SUBSTANTIAL ERRORS IN ESTIMATES OF SAP FLOW USING THE HEAT BALANCE TECHNIQUE ON WOODY STEMS UNDER FIELD CONDITIONS

The heat balance method was used to estimate transpirational sap flow through 60- to 75-mm-diameter stems (trunks) of 3-year-old peach (Prunus persica (L.) Batsch. cv. O'Henry) trees under field conditions. On rare occasions, heat balance estimates agreed well with independent lysimetric measurements, but on most occasions, heat balance estimates of sap flow were unrealistic in both direction and magnitude. In some cases, the errors in sap flow approached two orders of magnitude and were always the result of a calculation involving division by a very small and sometimes negative temperature differential between the stem surface temperature above and below the gauge heater. The occurrence of negative temperature differentials under positive transpiration conditions may be incon- sistent with a fundamental assumption in the heat balance model, namely that temperature differentials are solely a consequence of the dissipation of energy supplied to the gauge heater. In the absence of heating power applied to the gauge, temperature differentials exceeding - 1C were correlated with the rate of change in stem temperature, indi- cating that ambient conditions themselves can impose a bias in gauge signals and, hence, influence gauge accuracy. Our results suggest that the effect of ambient conditions on gauge signals should be critically evaluated before considering heat balance estimates of sap flow as reliable under any given conditions. Some recent papers have reported accurate in situ estimates of sap flow (transpiration) through both herbaceous (Ham and Heilman, 1990) and woody (Steinberg et al., 1990b) plant stems using the heat balance technique. The most recent development of this technique has been based on a constant heating gauge described by Sakuratani (1981, 1984) and further modified by Baker and Van Bavel (1987). In most of these reports, the theory of operation for the gauge itself has been described, but only in terms of the numerical calculations needed to convert gauge signals (largely thermojunction-measured temperature differentials) into sap mass flow. Some of the assumptions un- derlying the heat balance technique have been evaluated with a finite element model (Baker and Nieber, 1989) that showed that gauge accuracy may be influenced by factors such as stem vas- cular anatomy, with potentially greater accuracy in dicots than in monocots. The same analysis indicated that overall gauge accuracy could be relatively good, however, depsite the pres- ence of systematic errors in estimates of some of the heat bal- ance components. In view of the accuracy reported in some studies (Steinberg et al., 1989) and reproducibility reported in others (Heilman et al., 1989), it is reasonable to assume that heat balance estimates of sap flow are reliable and that errors, if present, are of minor significance. One class of errors that has not been considered, however, are those associated with the measurement of the temperature differential on the stem surface from below to above the gauge heater. Baker and Nieber (1989) recognized that this temperature differential is critical in the calculation of sap flow, but also that errors in its determination could not be formally considered within the theoretical model of gauge function. In effect, the heat balance theory is limited to considering only temperature differentials that result from heat energy applied to the stem itself. Since large and diurnally fluctuating differences between soil and air temperature are commonplace under many field conditions, and since the plant represents a bridge between these two thermal regimes, it is reasonable to examine to what extent such environmental tem- perature differentials lead to differentials within the plant stem, and the impact of these on the measurement of sap flow. This study documents that such differentials do occur and may be potentially large enough to cause substantial errors in the mea- surement of sap flow by the heat balance technique.