Potential errors in measurement of nonuniform sap flow using heat dissipation probes

Abstract

The empirical calibration of Granier-type heat dissipation sap flow probes that relate temperature difference (DeltaT) to sap velocity (v) was reevaluated in stems of three tropical tree species. The original calibration was confirmed when the entire heated probe was in contact with conducting xylem, but mean v was underestimated when part of the probe was in contact with nonconducting xylem or bark. Analysis of the effects of nonuniform sap velocity profiles on heat dissipation estimates showed that errors increased as v and the proportion of the probe in nonconducting wood increased. If half of a 20-mm probe is in sapwood with a v of 0.15 mm s(-1) and the other half is in nonconducting wood, then mean v for the whole probe can be underestimated by as much as 50%. A correction was developed that can be used if the proportion of the probe in nonconducting wood is known. Even with the entire heated probe in contact with conducting xylem, v would be underestimated when radial velocity gradients are present. In this case, the error would be smaller except when velocity gradients are very steep, as can occur in species with ring-porous wood anatomy. Errors occur because the relationship between DeltaT and v is nonlinear. Mean DeltaT along the probe is therefore not a measure of mean v, and users of heat dissipation probes should not assume that v is integrated along the length of the probe. The same type of error can occur when DeltaT is averaged through time while v is changing, but the error is small unless there are sudden, step changes between zero and high sap velocity. It is recommended that relatively short probes (20 mm or less) be used and that probes longer than the depth of conducting sapwood be avoided. Multiple probes inserted to a range of depths should be used in situations where steep gradients in v are expected. If these conditions are met, heat dissipation probes remain useful and widely applicable for measuring sap flow in woody stems.

Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies

“…al., 2008) which investigated radial patterns and changes in sapwood area with tree size of the same tree species at the same forest site. No corrections of sap fluxes as suggested by Clearwater et al (1999) were necessary for our study trees.…”

Response of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits under Mediterranean mountain climate

“…I). Probes installed in pines were 20 mm long, whereas those used in oaks were 10 mm to minimize the errors due to steep gradients in sap flow density along the depth of the sensor [8].…”

Evaluation of heat balance and heat dissipation methods for sapflow measurements in pine and spruce

“…As this necessitates a large number of sensors of variable length, an acceptable compromise would be to distribute two or more 2 cm long sensors throughout the entire depth of the sapwood [30]. It has been suggested that measurements with sensors that partly cover the conducting xylem should be scaled using a correction factor [13,25]. The relation between sapflow density at different depths can be used to estimate a linear relationship that can be used to scale measurements of sapflow density in trees where only singledepth sensors are used.…”

“…A cut-tree comparison on grapevine (Vitis vinifera L.) [2] and a lysimeter study on mango (Mangifera indica L.) [29] showed good agreement with the original calibration at flow rates up to 225 g m -2 s -1 . Clearwater et al [13] found that heat dissipation probes used with the original Granier calibration underestimated water flow through excised stems from several species if the probe was in partial contact with non-active xylem. The empirical calibration was re-evaluated and confirmed in stems of three tropical species, when the entire sensor was in contact with conducting xylem.…”