Transient thermal dissipation method for xylem sap flow measurement: implementation with a single probe.

Comparisons of tree water relations between treatments, species and sites are facilitated by the use of simple and low-cost measurements of xylem sap flow rates. The transient thermal dissipation (TTD) method is a variant of the constant thermal dissipation (CTD) method of Granier. It has the advantages of limiting thermal interference and of saving electrical energy. Here, our concern was to test a new step towards simplicity and low cost: the applicability of the TTD method with a single probe, i.e., without a reference sensor, following a cycle of 10 min heating and 10 min cooling, and using the same thermal index and multi-species calibration previously assessed with a dual probe. First, the responses of the dual and single probes were compared in an artificial hydraulic column of sawdust in the laboratory over a complete range of flux densities, from 0.3 to 4.0 l dm⁻² h⁻¹. Second, diurnal kinetics were compared in a young tree with rapid changes in the sapwood reference temperature of up to 5 °C h⁻¹ for 5 consecutive days. With a relatively stable reference temperature, laboratory results showed that a single probe yielded the same temperature signal and thermal index as a dual probe for the full range of sap flux densities. Within the tree, the cooled temperature of the heated probe, linearly interpolated, proved to be an accurate indicator of the change in the reference temperature over time. Logically, the temperature signals and estimates of sap flux density with the single probe did not differ from the dual-sensor measurements when the cooled temperature was interpolated. Additionally, the responses of the thermal index, yielded in the hydraulic experiment with the sawdust column, fell within the variability of the multi-species calibration. This result supports the previous assessment of a non-species-specific calibration for the TTD method with diffuse porous media. In conclusion, our results showed that the TTD method can be directly applied with a single probe. Limitations and possible future progress are pointed out. This measurement system is probably the simplest technique currently available to measure xylem sap flow.

Transient thermal dissipation method of xylem sap flow measurement: multi-species calibration and field evaluation.

The transient thermal dissipation (TTD) method developed by Do and Rocheteau (2002b) is a close evolution of the original constant thermal dissipation (CTD) method of Granier (1985). The TTD method has the advantage of limiting the influence of passive natural temperature gradients and of yielding more stable zero-flux references at night. By analogy with the CTD method, the transient method was first calibrated on synthetic porous material (sawdust) on the assumption that the relationship was independent of the woody species. Here, our concern was to test the latter hypothesis with a 10-min heating time in three tropical species: Hevea brasiliensis Müll. Arg., Mangifera indica L. and Citrus maxima Merr. A complementary objective was to compare the field estimates of daily transpiration for mature rubber trees with estimates based on a simplified soil water balance in the dry season. The calibration experiments were carried out in the laboratory on cut stems using an HPFM device and gravimetric control of water flow up to 5 L dm(-2) h(-1). Nineteen response curves were assessed on fully conductive xylem, combining 11 cut stems and two probes. The field evaluation comprised five periods from November 2007 to February 2008. Estimates of daily transpiration from the measurement of sap flow were based on the 41 sensors set up on 11 trees. Soil water depletion was monitored by neutron probe and 12 access tubes to a depth of 1.8 m. The calibrations confirmed that the response of the transient thermal index to flow density was independent of the woody species that were tested. The best fit was a simple linear response (R(2) = 0.88, n = 276 and P < 0.0001). The previous calibration performed by Do and Rocheteau (2002b) on sawdust fell within the variability of the multi-species calibration; however, there were substantial differences with the average curve at extreme flow rates. Field comparison with soil water depletion in the dry season validated to a reasonable extent the absolute estimates of transpiration acquired with the 10-min TTD method. In conclusion, evidence for the independence of calibration from woody species and the simple linear response of the thermal index strengthen the interest of the TTD method with 10-min heating.