Sabine Rosner

Genetic parameters of growth and wood quality traits in Picea abies

Genetic parameters were estimated for wood and growth traits in two 19-yr-old clonal trials and a 40-yr-old full-sib progeny trial of Norway spruce [Picea abies (L.) Karst.]. In the clonal trials high (>0.4) broad-sense heritabilities were found for wood density traits, lignin content, number of internal cracks, growth traits, spiral grain and number of resin canals. Moderate (0.2–0.4) heritabilities were found for tracheid lumen diameter and cell wall thickness, microfibril angle and tracheid length, while low heritabilities (<0.2) were found for pulp yield, fibre strength, wood stiffness and wood colour. Lignin content and pulp yield showed low genetic variation, whereas the genotypic coefficient of variation for most other traits ranged between 5 and 15%. Most traits showed low levels of genotype by environment interaction. Among the wood properties, latewood proportion, earlywood density and ring density showed significant, adverse correlations with volume in both clonal trials.

Uptake of Water via Branches Helps Timberline Conifers Refill Embolized Xylem in Late Winter

Timberline conifers, which exhibit potentially lethal winter embolism, refill stem xylem with water taken up via branches and water transport to isolated, embolized conduits by active, cellular processes. Xylem embolism is a limiting factor for woody species worldwide. Conifers at the alpine timberline are exposed to drought and freeze-thaw stress during winter, which induce potentially lethal embolism. Previous studies indicated that timberline trees survive by xylem refilling. In this study on Picea abies, refilling was monitored during winter and spring seasons and analyzed in the laboratory and in situ experiments, based on hydraulic, anatomical, and histochemical methods. Refilling started in late winter, when the soil was frozen and soil water not available for the trees. Xylem embolism caused up to 86.2% ± 3.1% loss of conductivity and was correlated with the ratio of closed pits. Refilling of xylem as well as recovery in shoot conductance started in February and corresponded with starch accumulation in secondary phloem and in the mesophyll of needles, where we also observed increasing aquaporin densities in the phloem and endodermis. This indicates that active, cellular processes play a role for refilling even under winter conditions. As demonstrated by our experiments, water for refilling was thereby taken up via the branches, likely by foliar water uptake. Our results suggest that refilling is based on water shifts to embolized tracheids via intact xylem, phloem, and parenchyma, whereby aquaporins reduce resistances along the symplastic pathway and aspirated pits facilitate isolation of refilling tracheids. Refilling must be taken into account as a key process in plant hydraulics and in estimating future effects of climate change on forests and alpine tree ecosystems.

Defence reactions of Norway spruce against bark beetles and the associated fungus Ceratocystis polonica in secondary pure and mixed species stands

Abstract The effects of restoration of secondary pure Norway spruce stands on defence mechanisms against bark beetle attack were tested. The test trees have been growing in forest stands with contrasting structure, stand density (thinned/unthinned) and species composition (pure/mixed). Seasonal differences in defence mechanisms were also analysed. Anatomical characteristics of the resin duct system of the stem bark, constitutive resin flow rate and preformed monoterpene content, wound reaction after low density inoculation with Ceratocystis polonica, induced resin flow and monoterpene content, were used as indicators of tree resistance. Resin flow was closely related to anatomy of radial resin ducts of the secondary phloem. Differences in constitutive resinosis between spring and summer were mainly affected by different air temperatures during resin flow measurements. Wounding and fungus inoculations induced higher resin flow rates and also an increase in monoterpene content of unaffected bark tissues. Monoterpene composition also changed considerably. Induced defence was more different between stands than constitutive defence. Trees from the mixed species stand showed higher primary resin flow, lower lesion length and lower induced resin flow compared with trees from unthinned/thinned pure spruce stands. However, only the relative increase in resin flow was significantly higher in trees from the unthinned pure spruce stand. Constitutive as well as inducible defence mechanisms were affected by tree growth. Initial monoterpene content, cross-sectional area of injured resin ducts and primary resin flow rate decreased and the length of lesions increased with increasing radial growth indices. These results support the hypothesis that there is an inverse relationship between growth and defence. Both primary defence, as well as secondary induced defence, were affected by tree-individual growth history.

Do water‐limiting conditions predispose Norway spruce to bark beetle attack?

Drought is considered to enhance susceptibility of Norway spruce (Picea abies) to infestations by the Eurasian spruce bark beetle (Ips typographus, Coleoptera: Curculionidae), although empirical evidence is scarce. We studied the impact of experimentally induced drought on tree water status and constitutive resin flow, and how physiological stress affects host acceptance and resistance. We established rain-out shelters to induce both severe (two full-cover plots) and moderate (two semi-cover plots) drought stress. In total, 18 sample trees, which were divided equally between the above treatment plots and two control plots, were investigated. Infestation was controlled experimentally using a novel ‘attack box’ method. Treatments influenced the ratios of successful and defended attacks, but predisposition of trees to infestation appeared to be mainly driven by variations in stress status of the individual trees over time. With increasingly negative twig water potentials and decreasing resin exudation, the defence capability of the spruce trees decreased. We provide empirical evidence that water-limiting conditions impair Norway spruce resistance to bark beetle attack. Yet, at the same time our data point to reduced host acceptance byI. typographus with more extreme drought stress, indicated by strongly negative pre-dawn twig water potentials.

