No 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.

The decay resistance of black locust ( Robinia pseudoacacia L.) heartwood was evaluated on standardized test specimens taken from mature and juvenile wood in 25 trees distributed over five sites in Wallonia, Belgium. The percentage mass losses caused by the wood-destroying fungi Coriolus versicolor CTB 863 A and Coniophora puteana BAM Ebw.15 were evaluated according to Cen/ts 15083-1 (Cen/ts 15083-1. 2005. Durability of wood and wood-based products — determination of the natural durability of solid wood against wood-destroying fungi, test methods — Part 1: Basidiomycetes. European Comm. for Standardization, Brussels). Black locust is classified as a “very durable” wood (class 1), while the juvenile heartwood (located near the pith) is classified in classes 1 to 2. The statistical analysis does not reveal any significant differences among the sites, suggesting that the growth environment of the trees has no effect on their natural durability. However, statistical analysis shows very highly significant differences among trees and between the two radial positions in the tree, with mature wood being more resistant than juvenile wood. Using such a naturally very durable species could reduce reliance on preservative treatment and the use of tropical species generally recommended for outdoor woodwork.

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...