Abstract: Planting genetically improved, fast-growing tree seedlings is gaining importance as a strategy to enhance forest productivity and reduce labor requirements during plantation establishment. In this study, we evaluated the early growth and survival of advanced-generation Cryptomeria japonica seedlings compared to conventional stock, under varying planting densities and cultivation methods. A field experiment was conducted over 5 years using container-grown and bare-root seedlings derived from first- and second-generation plus trees, alongside traditional seedlings. The results showed that advanced-generation seedlings exhibited higher growth in tree height, stem diameter, and crown development than traditional seedlings, particularly when planted as container stock. These seedlings also had higher survival rates, likely due to their rapid initial height growth, which reduced the risks of accidental damage during weeding operations. Wider planting intervals increased the risk of man-made injury and seedling mortality, while faster-growing seedlings were more likely to escape from competing vegetation. Our findings highlight the potential of improved seedling stock to enhance early plantation success and reduce management inputs in the critical establishment phase of forestry.

Key words: Tree breeding, Planting density, Seedling survival, Competition · Weeding