Optical observations of embolism in three conifers overestimate the vulnerability of stem xylem to hydraulic dysfunction.

Hydraulic failure due to drought stress is a major cause of forest decline. Therefore, many techniques have been developed to test the vulnerability of trees to drought-induced xylem embolism, each with advantages and limitations. We quantified drought vulnerability using the optical vulnerability (OV) and ultrasonic acoustic emission (UAE) techniques by performing simultaneous measurements on branches of three conifers (Picea abies, Pinus sylvestris and Pinus cembra). Results were compared with vulnerability curves obtained using the flow-centrifuge (FC) technique. With respect to the OV method, the light transmission properties of the samples were analyzed to determine the xylem fraction observed. OV vulnerability thresholds were 0.3-1.4MPa higher than those obtained with UAE and FC, which were similar overall. Xylem depths observed by OV were limited to peripheral 0.15-0.20mm, as no light transmission was detected deeper. Light transmission was higher in saturated than in dry xylem. Results suggest that xylem embolism detection by OV is limited to thin, peripheral xylem layers, and that vulnerability thresholds may differ from thicker samples (>0.4mm) measured with UAE and FC. Therefore, OV may not accurately reflect the general vulnerability of thick branches, as only peripheral xylem layers are analyzed. Changes in light transmission during dehydration must be considered.