Edinburgh Napier University

  Timber is a local, sustainable and valuable building material, but it is highly variable compared to other building materials (e.g. concrete, steel). The quality of wood is its suitability for the end-user, in this case the construction industry (via timber processors).
Douglas-fir is a tall conifer capable of producing high construction grade timber. Native to the north-western Pacific regions of America and Canada, Douglas-fir was introduced to the UK in 1827. After World War 1, the planting of conifers greatly increased due to the establishment of the Forestry Commission. Despite being a high value timber crop in North America, Douglas-fir was not highly utilised in Great Britain due to a perceived lack of suitable growing sites (requiring nutrient-rich soil) and a lack of knowledge on its qualities (mechanical). Consequently, it still to this day covers a relatively small amount of the total UK conifer plantation area, but under predicted climate change projections an increased range of sites will become more suitable for Douglas-fir, thus investigation now is imperative.
To investigate the quality of Douglas-fir timber and its biological variation, a variety of sites were sampled in Scotland and Wales. The variation in the physical and mechanical properties of UK-grown Douglas-fir were investigated to determine how strength and stiffness of Douglas-fir compares to other commercially important timber species in the UK (as well as compared to Douglas-fir grown in different countries). Standing and felled tree measurements relating to tree architecture and important for timber volume (e.g. size, height, branching habits and taper) were collected in the forest. This was followed by laboratory testing of wood samples obtained from those trees to determine important raw material properties. Ultimately this will enable some explanation and prediction of the variation in mechanical and physical properties in Douglas-fir.
It was found that Douglas-fir is stronger, stiffer and denser than the UK’s most planted conifer, Sitka spruce. Wood adjacent to the pith (middle of tree) termed as juvenile was weaker, less stiff and less dense. Within-tree variation accounted for most of the variation for the key properties of strength, stiffness and density. It was possible to build models for some of these properties based on cambial age (ring number from the pith). Considering branches, it was found that within-tree variation in size, frequency, angle and status (alive or dead) were highly variable but it was possible to build empirical models to describe branch architecture for a typical tree. It was possible to measure the rate of swelling in oven dry Douglas-fir in the radial and tangential dimensions, but swelling of the longitudinal dimension was below the limit of detection for the apparatus. Heartwood area can be successfully predicted from the diameter of tree at a given point. It is hoped the information in this study will detail some characteristic Douglas-fir traits that may be deemed beneficial for the timber construction industry and allow understanding of its variability plus provide important models to use in helping to describe Great Britain’s forest resource.