Bacterial modification of Douglas fir roundwood permeability

Douglas fir roundwood in New Zealand is currently not used in high decay situations because the heartwood is not sufficiently durable and the sapwood is not permeable to water-borne preservatives. The results in this study have confirmed that during water-storage of wood, bacteria selectively attack pit membranes, improving permeability, which leads to enhanced preservative uptake. To optimise conditions for bacterial growth and distribution within wood, the effects of temperature, nutrient supplements, pH and incising/kerfing were investigated during the sprinkling of short Douglas fir bolts in small tanks. For water-sprinkling to be commercially acceptable the treatment time must be as brief as possible. Bacterial access to the wood is an important limiting factor. Tangential movement of bacteria in wood was found to be more rapid than radial movement and so the effect of incising or kerfing on bacterial access was investigated. Kerfing provided an ideal pathway for tangential entry into wood. Nutrient supplements in the sprinkling solution, particularly nitrogen and phosphate together, enhanced bacterial growth and enzyme activity, reducing the time required to achieve total sapwood preservative penetration. Sprinkling had no effect on heartwood permeability. Sprinkling treatments also influenced the drying characteristics of Douglas fir. Short sprinkling times (1-4 weeks) reduced radial and tangential drying rates. Longer sprinkling times improved the rate of drying. Shrinkage measurements were made on small cubes cut from sprinkled wood. Tangential shrinkage was reduced but radial shrinkage was unaffected; this should reduce the incidence of checking in water-sprinkled wood