Baseline Survey and the Immediate Thinning Impact on the Stand Composition of Woody Plants and Overstory Structure of a Sugi Plantation (Cryptomeria japonica) in the Zenlen Area

The practice of forest thinning can influence the functions and structure of forest ecosystems. To investigate the effects of alternative thinning strategies on the stand structure and tree composition of a sugi (Cryptomeria japonica) plantation, this study was conducted on national forests in the Nandai Working Circle Area. In a 35-yr-old plantation, the baseline stand status was surveyed to determine the inventory of timber resources in the study area on 12 plots with a size of 1 ha each. Among them, a randomized block design was adopted for 3 treatments with 4 replications for each treatment in 1 ha. A gap thinning rule was used to remove trees with 3 levels of thinning intensity in terms of removing 0, 25, and 50% area of sugi trees in a plot. In each plot, all woody plants with a diameter at breast height (dbh) of ＞1.0 cm were tallied, tagged, identified to species, the position was recorded, and dbh was measured. Baseline survey results showed that due to variations in growth among trees in the past, inconsistencies in tree densities were evident among plots. While the basal area shared by understory woody plants was quite small in the plantation (i.e., ＜5%), the enhancement of biodiversity was obvious. In terms of Shannon diversity index, the biological diversity of all woody plants increased after thinning, but there was little change in overstory trees. Vertical evenness of the overstory tree canopy was reduced after thinning and there was a not noticeable left-truncated Weibull dbh distribution after thinning.