The functional characteristics of plant cell walls depend on the composition of the cell wall polymers, as well as on their highly ordered architecture at scales from a few nanometres to several microns. Raman spectra of wood acquired with linear polarized laser light include information about polymer composition as well as the alignment of cellulose microfibrils with respect to the fibre axis (microfibril angle). By changing the laser polarization direction in 3° steps, the dependency between cellulose and laser orientation direction was investigated. Orientation-dependent changes of band height ratios and spectra were described by quadratic linear regression and partial least square regressions, respectively. Using the models and regressions with high coefficients of determination (R2 > 0.99) microfibril orientation was predicted in the S1 and S2 layers distinguished by the Raman imaging approach in cross-sections of spruce normal, opposite, and compression wood. The determined microfibril angle (MFA) in the different S2 layers ranged from 0° to 49.9° and was in coincidence with X-ray diffraction determination. With the prerequisite of geometric sample and laser alignment, exact MFA prediction can complete the picture of the chemical cell wall design gained by the Raman imaging approach at the micron level in all plant tissues.

Top dieback in 40–60 years old forest stands of Norway spruce [Picea abies (L.) Karst.] in southern Norway is supposed to be associated with climatic extremes. Our intention was to learn more about the processes related to top dieback and in particular about the plasticity of possible predisposing factors. We aimed at (i) developing proxies for P50 based on anatomical data assessed by SilviScan technology and (ii) testing these proxies for their plasticity regarding climate, in order to (iii) analyze annual variations of hydraulic proxies of healthy looking trees and trees with top dieback upon their impact on tree survival. At two sites we selected 10 tree pairs, i.e., one healthy looking tree and one tree with visual signs of dieback such as dry tops, needle shortening and needle yellowing (n = 40 trees). Vulnerability to cavitation (P50) of the main trunk was assessed in a selected sample set (n = 19) and we thereafter applied SilviScan technology to measure cell dimensions (lumen (b) and cell wall thickness (t)) in these specimen and in all 40 trees in tree rings formed between 1990 and 2010. In a first analysis step, we searched for anatomical proxies for P50. The set of potential proxies included hydraulic lumen diameters and wall reinforcement parameters based on mean, radial, and tangential tracheid diameters. The conduit wall reinforcement based on tangential hydraulic lumen diameters ((t/bht)2) was the best estimate for P50. It was thus possible to relate climatic extremes to the potential vulnerability of single annual rings. Trees with top dieback had significantly lower (t/bht)2 and wider tangential (hydraulic) lumen diameters some years before a period of water deficit (2005–2006). Radial (hydraulic) lumen diameters showed however no significant differences between both tree groups. (t/bht)2 was influenced by annual climate variability; strongest correlations were found with precipitation in September of the previous growing season: high precipitation in previous September resulted in more vulnerable annual rings in the next season. The results are discussed with respect to an “opportunistic behavior” and genetic predisposition to drought sensitivity.

The potential of Fourier transform near-infrared (FT-NIR) spectroscopy to predict hydraulic traits in Norway spruce (Picea abies (L.) Karst.) sapwood was evaluated. Hydraulic traits tested were P 50 (applied air pressure causing 50% loss of hydraulic conductivity) and RWL 50 (applied air pressure causing 50% relative water loss). Samples came from 24-year-old spruce clones. FT-NIR spectra were collected from the axial (transverse) and radial surface of each solid wood sample for the prediction of P 50 and RWL 50 . Partial least squares regression (PLS-R) models with cross validation were used to establish relationships between the FT-NIR spectra and the reference data from hydraulic properties analysis. The impact of the wavenumber range and the pretreatment during the PLS-R model development and the differences between the axial and radial surfaces were shown. Based on the values of the coefficient of determination (r 2 ) and the root mean square error of cross validation, predicted results were evaluated as acceptable. The models from the axial surface gave better results than the models from the radial surface for P 50 (r 2 = 0.65), as well as for RWL 50 (r 2 = 0.77). The first approach to predict hydraulic properties such as P 50 and RWL 50 by FT-NIR spectroscopy can be regarded as successful. We conclude that the method has high potential to be put into practice as a rapid, reliable, and nondestructive method to determine P 50 and RWL 50 .Résumé : Le potentiel de la spectroscopie dans le proche infrarouge à transformée de Fourier (PIR-TF) pour prédire les traits hydrauliques du bois d'aubier d'épicéa commun (Picea abies (L.) Karst.) a été évalué. Les traits hydrauliques qui ont été testés sont P 50 (la pression d'air qui cause une perte de conductivité hydraulique de 50 %) et RWL 50 (la pression d'air nécessaire pour causer une perte relative d'eau de 50 %). Les échantillons provenaient de clones d'épicéa âgés de 24 ans. Les spectres PIR-TF ont été collectés à partir de la face axiale (transverse) et de la face radiale de chaque échantillon solide de bois pour prédire P 50 et RWL 50 . L'analyse de régression partielle par les moindres carrés avec validation croisée a été utilisée pour établir les relations entre les spectres PIR-TF et les données de référence obtenues par l'analyse des propriétés hydrauliques. L'impact de la plage de nombres d'ondes et du prétraitement durant l'élaboration du modèle de régression ainsi que les différences entre les faces axiale et radiale ont été démontrés. Sur la base des valeurs du coefficient de détermination (r 2 ) et de l'écart moyen quadratique de la validation croisée, les résultats prédits ont été jugés acceptables. Les modèles élaborés à partir de la face axiale ont produit de meilleurs résultats que ceux qui ont été élaborés à partir de la face radiale tant dans le cas de P 50 (r 2 = 0,65) et que de RWL 50 (r 2 = 0,77). La première tentative de prédire les propriétés hydrauliques telles que P 50 et RWL 50 en utilisant la spectroscopie PIR-TF peut être considérée co...

<p style="text-align: justify;">There is evidence that recently occurring top dieback of Norway spruce (<em>Piceaabies</em>(L.) Karst.) treesin southern Norway is associated with drought stress. We comparedfunctional wood traits of20 healthy looking trees and 20 trees with visual signs of top dieback. SilviScan technology was applied to measure cell dimensions (lumen and cell wall thickness) in a selected set of trunk wood specimens where vulnerability to cavitation (P₅₀) datawere available. The conduit wall reinforcement ((<em>t</em>/<em>b</em>)²) was a good proxy for P₅₀. Cell dimensions were measured on wood cores of all 40 trees; theoretical vulnerability of single annual rings could bethus estimated. Declining trees tended to have lower (<em>t</em>/<em>b</em>)² before and during a period of water deficit (difference between precipitation and potential evapotranspiration)that lasted from 2004 to 2006. The results are discussed with respect to genetic predisposition.</p>

Relationships between hydraulic vulnerability expressed as P50 (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 P50 relating to functional aspects. WD and tracheid dimensions were measured with SilviScan on Norway spruce (Picea abies (L.) Karst.) trunk wood.P50 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/bt)2) gave better results (r = -0.54) than radial lumen width (r = -0.31). The correlations of P50 with earlywood (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 WDLW) 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 P50 values of each specimen. Tightest correlations were found for (t/bt)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 P50, 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.