Tradeoffs between wood functions in tree stems

Norway spruce (Picea abies (L.) Karst.) is one of the commercially most important tree species across Europe. Under the pressure of the growing demand for wood products, strategies have been developed to use wood more efficiently. A main strategy is to grow wood with more uniform properties along the stem. Secondary xylem (wood) fulfills many of the functions required for tree survival, e.g. water transport and mechanical support. The evolutionary process has optimized tree structure to maximize survival of the species, but has not necessarily optimized the wood proper-ties needed for lumber. Breeding trees without understanding the relationships between wood structure and tree survival may result in enhanced wood properties, but poor survival prospects. Within the proposed project we state three main hypotheses: #1 Wood structure within and between trees differs due to changing demands on hydraulic efficiency, safety and mechanical performance. The optimum structures for achievement of these essential functions will most likely differ, leading to conflicting demands on wood structure for physiological fitness. #2 A tradeoff exists between hydraulic safety and efficiency at the pit level. Earlywood and late-wood in Picea abies may operate with different strategies of xylem safety, obviously related to pit anatomy. The question is raised how the arrangement of earlywood and latewood tracheids both guarantee high hydraulic efficiency under high water supply and xylem safety under a sudden drought stress event. #3 Superior wood quality is not always paralleled by superior hydraulic properties, xylem safety and mechanical performance. Uniformity and high wood density (thus more "mature-like" wood) are the most desired solid-wood properties. Our intention is to evaluate wood functions as to their anatomical and chemical implications and their consequence for wood quality. To test the hypotheses we will perform comprehensive investigations on anatomical and chemical parameters defining wood quality and their relevance for hydraulic efficiency, drought vulnerability, and mechanical performance within a stem, with reference to xylem characteristics that change with cambial age, tree height (juvenile vs. adult wood), season (earlywood vs. latewood), and with the influence of drought and growth stresses.

Under the aspect of the growing demand for wood products, in the past two decades strategies have been developed to use wood more efficiently. The main strategy is to grow wood with properties making it suitable for particular processing and products. As a consequence, the production of wood with more uniform properties is aimed at, because both yield and quality of the final product will respond positively to greater uniformity in the desired wood properties. Nevertheless, secondary xylem (wood) fulfills many of the functions required for tree survival, including water and nutrient transport, mechanical support and storage of water, carbohydrates and secondary compounds. The optimum tree structures for each of these functions will most likely differ, leading to conflicting demands on wood structure for physiological fitness. The evolution process does not necessarily optimize the desired properties of the resultant lumber such as stiffness, strength or uniformity. We investigated structure-function relationships in young and mature Norway spruce (Picea abies (L.) Karst.) clones concerning the behavior under hydraulic, mechanical bending and compression stress and related them to growth and wood quality aspects. Wood structure and hydraulic vulnerability of 25 year old spruces varied considerably between tree top and breast height. Hydraulic and mechanical stress behavior were strongly influenced by the growth characteristics of the trees. The strong relationship between wood density, hydraulic vulnerability and mechanical strength in mature spruce wood was masked in juvenile wood by the presence of compression wood. Compression wood is necessary in flexible young stems to keep the tree in the upright position. Breeding for high growth or more uniformity can thus result in trees with enhanced commercial wood properties, but poor survival prospects under drought stress, strong winds and heavy snow loads. Another aspect of our research was to develop a readily automated method for determining the hydraulic vulnerability of spruce trunkwood, because the conventional hydraulic method is very labor intensive. We managed to assess the hydraulic vulnerability by extracting useful waveform information from ultrasound acoustic signals emitted by dehydrating wood samples. Acoustic emission testing in combination with waveform feature extraction offers a readily automated and easily executed alternative to the hydraulic method and could be applied also in industrial lumber drying.