Provenance variation in heartwood natural durability of four seed origins of British grown Douglas fir (DF) (Pseudotsuga menziesii (Mirb.) Franco) has been studied. The seed origins included in the study were Darrington, J. Landing, Naselle and Hawkinsville. The trees were collected from IUFRO seed origin experimental plot at Radnor, Wales, planted in 1972; from each seed origin 30 i.e. (4 x 30) 120 trees were assessed. However, one tree was missing from the Hawkinsville seed origin.
Natural durability was evaluated by pure culture decay tests against basidiomycete fungi (Coniophora puteana and Postia (=Poria) placenta) after 4 months incubation and in a fungal cellar test after 12 months exposure. Weight loss and additionally loss in mechanical properties in the fungal cellar test were considered as a measure of decay.
In the test, the rate of decay of small stakes was monitored by the loss of modulus of elasticity (MOE) at different time intervals in a non-destructive test. At the end of the test the modulus of rupture (MOR) of the decayed samples was measured and compared with undecayed sets of samples.
To further the understanding of the effects of other properties on natural durability, some tree characteristics ( e.g. diameter, cross-sectional total area, sapwood and heartwood area, heartwood percentage, heartwood age, basic density and extractive contents and composition) were also measured. The major extractive component of acetone soluble fractions of Douglas fir was confirmed as taxifolin by HPLC analysis and quantitative estimation (mg g-¹ ) of the component was also performed.
The trees showed less durability than expected for the species. According to the BS EN 350-1, these trees can be classed as slightly to non-durable. In the fungal cellar test, soft rot attack was predominant and the rate of decay was much slower than in the pure culture decay tests. At low weight losses of about. 5%, bending properties (both MOE and MOR) lost 45-50%. MOE appeared to be a good indicator for monitoring the rate of decay as decay progressed. Both bending properties appeared to be good assessment methods to evaluate decay even at low weight loss.
Weight loss due to decay did not correlate with tree diameter. Negative correlations were found between weight loss and density, but in the seed origin Darrington the correlation was not significant. Although heartwood extractives showed a negative correlation with weight loss, the
relationships were not significant for all seed origins. The quantity of taxifolin was negatively related with weight loss, but in Darrington and J. Landing the correlations were not significant. No single factor provided a strong explanation of durability for all of the seed origins; the
natural durability varied widely and it is suggested this may depend on tree age. However, correlation between durability and other properiies of all 119 trees showed that durability of DF is dependent on density, extractives and taxifolin contents.
Quantitative variation in natural durability and other wood properties between the four seed origins and between trees within the origins were determined. Pairwise comparisons showed significant variation between some origins. These variations offer scope for selection of suitable origins or trees for desirable properties.
Pairwise comparisons between 4 origins showed that J. Landing gave better durability against C. puteana than the other origins; in terms of wood properties the origin showed higher density, bending properties and higher amount of extractives. Thus in terms of durability against C.
puteana, physical and chemical properties, J. Landing appeared as the best of the seed origins studied.
A high variation in decay studies and lower durability, physical and mechanical properties were unexpected. The lower values were explained by the predominance of juvenile wood in the sampled wood. The high variability in decay studies was attributed to sampling from different
diameter trees giving sampled wood of a variety of ages and degree of juvenility.