New Zealand Journal of Forestry (1967) 12(1): 54–62
©New Zealand Institute of Forestry
Wood Density as a Criterion for Thinning Douglas Fir
J. Madden Harris
The use of wood density as a criterion for selecting Douglas fir trees for thinning has been shown to be feasible for first thinnings of a well-stocked stand (399 stems per acre) aged 40 years. It proved possible to favour the inclusion of trees with high wood density, and to exclude 16 of the 80 trees per acre with lowest wood density, during a thinning which left 166 well-formed stems per acre for future growth.
The immediate effects of this selection on the average wood density for the site were not very marked, and resulted in an estimated increase of only 0.008 g/cc. The effect on future wood production (as estimated from wood density of the outer growth layers at breast height) should be more marked. 97% of the trees will be producing mature wood with basic density 0.40 g/cc or better, and the average modulus of rupture of mature wood produced from now on wilt be improved by approximately 5% (290 lb / sq. in.) as a result of the selective thinning. The development of new techniques to use wood density as a criterion for selection could well improve on these results, and, if malformation is not severe after the first thinnings, it should be possible to make even more effective use of wood density as a selection criterion during subsequent thinnings.
Selection to increase wood density would be of value when selecting trees for special purposes, such as seed production, even using the relatively costly methods currently available for estimating the wood density of standing trees. It is also believed that the results justify further experimental work being undertaken to develop instruments for the economical estimation of wood density so that it can be used for tree selection on a larger scale.