Windthrow damage in Picea abies is associated with physical and chemical stem wood properties

On 26 December 1999, the windstorm "Lothar" hit large parts of western and central Europe. In Switzerland, windthrow losses reached 12.7 Mio m(3) of timber, corresponding to 2.8 times the annual national timber harvest. Although these exceptional losses were due to extreme peak velocities, recent changes in tree nutrition may have increased forest susceptibility. Previous controlled environment experiments revealed that wood density (associated with wood stiffness) tends to increase in elevated CO2, and to decrease when N-availability is enhanced (e.g., by soluble N-deposition). Such changes in wood quality could theoretically influence the risk of wind damage. We used the "Lothar" windstorm as a "natural experiment" to explore links between damage and wood properties. In 104 windthrow sites across the Swiss Plateau, more than 1,600 wood cores from (1) broken, (2) uprooted and (3) still standing (not damaged) spruce trees (Picea abies) were collected in February and March 2000. Wood properties, treering width and chemistry of the wood samples were analysed. Broken trees showed wider treerings in the decade 1990-99 compared to non-broken trees (either uprooted or undamaged trees). Broken trees also showed lower non-structural carbohydrate (NSC) concentration in sapwood, reflecting active structural carbohydrate sinks associated with fast growth. There was also a trend for higher tissue N-concentrations in broken trees. No significant differences between damage types were found in wood density and wood shrinkage during desiccation. We conclude that stem breakage risk of P. abies is associated with a stimulation of growth in the past decade and with changes in tree nutritional status. However, the risk for windthrow of whole spruce trees (uprooted but not broken) was not related to the studied wood parameters

The influence of chemical degradation and polyethylene glycol on moisture-dependent cell wall properties of archeological wooden objects: a case study of the Vasa shipwreck

Cell wall measures allow for direct assessment of wood modification without the adverse effect of varying density and microstructure. In this study, cell wall properties of recent and archeological oak wood from the Vasa shipwreck were investigated for cell wall stiffness, hardness and creep with respect to effects of chemical degradation, impregnation with a preservation agent, namely polyethylene glycol, and moisture. For this purpose, nanoindentation tests were performed at varying relative humidity, leading to different moisture contents in the wood samples. Concurrently, microstructural and chemical characterization of the material was conducted. Impregnated and untreated recent oak wood showed a softening effect of both moisture and preservation agent at the wood cell wall level. On the contrary, increased stiffness was found for non-impregnated Vasa oak, which can be explained by aging-related modifications in cell wall components. These effects were counteracted by the softening effect of polyethylene glycol in the impregnated Vasa material, where a lower overall stiffness was measured. The reverse effect of the preservation agent and moisture, namely increased indentation creep of the cell wall material, was revealed. The loss of acetyl groups in the hemicelluloses explained the decreased hygroscopicity of the Vasa oak. In the impregnated Vasa oak, this effect seemed to be partly counteracted by the presence of low-molecular polyethylene glycol contributing to higher hygroscopicity of the cell wall. Thus, the higher overall sorptive capacity of the impregnated Vasa material, with respect to the non-impregnated material, was detected, which has resulted in a sorptive behavior similar to that of recent oak wood. The proposed approach requires only small amounts of material, making it especially suitable for application to precious historical wooden artifacts.2

Tree-rings and climate - Standardization, proxy-development, and Fennoscandian summer temperature history

Instrumental meteorological observation are too short for trying to estimate climate change and variability on multi-decadal and centennial time-scales, and when trying to evaluate the response of the climate system to human influence, such as raised concentrations of green house gases (GHG), altered land-use, black carbon etc. To access information about the climate system predating instrumental observations, reliable proxy records (natural archives) are necessary. These proxies include for example tree rings, ice cores, fossil pollen, ocean sediments, corals and historical documentary data. Tree rings is one of the most widely used proxy for high-resolution growing season temperature reconstructions during the last millennium, and in Fennoscandia some of the best-calibrated records in the world exist. Yet, in this available body of work, there is limited homogeneity on decadal to centennial scales. Since this tree-ring data is targeting growing-season temperatures and growing-season temperatures in this region are very well correlated on annual to decadal scales this is unexpected. This thesis is concerned with trying to address this issue by 1) developing existing standardization-tools in order to display centennial scale variability and at the same time reduce noise arising from internal and external disturbances and mismatches in actual growth trends compared to the expected growth trend. 2) By developing the new un-exploited ΔDensity and ΔBlue Intensity proxies (the difference between the latewood and earlywood for density and blue intensity respectively) to act as complement or quality control to the established maximum latewood density (MXD) which is the state of the art proxy for high latitude temperature reconstructions, and also to the Blue Intensity measurement scheme, that potentially could be an inexpensive complement to the radiodensitometric methodology. Results showed that using the Δ parameter for both density and Blue Intensity, give added value in a more focused annual scale summer temperature signal, and an improved coherence between different chronologies on decadal to centennial scales. Methodological protocols such as data analysis and standardization seem to be critical when trying to attain adequate low-frequency signals from tree-ring data. A more coherent view of the summer temperature history for the last 900 years in Fennoscandia is provided using the methodological improvements outlined in this thesis. Future challenges include trying to extend this excellent network back in time to not only cover the Little Ice Age (1450-1900 CE) but also to cover the debated Medieval Climate Anomaly (850-1250 CE)