Measuring the Diurnal Variation of Root Conductance in Olive Trees Using Microtensiometers and Sap Flow Sensors

Understanding the variation of root hydraulic conductance (L_p) is critical for the simulation of the soil–plant-atmosphere continuum (SPAC), but its monitoring remains challenging. In this study, we introduce a new non-destructive method for characterizing L_p dynamics in woody species through the combination of simultaneous determinations of sap flow and xylem water potential. Recent studies indicate that modern microtensiometers provide robust estimates of xylem water potential, but it is unknown whether they allow tracking rapid changes in water potential without significant time lags, which may have implications for the proposed methodology.

The impulse response of microtensiometers was measured in the lab, developing a procedure for correcting sensor data by deconvolution. Then, microtensiometers and compensation heat pulse sensors were used to evaluate the variations in L_p in two well-watered olive trees during the summer of 2022 in Cordoba, Spain.

Correcting microtensiometer outputs was critical to analyze our field data as strong stomatal oscillations occurred, with microtensiometers damping xylem water potential variations. By contrast, our results suggest that correction procedures may not be required for many practical applications like irrigation scheduling. The daytime values of L_p were close to those obtained in previous studies, while nighttime values were extremely low. Therefore, a proportionality between L_p and sap flow rate was observed, which agrees with previous studies, although it does not prove a causal relationship.

The methods proposed here could be applied to studying the temporal dynamics of root hydraulic conductance in other tree species.