Thinning Effect on Tree Growth in Secondary Betula ermanii Stands on Scarified Sites

Abstract

We conducted a thinning experiment in secondary Betula ermanii stands on scarified sites and examined the thinning effect on stand and tree growths after the thinning. The experiment was conducted in natural B. ermanii stands in the Uryu experimental forest of Hokkaido University, and 3 plots thinned to approximately 1,000, 2,500, 5,000 trees/ha and an unthinned, control plot were established at 16-year-old stand age. After 20 years of the thinning, mean diameter at breast height was 8.4-13.1 cm and larger in the order of thinning intensity, and therefore, thinning effect was recognized in the thinning experiment. In 1000-tree-plot, tree growth was apparently accelerated in 12 years after thinning, and thereafter, tree competition was enhanced. In 2500- and 5000-tree-plots, the relative yield indices were more than 0.9 at 12 years after thinnings, and they were considerably crowded. Relationships between mean diameter increment and tree diameter differed among plots after the thinning, but the differences diminished with tree growth. In conclusion, we recommend an intensive thinning in natural B. ermanii stands in young growth stage, because tree growth would be enhanced and thinning cost would be small in such a thinning system.