Instruction manual

9
At this point, we note that
a priori
the wound size is not known, although we might
expect it to be a little larger than the size of the drill hole. This is because of additional
damage which results from mechanical disruption of vessels at the edges of the drill hole.
Anatomical investigations by Barrett et al. (1995) indicated the total wound width is
likely to extend about 0.3 mm either side of the drill hole. Thus, a wound correction of
(1.8 + 2
×
0.3) mm seems appropriate for a drill hole of 1.8 mm (as used here). We have
used this wound width to successfully validate the heat-pulse method in apple, but found
a larger wound correction of some 3.2 mm was required for kiwifruit vines to bring the
heat-pulse measurement into line with actual flow rates (Green and Clothier, 1988).
Kiwifruit have very large xylem vessels and a substantial interstitial area of woody matrix
which affects the thermal homogeneity of the sapwood and, therefore, affects the
transmission and measurement of heat-pulse in kiwifruit. Our results (Green and Clothier,
1988) and the results of Barrett et al. (1995) show the importance of validating the heat-
pulse method in order to determine the appropriate wound factor. Such a validation is
useful, and sometimes necessary, in order to reach a good level of competence and
confidence in using the heat-pulse method.
We also issue a practical warning: if due care is not taken to align the centers of the drill
holes with the longitudinal direction of the sap flow, then the actual wound width could
well be higher than expected. In practice, we believe the 2.4 mm wound correction to be
the lower limit appropriate for probes of 1.8 mm diameter. We also stress the importance
of
being careful
when placing sensors in the tree stem since the calculations of total sap
flow are very sensitive to the wound factor.
A final point on the wound corrections: if the probes are left in place for an extended
period, e.g. many months, then the initial wounding may be expected to increase. This is
because of increased blockage due to tylose formation in the vessels close to the probes,
which may occur as the tree reacts to further isolate the ‘wound’. An increased wound
reaction could reduce the sensitivity of the sap flow measurements. However, we have
never experienced a significant decrease in the heat-pulse sensitivity over time, in either