50 years of low thinning in second growth Douglas-fir

Abstract

This paper presents the Port Blakely XT series, established in western Washington in the period 1948–1958. XT-1, 2 and 3 were designed to evaluate effects of repeated low thinning and extended harvest rotations. XT-7 compares the effects of different levels of removal on standing volume and wood quality.Periodic and mean annual increment trends have been examined for all four trials. Results from these trials indicate the extension of culmination of mean annual increment due to low thinning combined with extended rotations. Volume yield is reported for XT-1 and XT-2. The thinned plots reveal an increase in bole size and an increase in total harvested volume. Wood quality characteristics were measured in XT-1 and 7, evaluated for differences between thinned and non-thinned and compared to stands harvested at younger ages. Results indicate that extended rotations and low thinning increase the quantity of structural log grades when compared to stands harvested on shorter rotations.Such regimes should reduce conflicts between timber production and other values such as watershed, wildlife, and aesthetics and are particularly applicable to public ownership. Key words: growth and yield, mean annual increment, wood quality, alternative silviculture, extended rotations, low thinning