Fig. 1: Native Douglas fir is suitable for a variety of different purposes. (Photo: FVA/T. Weidner)
After more than 100 years of cultivation of Douglas fir in the forests of Germany the timber industry can be supplied with all types of roundwood from small-diameter wood to high-quality large-diameter timber in very large dimensions. However, the current stocks and uses of the Douglas fir constitute only a fraction of the stocks and uses of the spruce.
A regional market for Douglas fir timber
Fig. 2: Volume harvested without bark in accordance with the National Forest Inventory (BWI2) (former FRG)
Thus, Douglas fir timber has hitherto played a relatively large part only in the regional market. As well as the conventional use as construction timber it is used in particular in specialised applications, such as for plank floors, which require very large dimensions, or for high-value masts. The timber has several significant advantages as compared to other conifers, but at the same time demands specialised skills in processing and using it. Therefore a range of companies have become specialists in this type of timber. More recently, however, processors have set up businesses in modern sawmills to cut small and medium-diameter roundwood as sawn timber for the mass market. All in all, there is great potential for the use of this timber.
Features of the timber and technological properties
The native Douglas fir shows much greater variation in the features determining the quality of the timber and its technological properties than can be observed in any other of our commercially important conifers. In an analysis of the technological properties of the wood in Douglas firs ready for harvesting in Baden-Württemberg (Sauter 1992) the sawn timber in the form of squared timber and board lamellas showed mean values of between 1.5 and 8.2 mm in width of the annual rings per cross-section of sawn timber. On average in the timber examined the values lay around a moderate 4 mm. The width of the annual rings may be regarded as the indicators of significant properties of the wood which determine quality. As a rule, wide rings in the wood mean a low wood density.
Therefore it is not surprising that the wood density of the same sawn timber, measured across the cross-section of the trunks and for three trunk heights typical for the Douglas fir, also showed fluctuations across a wide range of values from 0.38 up to 0.68 g/cm³ for kiln-dry timber. The peak values at both ends of the scale remain concealed by investigation of sawn timber in line with practice in the dimensions in normal use. On average the wood density is in the region of 0.52 g/cm³ and is thus significantly greater than that of the average spruce and fir wood from the same region. The high density level of the Douglas fir compared to other major commercial tree species has also been demonstrated in summary form for Canadian conditions by Jozsa and Middleton (1994).
| Tab. 1: Properties of the Douglas fir timber. | ||||
| Source | Wood density (at 12% humidity) kg/m3 | Static modulus of elasticity N/mm2 | Bending strength N/mm2 | Tensile strength (lamella) N/mm2 |
| Fischer 1994 (n=415) | 471 | 10.603 | 27,1 | - |
| Glos et al. 1995 [10] (n=165) | 501 | - | - | - |
| Glos et al. 1995 (n=268) | 488 | 16.357 | 36,9 | - |
| Sauter 1992 (n=393) | 506 | 12.576 | 24,7 | 42,3 (n=129, s=25,5) |
| Pelz et al. 1998 (n=115) | 438 (s=33) | 9.158 (s=2.824) | 18,1 (s=7,4) | 22,9 (n=116, s=10,03) |
Branchiness
As well as the wood density, the form of the branches is another major factor in determining the mechanical properties of the Douglas fir wood. In this area also the native Douglas fir spans a range of values from the smallest to the largest on the sawn timber surface in accordance with sawn timber standard DIN 4074 for measurable relevant branch diameters of around 70 mm. On average the branches have a diameter of between 20 and 40 mm, in extreme cases, however, up to 90 mm.
Both of the features of the wood which determine its mechanical properties, wood density and branchiness, are affected to a high degree by differences in growing conditions. As well as the choice of location and provenance, the tending of the stand, in particular the control of the spacing conditions for individual trees, takes on major significance for the production of high-quality Douglas fir.
In general the native Douglas fir is characterised by its very good mechanical properties in bending and tensile strength, as they show in the case of typical loads as construction timber, as robust square timber or as laminated timber beams. Because of its high wood density in comparison with the other conifer species, the Douglas fir sawn timber reaches high load rating values despite having branches of comparably large dimensions and is thus normally associated with better load rating classes. This circumstance becomes highly relevant because it allows comparable small or slim sawn timber cross-sections for given branchiness to be used. This property makes Douglas fir timber very attractive for modern timber construction.
Juvenile wood
There are also risks. Rapid juvenile growth causes wide annual rings in the core and thus high proportions of juvenile wood in the roundwood harvested. About 20 annual rings around the pith are ascribed to juvenile wood in the Douglas fir. The negative impact of juvenile wood on the mechanical properties reduces the suitability of the Douglas fir sawn timber for construction purposes in a way that cannot be ignored. These effects cannot always be compensated by the typically smaller branch sizes near the medulla.
Automated sorting required
Nowadays the sorting of sawn timber according to its load ratings (DIN 4074) should without doubt be done by sorting machines. If the sawn timber is sorted by the admissible visual methods, the load rating potential of the Douglas fir wood cannot be used to its maximum. Among other functions, sorting machines are able to identify with certainty the basically high gross densities, which results overall in an increase in the commercially significant exploitation (of value).
Range of applications
As well as the applications described in the construction timber sector, native Douglas fir is popular and suitable for a wide range of other applications. Some examples are the constantly growing area of façades, balconies, car ports and timber for landscape gardening. Native Douglas fir is also being used increasingly for interior fitting in the form of floors, window scantling and wall cladding.