Effects of canopy on the sapling composition and structure in a subalpine old-growth forest, central Japan

Abstract

Species composition of the sapling bank and size structure of Abies saplings under different canopy conditions, such as the evergreen coniferous canopy of Abies spp. (evergreen canopy), the deciduous broad-leaved canopy of Betula spp. (deciduous canopy), and canopy gaps were examined in a subalpine old-growth forest of the northern Yatsugatake mountains, central Japan. Light levels tended to be lowest under evergreen canopies, higher under deciduous canopies, and highest in canopy gaps. The species composition and structure of conifer sapling (≥ 15 cm tall and dbh < 5 cm) banks varied with the canopy conditions. Abies mariesii saplings were the exclusive dominants under both evergreen and deciduous canopies and in canopy gaps. A. veitchii saplings were subdominant, and Tsuga diversifolia and Picea jezoensis var. hondoensis saplings were few under all canopy conditions. In canopy gaps, the relative density of A. veitchii saplings was highest and that of A. mariesii saplings was lowest under canopy conditions. A. veitchii saplings might thus outcompete A. mariesii saplings in canopy gaps. The sapling height distributions of Abies spp. were different depending on light conditions; they were L- or reverse J-shaped at lower light levels and bell-shaped at higher light levels. Species composition of the sapling bank and the size structure of Abies saplings are considered to be influenced not only by the difference between closed canopy and canopy gaps, but also by the difference between evergreen and deciduous canopies due to the different light conditions.