Role of thinning intensity in creating mixed hardwood and conifer forests within a Cryptomeria japonica conifer plantation: A 14-year study

Abstract

For managers of conifer plantations, conversion to mixed hardwood and conifer forests is an important management goal. However, the extent to which thinning intensity affects stand volume and timber quantity and quality remains unclear. We investigated the growth of conifer and hardwood trees over 14 years (at 5 and 9 years after the first and second thinning operations, respectively) under unthinned (Control), 33% thinned (Weak), and 67% thinned (Intensive) treatments in a Cryptomeria japonica plantation. The diameter growth of both conifers and hardwoods, stand volume of hardwoods, and relative increment of stand volume of conifers increased with greater thinning intensity in the order of Control < Weak < Intensive, whereas the reverse was observed for the stand volume of conifers, dead conifer trees, and total yield of stand volume (i.e., gross production). These results suggest that intensive thinning can create large conifers and hardwoods, but reduces gross production. Mean annual tree ring width (ATRW), consistency of ATRW value, and mean crown ratio increased with increasing thinning intensity in the order of Control < Weak < Intensive, while height-to-diameter ratio (i.e., slenderness) showed the opposite trend. These findings suggest that timber quality (e.g., wood density, stiffness, visual grading) was better under Weak than under Intensive thinning. However, the production of large conifer timbers with homogeneous ATRW may offset reductions in quality. This study suggests that intensive thinning is a reliable method for rapidly creating mixed hardwood–conifer forests at the canopy level and producing large timbers of both conifer and hardwood species, whereas weak thinning is suitable for producing high-quality conifer timbers and maintaining net primary production, although repeated thinning is necessary.