Effect of growth rate on the physical and mechanical properties of Douglas-fir in western Europe

Abstract

To quantify the impact of forest management practices and tree growth rate on the potential uses of Douglas-fir wood, nine physico-mechanical properties were studied on more than 1250 standardized clear specimens. These were collected from trees cut in 11 even-aged stands (six trees per stand) located in Wallonia (southern Belgium). Stands were 40 to 69 years old, and mean tree girth was ca. 150 cm. Mean ring width of the 66 trees ranged from 3 to more than 7 mm. Statistical analysis showed significant but weak effects of ring width on the studied properties. Considered jointly, mean ring width and cambial age of the test specimens only explained 28% to 40% of the variability of their properties. Also, when ring width increases, these properties display higher decreases in juvenile wood than in mature wood. From a technological standpoint, maintaining mean ring width under 4 mm in juvenile wood and 6 mm in mature wood should accommodate all potential uses of Douglas-fir wood. However, considering that density appeared to be the main driver of wood properties, our results and the literature corroborate the importance of genetic selection as a complement to silvicultural measures to improve or guarantee the technological properties of Douglas-fir wood.

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

“…The lower density values presented by the 70 × 180 mm 2 boards are therefore logical since, for a given ring width, corewood density is ca. 10% lower than that of outerwood [13,20].…”

“…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.…”

“…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 determines lower mechanical properties of Douglas fir wood [12][13][14]17,20], this trend is less pronounced than in other softwood species [17].Additionally, given the differences in mechanical properties of core-and outerwood (As defined in [23]) [13,15,20,24], the type of wood should also be taken into account as much as possible in the analysis of the impact of tree growth rate. In this respect, it is important to note that the age at which wood exhibits its maturity characteristics notably relies on the property considered [14]: for example, microfibril angle, tracheid length, or density do not necessarily stabilize from the same growth ring [25].…”

“…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 determines lower mechanical properties of Douglas fir wood [12][13][14]17,20], this trend is less pronounced than in other softwood species [17].…”

“…Additionally, given the differences in mechanical properties of core-and outerwood (As defined in [23]) [13,15,20,24], the type of wood should also be taken into account as much as possible in the analysis of the impact of tree growth rate. In this respect, it is important to note that the age at which wood exhibits its maturity characteristics notably relies on the property considered [14]: for example, microfibril angle, tracheid length, or density do not necessarily stabilize from the same growth ring [25].…”