Using branch and basal trunk sap flow measurements to estimate whole-plant water capacitance: a caution

Abstract

Thermometric sap flow sensors are widely used to measure water flow in roots, stems and branches of plants. Comparison of the timing of flow in branches and stems has been used to estimate water capacitance of large trees. We review studies of sap flow in branches and present our own data to show that there is wide variation in the patterns and timing of sap flow of branches in different parts of the crown, owing to the course of daily solar illuminance. In contiguous forest, east-facing and upper branches are illuminated earlier than west-facing and lower branches and most capacitance studies do not include adequate information about branch sampling regimes relative to the overall pattern of crown illuminance, raising questions about the accuracy of capacitance estimates. Measuring only upper branches and normalising these results to represent the entire crown is dangerous because flows at the stem base likely peak in response to maximum crown illuminance (and transpiration) and this will differ compared to the timing of peak flows in upper branches. We suggest that the magnitude of flow lags between branches and stems needs further study, with careful attention to branch position and method application before a robust understanding of capacitance, particularly in woody tissues of large trees, can be formed. We did not detect flow lags in the world's tallest and largest tree species Sequoia sempervirens and Sequoiadendron giganteum, despite measurement along large pathlengths (∼57 and 85 m), which raises questions as to why large flow lags are often recorded for much smaller species. One conspicuous possibility is the different methods used among studies. Constantheating methods such as the thermal dissipation probe (and also heat balance methods) include heat capacitance behaviour due to warming of wood tissues, which delays the response of the sensors to changing sap flow conditions. We argue that methods with intrinsic heat-capacitance present dangers when trying to measure water-capacitance in trees. In this respect heat pulse methods hold an advantage.

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

“…For example, Doronila and Forster [57] demonstrated how peak diurnal sap flow differed across three co-occurring Eucalyptus species in response to heat waves, vapor pressure deficit (VPD), and temperature. Burgess and Dawson [58] used normalized sap flow rates to determine the capacitance of trees. In these and similar studies, the relative, directional rate, or timing of sap flow was of primary interest and negated the need for sensor calibration.…”

Observations of stem water storage in trees of opposing hydraulic strategies

“…It follows that the role of storage in both the hydraulic limitation of stomatal aperture as well as the response to soil conditions is nontrivial. These complexities make storage a critical, dynamic and challenging variable to estimate (Burgess andDawson 2008, Phillips et al 2009). …”

Use of the heat dissipation method for sap flow measurement in citrus nursery trees1

“…Although the leaves did not show any symptoms of wilting, the stem injuries probably affected sap flow and significantly altered measurements, resulting in a potential constraint for this method. Trunk or branch segments with diameters of less than 5 cm are usually not used for the heat dissipation method, as a result of both inadequate determination of effective xylem section and greater influence of the natural thermal gradient (CERMÁK et al, 2004;JONES, 2004;DELGADO-ROJAS et al, 2007;BURGESS;DAWSON, 2008). Sap flow measurements may also be influenced by the relation between the interfered section by the probes and the trunk section area.…”

“…Crops physiological variables measured in vivo are potential monitoring methods for irrigation scheduling (JONES, 2004). Sap flow measurement is among the most promising techniques and has been particularly studied in citrus species to estimate transpiration rates and to indicate water relations and drought effects (ORTUÑO et al, 2004a(ORTUÑO et al, , 2006ALARCÓN et al, 2005;DELGADO-ROJAS et al, 2007). These methods led to satisfactory comparisons with alternatives, such as trunk diameter fluctuations (ORTUÑO et al, 2004b), despite limitations which included errors inherent to sampling, scaling up of data to entire plant populations, strong influence of atmospheric conditions on sap flow, and need of constant calibrations (CERMÁK et al, 2004;JONES, 2004;BURGESS;DAWSON, 2008).…”