Douglas-fir is the second most commonly planted commercial tree species in New Zealand. Forest owners and managers who grow Douglas-fir now have a better understanding of how they might improve the performance of its timber thanks to work carried out at Scion.

The research team looked at site-related factors that could influence wood density such as climate and soil fertility. They pulled together data on wood density collected over more than 50 years to assemble a dataset with data from around 10,800 trees. These data were “crunched” to develop models to explain the variation in wood density that occurs within and among trees.

They found that breast height outerwood density was higher in warmer areas. A similar relationship is seen for radiata pine where it is thought that warmer temperatures allow a longer period of production of denser latewood during the growing season.

Soil fertility (ratio of carbon to nitrogen) was also found to affect wood density with trees grown on more fertile soil tending to have less dense wood. A similar relationship is also seen for radiata pine.

Silviculture factors such as stand density did not seem to have a great effect on wood density, which was a somewhat unexpected result. However, stand density is still expected to affect the mechanical properties of sawn timber, particularly through its effect on branch size.

Looking within trees, the pith to bark profile of wood density differs from that in radiata pine. It is high near the pith, decreases over the first few rings before increasing again and stabilising at around ring 30 from the pith. Density also tends to be higher at the tree base than at its top.

The wood density models developed in this work have been incorporated into the Forecaster growth and yield simulator. Using this, forest managers will be able to devise strategies that take into account the effects of site and silviculture on Douglas-fir wood density to achieve desired end product outcomes.

Contact Peter Clinton for more information

Kimberley, M. O., McKinley, R. B., Cown, D. J., & Moore, J. R. (2017). Modelling the variation in wood density of New Zealand-grown Douglas-fir. New Zealand Journal of Forestry Science, 47(1), 15. https://doi.org/10.1186/s40490-017-0096-0