The effects of transformation of even-aged stands to continuous cover forestry on conifer log quality and wood properties in the UK

There is an increasing move in the UK to transform even-aged, single-species conifer plantations to continuous cover forest, i.e. more diverse and irregular stand structures. However, experience of doing this is limited and research to date has not addressed the consequences of this change on timber quality. This paper reviews the impact of transformation on timber quality and wood properties and uses coupled growth and timber property model simulations to examine the effects of different transformation scenarios on Sitka spruce (Picea sitchensis (Bong.) Carr.). The results of the modelling broadly confirmed the conclusions from the literature review. Five key aspects of transformation are considered. Retaining trees to older ages can produce higher quality timber with improved mechanical properties. Regular selective thinning and increased use of crown thinning will improve timber quality, but timing is critical to avoid producing highly tapered trees with heavy branches. Creating gaps in a uniform canopy will generally have a negative impact on the timber quality of the trees around the gap edges. Increased variation in tree age, size, spacing and species will result in greater variation in log quality and wood properties. Using natural regeneration reduces the opportunities for improved growth and timber quality offered by selectively bred planting stock but can deliver good-quality timber if the characteristics of the original stand are suitable and adequate stocking is achieved. The main conclusion of this review is that transformation to continuous cover forestry will not lead to a significant reduction or improvement in the quality of timber being produced in forests in the UK. The main effect will probably be to increase the variation of log sizes and wood properties that are available in the market.