Impact of Tree Growth Rate on the Mechanical Properties of Douglas Fir Lumber in Belgium

Abstract

In the context of questioning the relevance of making Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) silviculture more dynamic in Wallonia, we evaluated the influence of growth rate on the potential of Douglas-fir lumber for structural uses. Therefore, six trees 120 to 180 cm in circumference at 1.5 m were felled in 11 stands whose age varied from 40 to 69 years (mean circumference of the trees ≈150 cm; initial planting density from ≈2200 to 4400 seedlings/ha). In total, 706 boards (38 × 100 mm 2 and 70 × 180 mm 2 in cross section) were cut from these trees, whose average ring width ranged between 3 and 7 mm. The density of the wood (ρ) always appeared compatible with the mechanical class C30, regardless of the growth rate of the trees from which the lumber originated. The modulus of elasticity (E) and the modulus of rupture (f m ) displayed by the 38 × 100 mm 2 boards cut from corewood were respectively 30% and 41% lower than those observed in outerwood. The latter did not seem affected by growth rate: E and f m characteristic values remained compatible with structural use, regardless of the mean ring width. Growth rate considerably affects the characteristic values of these mechanical properties when boards are made from corewood. Juvenile growth should therefore be limited.American species, as shown by data from the Walloon Permanent Forest Inventory (WPFI) according to which the mean annual circumference increment (MACI) in Walloon Douglas fir stands is about 3.2 cm/year (Lecomte, 2010, pers. com.), while with ad hoc silvicultural management, periodic increments of up to 4 cm/year can easily be achieved, at least in young stands [11]. As recent research in Germany [12,13] or in the United Kingdom [14,15] has shown, European forest managers are therefore questioning the impact of silviculture on the characteristics of the Douglas fir resource. In France, research on this issue has been carried out longer ago [16][17][18]. Moved by the same questions, the Public Service of Wallonia has therefore subsidized a vast study in order to assess to what extent it would be opportune to boost Douglas fir silviculture in Wallonia while ensuring the production of a material that offers the widest potential of uses. Hence, the influence of growth rate was evaluated on certain morphological characteristics (bark thickness, taper, branchiness, etc.), on the natural durability of the wood [19], on the physico-mechanical properties of clear wood specimens [20], as well as on the mechanical and visual characteristics of lumber destined for structural applications or cladding [21,22]. In addition to the intrinsic mechanical properties of clear wood, it is also important to ensure that the final product does not suffer excessively from an increase in the growth rate of the trees. Indeed, lumber presents features (knots, resin pockets, etc.) which, despite the valuable properties of the material per se, can prove to be prohibitive in a particular context of use. Moreover, although it is known that a higher growth rate determine...

The effect of thinning on mechanical properties of Douglas fir, Norway spruce, and Sitka spruce

“…Timber from unthinned Norway spruce stands had better mechanical properties than timber from unthinned Sitka spruce stands, while an opposite trend could be found in thinned stands, in both specimen sizes. The presence of juvenile wood lowers all grade-determining properties, in line with previous studies on softwoods (Brüchert et al 2000;Henin et al 2018;Cameron et al 2015). Although the longitudinal position of the sample had effects on wood properties, the direction of the effects in this study was not as uniform as in previous studies (Brüchert et al 2000;Lindström et al 2009;Rais et al 2014;Simic et al 2018).…”

“…Compared with the findings of previous studies (Roblot et al 2008;Henin et al 2018;Simic et al 2018;Gardiner et al 2011;Larsson et al 1998;Wilhelmsson and Arlinger 2002) on the same species, all of the measured physical and mechanical properties were relatively high. All sample trees in this study from thinned stands represented final crop trees; therefore, the best material available at the end of the rotation of the stands due to the type of thinning implemented, which was thinning from below with systematic row thinning.…”

Relationships between wood properties of small clear specimens and structural-sized boards in three softwood species

“…While the mechanical and physical properties examined in the current study were shown by previous studies to vary with various different factors (such as specimen size, climate, silvicultural management…), the values of mechanical and physical properties reported in the current study were mostly in the range of previously reported values for the same species (Lavers, 2002;Grottal et al, 2005;Verkasalo and Leban, 2002;Moore et al, 2009;Gardiner et al, 2011;Larsson et al, 1998). The one exception were mechanical and physical properties of Douglas-fir boards, which were slightly higher than previously reported (Henin et al, 2018).…”

Impact of spacing and pruning on quantity, quality and economics of Douglas-fir sawn timber: scenario and sensitivity analysis

“…Empirical studies exhibited the potential of Douglas-fir's wood quality focusing on old (future-crop) trees (von Pechmann and Courtois 1970;Möhler and Beyersdorfer 1987;Sauter 1992). The simulated planting densities reflected the recommended current densities (Bayerische Staatsforsten 2012) and were comparable to planting densities from the literature for the Pacific Northwest from 300 to 2960 trees ha −1 (Scott et al 1998), for Switzerland from 1346 to 2790 trees ha −1 (Schütz et al 2015), for Belgium from 2200 to 4400 trees ha −1 (Henin et al 2018) and for Germany from 1000 to 4000 trees ha −1 (Weller and Spellmann 2014;Rais et al 2014) as well as from 500 to 4000 trees ha −1 (Klädtke et al 2012).…”