Managing sun-exposed crowns to produce high-quality large timber under long-term forest management in the Yoshino Forestry Area

Abstract

We analyzed the relationships between the annual growth rate of the diameter at breast height (ADBH) and the sun-exposed crown size (sun-exposed crown diameter, length, and surface area) of trees in 38 to 210 year-old stands in the Yoshino Forestry area to propose quantitative criteria for sun-exposed crown management for the production of high-quality timber under long-rotation forestry. We obtained significant regressions of ADBH against the sun-exposed crown diameter and surface area. The reliability of the regression and convenience of field measurements suggest that sun-exposed crown diameter (Dsc) can be used as a useful criterion for sun-exposed crown management. The ADBH-Dsc relationship varied with stand age, indicating a decline in the efficiency of stem diameter growth per unit of sun-exposed crown diameter. This suggests that differences in the ADBH-Dsc relationship should be considered in long-term forest management, which includes a wide range of stand ages and tree sizes. We used the ADBH-Dsc relationship to estimate the Dsc ranges that corresponded to ADBH of 0.25∼0.54 cm/year (approximately 2-mm annual growth ring widths) for each stand age. The estimated Dsc ranges predicted annual ring widths within approximately 2 mm with 74∼88% accuracy. We conclude that this approach provides accurate quantitative criteria for a given stand age for the selection of which trees to leave for the production of high-quality timber with evenly spaced annual rings.