Cavitation in dehydrating xylem of Picea abies: energy properties of ultrasonic emissions reflect tracheid dimensions

Ultrasonic emission (UE) testing is used to analyse the vulnerability of xylem to embolism, but the number of UEs often does not sufficiently reflect effects on hydraulic conductivity. We monitored the absolute energy of UE signals in dehydrating xylem samples hypothesizing that (i) conduit diameter is correlated with UE energy and (ii) monitoring of UE energy may enhance the utility of this technique for analysis of xylem vulnerability. Split xylem samples were prepared from trunk wood of Picea abies, and four categories of samples, derived from mature (I: earlywood, II: 30-50% latewood, III: >50% latewood) or juvenile wood (IV: earlywood) were used. Ultrasonic emissions during dehydration were registered and anatomical parameters (tracheid lumen area, number per area) were analysed from cross-sections. Attenuation of UE energy was measured on a dehydrating wood beam by repeated lead breaks. Vulnerability to drought-induced embolism was analysed on dehydrating branches by hydraulic, UE number or UE energy measurements. In split samples, the cumulative number of UEs increased linearly with the number of tracheids per cross-section, and UE energy was positively correlated with the mean lumen area. Ultrasonic emission energies of earlywood samples (I and IV), which showed normally distributed tracheid lumen areas, increased during dehydration, whereas samples with latewood (II and III) exhibited a right-skewed distribution of lumina and UE energies. Ultrasonic emission energy was hardly influenced by moisture content until ∼40% moisture loss, and decreased exponentially thereafter. Dehydrating branches showed a 50% loss of conductivity at -3.6 MPa in hydraulic measurements and at -3.9 and -3.5 MPa in UE analysis based on cumulative number or energy of signals, respectively. Ultrasonic emission energy emitted by cavitating conduits is determined by the xylem water potential and by the size of element. Energy patterns during dehydration are thus influenced by the vulnerability to cavitation, conduit size distribution as well as attenuation properties. Measurements of UE energy may be used as an alternative to the number of UEs in vulnerability analysis.

Shrinkage processes in standard-size Norway spruce wood specimens with different vulnerability to cavitation

The aim of this study was to observe the radial shrinkage of Norway spruce [Picea abies (L. Karst.)] trunkwood specimens with different hydraulic vulnerability to cavitation from the fully saturated state until the overall shrinkage reaches a stable value, and to relate wood shrinkage and recovery from shrinkage to cavitations of the water column inside the tracheids. Radial shrinkage processes in standard-size sapwood specimens (6 mm x 6 mm x 100 mm; radial, tangential and longitudinal) obtained at different positions within the trunk, representing different ages of the cambium, were compared. Cavitation events were assessed by acoustic emission (AE) testing, hydraulic vulnerability by the AE feature analysis and shrinkage was calculated from the changes in contact pressure between the 150 kHz AE transducer and the wood specimen. Two shrinkage processes were observed in both juvenile (annual rings 1 and 2) and mature wood (annual rings 17-19), the first one termed tension shrinkage and the second one cell wall shrinkage process, which started when most of the tracheids reached relative water contents below fiber saturation. Maximum tension shrinkage coincided with high-energy AEs, and the periods of shrinkage recovery could be traced to tension release due to cavitation. Juvenile wood, which was less sensitive to cavitation, had lower earlywood tracheid diameters and was less prone to deformation due to tensile strain than mature wood, showed a lower cell wall shrinkage, and thus total shrinkage. Earlywood lumen diameters and maximum tension shrinkage were strongly positively related to each other, meaning that bigger tracheids are more prone to deformation at the same water tension than the smaller tracheids.

Comparison of methods to quantify loss of hydraulic conductivity in Norway spruce

