Within-ring variability of wood structure and its relationship to drought sensitivity in Norway spruce trunks

Relationships between hydraulic vulnerability expressed as P-50 (the air pressure causing 50% loss of hydraulic conductivity) and within-ring differences in wood density (WD) and anatomical features were investigated with the aim to find efficient proxies for P-50 relating to functional aspects. WD and tracheid dimensions were measured with SilviScan on Norway spruce (Picea abies (L.) Karst.) trunk wood. P-50 was strongly related to mean WD (r = -0.64) and conduit wall reinforcement ((t/b)(2)), the square of the ratio between the tracheid double wall thickness (t) and the lumen width (b), where use of tangential lumen width ((t/b(t))(2)) gave better results (r = -0.54) than radial lumen width (r = -0.31). The correlations of P-50 with carlywood (EW), transition wood (TW) and latewood (LW) traits were lower than with the specimen averages, both for WD (r = -0.60 for WDEW , r = -0.56 for WDTW, r = -0.23 for WDTW) and all anatomical traits. The loss of hydraulic conductivity was addressed as a dynamic process and was simulated by defining consecutive phases of 5% theoretical conductivity loss. WD and tracheid traits were calculated and correlated with P 50 values of each specimen. Tightest correlations were found for (t/b(t))(2), at relative cumulated theoretical conductivities until 45 to 50% (r = -0.75). We conclude that WD is one of the best available proxies for P-50, but does not necessarily reflect the mechanism behind resistance to cavitation. The new trait, based on estimation of conductivity loss as a dynamic process, provided even stronger correlations.