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New Zealand Journal of Forestry (1977) 22(1): 24–44
©New Zealand Institute of Forestry

Research article
New Zealand Douglas fir timber quality in relation to silviculture

I.D. Whiteside , M.D. Wilcox and J.R. Tustin

The results of a number of recent timber and wood quality studies on New Zealand-grown Douglas fir have some important silvicultural implications. Branch size is by far the most important factor influencing timber stiffness and strength, and density is the next most important. In a recent study they together accounted for about 80% of the variation that occurred in timber stiffness as revealed by machine stress-grading. The most satisfactory multiple regression equation linking modulus of elasticity of 100 X 50 mm timber loaded as a plank (Ep) with branch size and wood density was EP (gigapascals) =2.9014— 0.07048 branch index (mm) + 0.01269 density (kg/m3) where Ep is the mean Ep of all pieces cut from a log, density is the mean density of these pieces, and branch index is that of the log concerned. Branch index was determined by measuring the largest branch per quartile per 1.2 m length, and averaging the 16 measurements per log so obtained.
Contrary to earlier indications, radial growth rate in itself has been shown to have only a small effect on density in wood of the same cambial age, particularly in wood more than 12 growth rings from the pith.
With machine stress-grading, much larger knot sizes can be tolerated than with visual grading to current rules, particularly when wood density is high. It is concluded that only if the sawn output is machine graded can further planting of Douglas fir for production of timber for engineering and framing uses possibly be justified.
It is considered that the objective in management of Douglas fir in New Zealand should be to grow trees as rapidly as possible and on as short a rotation as possible consistent with keeping branch index in the bottom two 4.8 m log lengths down to 36 mm, assuming that mean wood density will be approximately 400 kg/m3. If this is achieved, at least 60% of the sawn timber volume produced from the bottom two logs should qualify for exacting engineering uses with machine stress-grading. This is far superior to what can be achieved with radiata pine grown on current silvicultural schedules.
*Principal Forester and Scientists, Forest Research Institute, Rotorua

A silvicultural regime which should maintain branch size in the bottom two logs within the required limits is given. It involves a relatively wide initial spacing, heavy early thinning, and a short rotation age, and is radically different from regimes currently practised. No attempt is made to provide an economic justification for this regime.

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