Abstract• Percent loss of hydraulic conductivity (PLC) is an important measure in plant water relations, but the available methods differ and results have rarely been compared.• We compared PLC measured in Norway spruce (Picea abies) by quantifying conductivity before and after removing emboli, either by flushing with high water pressure or by infiltration under a partial vacuum, with relative water loss and staining conductive xylem to test advantages and possible problems of commonly applied methods.• Infiltration removed nearly all gas from sections of drought-stressed and unstressed twigs, and flushing and infiltration efficiently removed emboli. Infiltration appears less prone to producing clogged xylem elements than high pressure flushing. Relative water loss (RWL) and the proportion of xylem stained with phloxine B (PSX) was also highly correlated with PLC, the latter can be quantified by image analysis, and also shows the pattern of xylem dysfunction. Loss of conductivity was more common in the inner annual rings, in early wood within an annual ring, and in compression wood, though pattern differed substantially between branches.• Advantages and potential problems of these methods are discussed and it is suggested that RWL or PSX may be used as proxy measures for PLC in species when the correlations have been established.Résumé• Le pourcentage de perte de conductivité hydraulique (PLC) est un paramètre important des relations hydriques des plantes, mais les méthodes de mesure disponibles diffèrent et les résultats ont rarement été comparés.• Nous avons comparé des valeurs de perte de conductivité chez Picea abies obtenues en quantifiant la conductivité hydraulique avant et après suppression de l’embolie, soit par un flush d’eau sous haute pression soit par infiltration sous un vide partiel, avec les pertes relatives en eau et avec des méthodes de coloration du xylème conducteur coloré; cette comparaison a permis de tester les avantages et les inconvénients potentiels des méthodes couramment utilisées.• L’infiltration élimine presque tous les gaz des sections de petites branches, alors que de l’eau sous pression et l’infiltration suppriment l’embolie. L’infiltration paraît moins susceptible de provoquer une obstruction des éléments de xylème, qu’un flush à haute pression. Perte relative en eau (RWL) et proportion de xylème coloré avec de la phloxine B (PSX) étaient hautement corrélées avec la perte de conductivité. Une analyse d’image de xylème permet également de quantifier la perte de conductivité et de mettre en évidence la distribution spatiale des vaisseaux embolisés. La perte de conductivité a été très visible dans les cernes interne du tronc, dans le bois initial du cerne de l’année et dans le bois de compression, quoique les patrons diffèrent considérablement entre branches.• Les avantages et les inconvénients potentiels de ces méthodes sont discutés et il est proposé que RWL ou PSX puissent être utilisées comme des estimateurs indirects de perte de conductivité une fois les corrélations établies pour chaque espèce.

Osmotic potential of Norway spruce [Picea abies (L.) Karst.] secondary phloem in relation to anatomy

Abstract. Variations in water status of secondary phloem of Picea abies (L.) Karst., caused by (1) radial and (2) vertical differences within the tree, (3) seasonal influences, and (4) tree class, and their relation to bark anatomy were investigated. The water status parameters measured were the osmotic potential at full saturation [Ψo (sat)], the in situ osmotic potential [Ψo (in situ)], the in situ water content (Cin situ), and the in situ relative water content (Rin situ). Ψo (sat) reached most negative values in the conducting part of the secondary phloem, whereas Ψo (in situ) was similar in conducting (PC) and non-conducting secondary phloem (PN). The remarkable discontinuity in the radial course of Cin situ and Ψo (sat) at the transition from PC to PN can be attributed to the degeneration of sieve cells and Strasburger cells. In PC, the vertical decrease of Ψo (sat) towards the crown was compensated by an increase in Rin situ, so that Ψo (in situ) did not change along the stem. With stem height, Ψo (sat) decreased and PC width increased. The determining factor for vertical gradients in Ψo (sat) was the distance to the sources; similar gradients were also measured in PN. Seasonal differences in Ψo (sat) could only be detected in PC, where they corresponded exactly to changes of Ψo (sat) in needles. Suppressed trees showed less negative Ψo (sat) values in PC, smaller annual secondary phloem increments and smaller radial lumen diameter of living sieve cells than predominant or dominant trees.

An improved method and data analysis for ultrasound acoustic emissions and xylem vulnerability in conifer wood

The vulnerability of the xylem to cavitation is an important trait in plant drought resistance and has been quantified by several methods. We present a modified method for the simultaneous measurement of cavitations, recorded as ultrasound acoustic emissions (UAEs), and the water potential, measured with a thermocouple psychrometer, in small samples of conifer wood. Analyzing the amplitude of the individual signals showed that a first phase, during which the mean amplitude increased, was followed by a second phase with distinctly lower signal amplitudes. We provide a method to separate the two groups of signals and show that for many samples plausible vulnerability curves require rejecting late low-energy UAEs. These very likely do not result from cavitations. This method was used to analyze the differences between juvenile wood, and early and late mature wood in Picea abies (L.) Karst. Juvenile earlywood was more resistant to cavitation than mature earlywood or latewood, which we relate to the tracheid anatomy of the samples.

Hydraulic and biomechanical optimization in norway spruce trunkwood – a review

Secondary xylem (wood) fulfills many of the functions required for tree survival, such as transport of water and nutrients, storage of water and assimilates, and mechanical support. The evolutionary process has optimized tree structure to maximize survival of the species, but has not necessarily optimized the wood properties needed for lumber. Under the impact of global warming, knowledge about structure-function relationships in tree trunks will become more and more important in order to prognosticate survival prospects of a species, individuals or provenances. Increasing our knowledge on functional wood anatomy can also provide valuable input for the development of reliable, fast, and at best quasi-non-destructive (e.g. wood coring of mature trunks) indirect screening techniques for drought susceptibility of woody species. This review gives an interdisciplinary update of our present knowledge on hydraulic and biomechanical determinants of wood structure within and among trunks of Norway spruce (Picea abies (L.) Karst.), which is one of Europe’s economically most important forest tree species. It summarizes what we know so far on 1) withinring variability of hydraulic and mechanical properties, 2) structure-function relationships in mature wood, 3) mechanical and hydraulic demands and their tradeoffs along tree trunks, and 4) the quite complex wood structure of the young trunk associated with mechanical demands of a small tree. Due to its interdisciplinary nature this review is addressed to physiologists, foresters, tree breeders and wood technologists.