Calibrating sapflow sensors has become increasingly important to accurately estimate plant water use. A number of calibration approaches exist in the literature and fundamental differences among them suggest they may not provide the same estimates of sapflow; however, these fundamental differences have never been explicitly discussed or tested. Here we compare the two most common calibration methods—gravimetric and potometer. The gravimetric method involves using an excised stem section without leaves or branches and pushes water through the stem using gravity, a positive force. The potometer method requires severing a tree stem at its base, placing it upright with the exposed stem in a reservoir, and allowing it to pull water through the stem via transpiration, a negative force. Gravimetric and potometer sapflow parameters were determined using Eucalyptus benthamii trees (N=12). Conducting sapwood area was also determined during each calibration method. Additional independent trees (N=3) were used to compare actual plant water use with water use calculated using calibrated sapflow parameters. Calibrated sapflow parameters were also applied to intact trees (N=5) to compare how applying sapflow parameters from each calibration methods effect cumulative water use calculated over a 31-day period.
Results/Conclusions
We found that the different calibration approaches resulted in different sapflow parameters. Conducting area was 11% larger for gravimetric calibration than for potometer calibration. Sapflow estimated from gravimetric parameters underestimated sapflow, accounting for only 28% of actual sapflow, whereas potometer parameters yielded estimates similar to actual sapflow. These differences in estimated sapflow between approaches cannot be fully attributed to differences in conducting area during calibration, suggesting additional factors affect sapflow parameter calculation. When calibration parameters were applied to intact trees, cumulative sapflow calculated with potometers parameters was 448% lager than cumulative sapflow calculated with gravimetric parameters. Our results indicate that calibration method influences sapflow estimates. Future research should further explore physical and physiological reasons why these calibration approaches differ and seek to develop standardized calibration protocol that can be used within the sapflow community.