The measurement of plant hydraulic functioning is a tool for understanding plant responses to extreme climate events, landscape water fluxes, and species distributions. Common assumptions and methodologies related to empirical measurements of hydraulic conductivity have recently come under scrutiny. Specifically, debates within our field have centered around the process of embolism-induced loss of hydraulic capacity, when air bubbles are aspirated into water-filled conduits under tension. Several studies have suggested that embolism may be artificially introduced into the xylem using typical procedures for harvesting branches from woody plants and when developing branch vulnerability curves with centrifuge techniques, and that such artefacts are more common in species with longer vessels than shorter ones. If artefactual emboli are formed, then the measured hydraulic conductivities are not indicative of those in vivo, which also impacts interpretation of embolism repair processes. Here, we used microCT images of intact and excised Castanea dentata (mean vessel length = 0.14 m) stem segments combined with active xylem staining and empirical measurements of hydraulic conductivity to examine if (1) there were any artefacts when harvesting segments under water and (2) how theoretical conductivity derived from microCT images compare to hydraulic conductivity measured on the same branch segments.
Branch segments harvested under water following a 10-minute tension relaxation had similar levels of emboli compared to microCT data, suggesting no harvesting artefact for our samples. Anatomical-based conductivity (Kt) and counts of air- and water-filled vessels were nearly identical (r2 > 0.90, p < 0.05) in microCT scans before and after harvesting. Further, percent loss of conductivity estimates using traditional hydraulic conductivity measurements (Kh) and Kt derived from xylem staining and microCT images showed good agreement. Absolute conductivities found that Kt values were greater than Kh measurements. Estimates of Kt typically overestimate conductivity due to uncalculated interconduit pit membrane resistance, hydraulically isolated or non-conductive vessels, and variable vessel shapes producing altered flow rates. This study highlights the complexities of plant water transport by demonstrating how, even when carefully conducted, multiple methods can produce consistencies and discrepancies based on the species, measurements and methods used. To move forward, great care should be taken when harvesting and performing conductivity measurements with all species and potential errors may be magnified for long-vesselled species with wide vessel diameters. We recommend that comparing multiple independent methods is a way to ensure reliability of results.