asymmetrical temperature differences around a measured linear heater (dTsym and dTas) relative to their ratio dTsym/dTas (so called a K-diagram) is at the heart of this methodology. This key concept, however, has to date only been generally described in previous works on the HFD technique. My objective here is to provide a comprehensive overview describing different types of K-diagrams, their interpretation and application for determining Kvalues or dTas for a zero flow condition. The K-value is a measured parameter which is particularly important for objectively characterizing heat conducting properties at the sensor insertion point under specific local measurement conditions. Correctly determining the K-value is critical for accurately calculating sap flow based on recorded temperature measurements. I have included in this review several examples demonstrating how the K-value is dependent upon changes to the environment and its important role in sap flow estimation. Keywords: K-diagram, K/R-diagram, K-value, Sap Flow per Section, Sap Flux Density, Sensor IntroductionThermodynamic methods for measuring sap flow are widely used in forestry, hydrology, agronomy and eco-physiological studies to accurately estimate tree water use, to identify specific drought stress conditions or to understand plant functionality. These methods which depend on heat transfer, however, differ fairly widely in their capabilities and limitations, notably in their sensitivity to low, zero and reverse flow rates. Reverse sap flow is particularly important for understanding the whole plant physiology and tree architecture, and acts as an indicator of hydraulic redistribution (Nadezhdina 1999, 2010, Burgess et al. 1998. In thermodynamic methods, thermometers are commonly used to gauge temperature changes of the heat field around a heater in response to sap movement. Thermometers may be arranged around the heater either symmetrically or asymmetrically. For symmetrical measurements, they are placed above and below and at equal distance from the heater in the axial direction (Daum 1967, Vieweg & Ziegler 1960, Sakuratani 1981, Tikhov 1979, Nadezhdina 1989, 1999, Burgess et al. 2001, while for asymmetrical measurements, the above thermometer is substituted by one placed together with the heater. (Ittner 1968, Cermák et al. 1973, Granier 1985.The majority of methods using thermometers arranged symmetrically around a heater produce accurate sap flow measurement with good dynamic resolution with the true sap flow under low flow conditions (Vieweg & Ziegler 1960, Leyton 1970, Saddler & Pitman 1970, Nadezhdina 1999. The situation can be different, however, under hig...

Below-ground hydraulic constraints during drought-induced decline in Scots pine

“…Data processing of heat dissipation sensor output included correction for natural temperature gradients in the stem, and zero flow determination was based only on nights with low and stable evaporative demand (Poyatos et al 2013). Sap flow density calculation in the outer sapwood followed the original calibration (Granier 1985) and was then integrated over the entire sapwood using radial profiles of sap flow (six depths) measured in three trees using the heat field deformation method (Nadezhdina 2018), during at least 7 days per tree. This sap flow per unit sapwood area was expressed on a leaf area basis (J L ) after estimation of tree leaf area using site-specific allometries for Scots pine and accounting for seasonal variations of leaf area (see Poyatos et al 2013 for additional methodological details).…”

What We Know and What We Do Not Know about Dragon Trees?

“…The research was based on analyses of sap flow response of in-situ measured adult trees to wide-scale variations in environmental conditions and to several treatments of young plants growing ex-situ. The same sap flow method, namely the Heat Field Deformation (HFD) method [121][122][123] was used, enabling the exploration of radial sap flow variations, high time resolution, and bidirectional flow rates.…”

Tree-water interactions at varying spatiotemporal scales in a water limited environment

“…A set of methodologies have been proposed to extrapolate sap flow data from single trees to higher spatial scales (Hatton and Wu, 1995;Čermák and Nadezhdina, 1998;Köstner et al, 1998;Cermák et al, 2004). These methods are generally known as "scaling-up" methodologies (Lubczynski, 2009) and have the following aspects in common: (1) they require the application of an appropriate sampling methodology to account for the tree size variability when measuring the sap flow, (2) they establish a mathematical relation (called a scaling-up function) of a scaled-up parameter (sapwood area or sap flow) with an easily measurable tree characteristic (called a scalar), and finally (3) they measure scalars at spatial scales larger than the scales of data acquisition to be converted to scaled parameter applying a scaling-up function.…”

“…flowing water in conductive tissues of plants. The sap flow methods are typically applied to understand the wateruse of a diversity of plants, but specifically to estimate transpiration fluxes from trees (Granier, 1987;Köstner et al, 1998;Oltchev et al, 2002b;Lubczynski, 2009) in various fields such as: plant physiology (Nadezhdina, 1999;Čermák et al, 2000Nadezhdina et al, 2009), forest management (Čermák et al, 1998;Köstner et al, 1998;Wilson et al, 2001;Hernández-Santana et al, 2008a), hydrology (Lubczynski, 2000;David et al, 2004;Lubczynski and Gurwin, 2005), ecology (Wullschleger et al, 2001;Burgess and Dawson, 2004), etc. However, there is a general concern about the accuracy and adequacy of these methods when used to quantify the actual amount of water used by plants or in higher levels of organization such as tree stands or catchments.…”

“…The HFD method has three advantages as compared to TDP, it can better detect night flow, it provides radial variability of J p , in current implementation up to 8 cm of xylem depth and it measures bi-directional flows (Nadezhdina et al, 1998;Čermák et al, 2004). The HFD method has already been used in studies involving species from the Quercus genera (David et al, 2007;Poyatos et al, 2007;Nadezhdina et al, 2008), showing good results.…”