Abstract
Douglas-fir is the most commercially important timber species in the US Pacific Northwest due to its ecological prevalence and its superior wood attributes, especially strength and stiffness properties that make it highly prized for structural applications. Its economic significance has led to extensive establishment and management of plantations over the last few decades. Cultural treatments and genetic improvement designed to increase production of utilizable wood volume also impact tree morphology and wood properties. Many of these impacts are mediated by crown development, particularly the amount and distribution of foliage and size and geometry of branches. Natural selection for branch architecture that optimizes reproductive fitness may not necessarily be optimal for stem volume growth rate or for wood properties controlling the quality of manufactured solid wood products. Furthermore, Douglas-fir does not self-prune within the rotation lengths currently practiced. This paper synthesizes extensive Douglas-fir research in the Pacific Northwest addressing: (1) the effects of silviculture and genetics on branch structure and associated consequences for wood quality and the product value chain; and (2) methods to measure, monitor, modify, and model branch attributes to assist managers in selecting appropriate silvicultural techniques to achieve wood quality objectives and improve the value of their Douglas-fir resource.
Highlights
The natural dynamics of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands lead successively through stages of crown closure, branch suppression, and branch mortality near the crown base, with gradual rise in the base of the live crown, slow sloughing of dead branches, and eventual production of clear, knot-free wood typified by the clear bole surfaces of relatively old trees
We focus on Douglas-fir as the economically most important species in the Pacific Northwest of North America (PNW), but the concepts and approaches are applicable to characterizing primary branches and their implication for knot attributes, wood quality, and productive value in other softwood timber species as well
No PNW Douglas-fir growth model is linked to this sawing simulator, so economic analyses based on product recovery from sawing simulation are possible, but considerable post-processing of model output is necessary to provide input to the sawing simulator
Summary
The natural dynamics of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stands lead successively through stages of crown closure, branch suppression, and branch mortality near the crown base, with gradual rise in the base of the live crown, slow sloughing of dead branches, and eventual production of clear, knot-free wood typified by the clear bole surfaces of relatively old trees. A wide variety of products are manufactured from Douglas-fir These include appearance grade lumber where quality is based on the relative amount of clear wood (free of knots) and is influenced more by knot placement/pattern than size. A knot formed from the dead portion of a branch has tracheids that are structurally disjunct from the tracheids in the tree stem, so is referred to as dead, unsound, loose, or black Along with this distinction between live and dead, knots are described as star-checked (containing radial checks); firm (containing incipient decay); pith; or encased (not intergrown with surrounding wood) [7]. Knot size and placement have an effect on where failure under loading occurs, because grain distortion around knots represents a source of weakness (Figure 3)
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