Effect of growth rate and radial position on the natural durability of Douglas-fir

In terms of natural durability, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) fame owns to the favourable characteristics of centuries-old trees harvested in old-growth North American forests. The properties of wood coming from plantations harvested between 50 and 100 years-old are likely to be different. In such stands, plantation density and thinning intensity may have a large impact on the trees growth rate. Since this parameter is known to affect some properties of the wood, it was decided to assess to what extent an increase in Douglas fir growth rate affects the natural durability of its wood. This issue is indeed poorly documented in the scientific literature.
This parameter was evaluated on standardized heartwood specimens taken from 60 trees originating from 10 stands in Wallonia (Southern Belgium). In all these stands, the average girth of the trees ranged between 140 and 160cm, whilst their age (from plantation) ranged from 38 to 66 years old: These stands are thus representative of very contrasted silvicultural management practices. In terms of individual growth, the Mean Ring Width of the trees ranges between ca 3 and 7mm. Globally, 600 tests specimens were taken from two radial positions in the heartwood of each tree. Half of the specimens were taken in the heartwood at the border of the sapwood; the other half encloses the 20 years old ring, counting from the pith. The mass losses caused by the wood decaying fungus Poria placenta were assessed according to Cen/ts 15083-1 (2005).The natural durability of the wood is discussed as affected by sites, trees, radial positions in tree and tree growth rate.