Sap Flow & Dynamic Stem Water Content Measurements.

 

Most researchers take great care when collecting sap flow data. However, one critical source of error is rarely addressed in peer-reviewed literature: sapwood moisture content, also referred to as stem water content.

Key parameters influencing sap flow measurements are inherently dynamic—varying on daily, weekly, and seasonal timescales in response to plant physiological processes and environmental conditions. Despite this, many researchers treat these parameters as static, often measuring them only once at the beginning or end of a study.

This issue is particularly problematic for heat pulse velocity (HPV) sap flow methods including the Dual Method Approach (DMA), heat ratio (HRM), Tmax, and compensation heat pulse (CHP). Sapwood moisture content is highly variable, changing not only throughout the day but also in response to irrigation regimes, environmental changes, and experimental treatments. Nonetheless, in nearly all published HPV studies, this parameter is measured only once.

The Implexx Sap Flow Sensor offers a solution. It is capable of simultaneously measuring sap flow and stem water content, thereby enabling a more dynamic and accurate approach to sap flow quantification. This article discusses how sapwood water content is commonly treated as a static parameter and presents a method to overcome this limitation.

 

Static vs. Dynamic Parameters in Sap Flow Equations.

A common equation used to calculate sap flux density (SFD; cm³/cm²/hr) is as follows:

SFD Equation:

SFD = Vc × [((cd × pd) + (cw × pw × mc)) / (pw × (1 + mc))]

Where:

  • Vc = heat velocity

  • pd, pw = basic densities of dry wood and water, respectively (kg/m³)

  • cd, cw = specific heat capacities of dry wood and sap (J/kg/°C)

  • mc = sapwood water content (kg/kg)

In this equation, Vc is directly measured and inherently dynamic.
pd and pw are treated as constants, with pw typically assumed to be 1000 kg/m³.
cd and cw are also often treated as constants (1200 and 4182 J/kg/°C, respectively), but these values are temperature-dependent. Becker and Edwards (1999) showed that cd can range between 1100 and 1350 J/kg/°C from 0 to 50°C, whereas cw remains relatively stable.

The most critical and variable parameter, however, is mc. It is well documented that sapwood moisture content varies diurnally, seasonally, and under different environmental conditions. For example, Constantz and Murphy (1990) reported a 67% change in mc in Aesculus californica. Yet, most studies only measure mc once, despite its demonstrated variability.

This treatment of dynamic parameters—particularly mc—as static introduces uncertainty into sap flow estimations. The full implications of this practice remain unquantified, but existing studies suggest the impact may be significant.

 

The Role of Stem Water Content in Measurement Accuracy.

Steppe et al. (2010) evaluated the accuracy of three sap flow techniques and found that all methods underestimated true sap flow by 35–60%. Through a parameter-specific error analysis, they identified sapwood moisture content (specifically, fresh weight) as the dominant source of error for the heat pulse method.

Lopez-Bernal et al. (2014) also highlighted this issue. Using desorption curves, they demonstrated that fluctuations in mc significantly influence thermal diffusivity and sap flux density. They concluded that “ignoring seasonal and daily variations in [mc] might result in large errors in calculated sap flux.”

 

A More Dynamic Approach to Sap Flow Monitoring.

To address these limitations, the Implexx Sap Flow Sensor provides a means to continuously measure heat velocity, sap flux density, and stem water content. This allows researchers to incorporate real-time values into their calculations, improving accuracy.

The Implexx Sap Flow Sensor enables the derivation of additional parameters—such as specific heat capacity and maximum temperature rise—used in the thermal conductance and convection equations originally proposed by Marshall (1958). These data outputs support dynamic calculation of sap flow and related physiological variables.

 

Further Reading.

 

References.

Becker, P.; Edwards, W.R.N. Corrected heat capacity of wood for sap flow calculations. Tree Physiology 1999, 19, 767-768.

Constantz, J. and F. Murphy. 1990. Monitoring moisture storage in trees using time domain reflectometry. J. Hydrology 1990, 119:31--42.

López-Bernal, Á.; Alcántara, E.; Villalobos, F.J. Thermal properties of sapwood fruit trees as affected by anatomy and water potential: errors in sap flux density measurements based on heat pulse methods. Trees 2014, 28, 1623–1634.

Marshall, D.C. Measurement of sap flow in conifers by heat transport. Plant Physiology1958, 33, 385–396.

Steppe, K.; De Pauw, D.J.W.; Doody, T.M.; Teskey, R.O. A comparison of sap flux density using thermal dissipation, heat pulse velocity and heat field deformation methods. Agricultural and Forest Meteorology 2010, 150, 1046–1056.