Abstract
To examine the conversion of conifer plantations to mixed forests comprising coniferous and broad-leaved trees, we investigated responses of understory vegetation to variable-intensity thinning in 39-year-old Todo fir (Abies sachalinensis) stands 8 and 11 years after thinning, with 28-year-old unthinned stand for comparison. The following three operational thinning levels were used: one-row thinning and four-rows leaving (Th1), two-rows thinning and three-rows leaving (Th2), and three-rows thinning and two-rows leaving (Th3). Understory relative Photosynthetic Photon Flux Density increased with thinning intensity. Species richness and density of broad-leaved trees were positively influenced by thinning. However, in thinned stands, tree density decreased with thinning intensity. The maximum height and cover of both shrubs and herbs were positively affected by thinning, with the highest thinning level resulting in the highest maximum height and cover. The maximum height of herbs was higher than the mean height of trees under the Th2 and Th3, which may have hindered tree growth. However, 2,500∼3,750 seedlings/ha of ≥100 cm in height occurred under the Th2 and Th3 regimes, which may be sufficient for regeneration. Therefore, it may be feasible to apply thinning to convert conifer plantations to mixed forests.
