Comparison of Two Site Preparation Treatments for the Growth of Direct-Seeded Fraxinus chinensis subsp. rhynchophylla Seedlings and Their Effects on Soil Temperature and Understory Vegetation

Direct seeding is considered a versatile and cost-effective approach to forest regeneration; however, its broader application is limited by low seedling survival rates and species-specific regeneration requirements, which often necessitate site preparation. We investigated the emergence, survival, and growth of Korean ash (Fraxinus chinensis subsp. rhynchophylla (Hance) A.E.Murray) seedlings regenerated by direct seeding over six years following two site preparation treatments—scarification and mixing—to determine appropriate site preparation methods for direct seeding and to assess the effects of site preparation treatments on soil, understory vegetation, and seedling growth. Additionally, the seed germination, shoot and root lengths, and biomass of the seedlings were investigated over 50 days in a growth chamber using soils from each site preparation treatment to examine early-stage growth responses. Both scarification and mixing treatments enhanced seed germination and seedling establishment. Seedling emergence rates were similar between treatments; however, the seedling mortality and the height and coverage of competing understory vegetation were significantly greater at the scarification treatment than at the mixing treatment during the first year (p < 0.05). Both treatments reduced minimum winter soil temperatures during the first two years, with frost heaving identified as a primary cause of early seedling mortality. From the second year onward, seedling growth was significantly greater in the mixing treatment (p < 0.05), which also more effectively suppressed competing vegetation. A shallow depth mixing treatment (<5 cm) is recommended for direct seeding of Korean ash, as it reduces frost heaving damage and facilitates seedling survival and growth by minimizing understory competition.