Seedling establishment and regeneration of Korean red pine (Pinus densiflora S. et Z.) forests in Korea in relation to soil moisture

To identify limitations on the regeneration of Korean red pine (Pinus densiflora S. et Z.) in its native range, we evaluated seed production, dispersal, germination, and seedling establishment under a range of canopies that intercepted 0% (open ground, OG), 70% (canopy gap, CG), and 97% (closed canopy, CC) of the light. To evaluate the importance of drought we added a treatment, which supplemented soil depth and irrigated to the most open setting (IO) as a reference plot. Depletion in soil moisture content was measured gravimetrically over a range of depths in the laboratory as well as over the specified site conditions where seeds (100 per 90 cm2) had been sown. Seedling germination, survival, water potentials ((psi)), and relative water contents (RWC) were also monitored. Seed production proved stable interannually, averaging 25–27 m^2 year^-1 with 42–44% viability. Seed gravity dispersal fell off exponentially from the edge of a 70-year-old pine stand, approaching zero at a distance of 80 m based on the relationship equation developed between seed number and distance from the forest edge. With irrigation, 90% of the sown seed germinated; without irrigation <40% germinated. Subsequently, seedling survival following germination was 99% in the IO treatment, 0% on OG, 32% on CC, and 8% on CG. Differences in seedling mortality were attributed to the degree that they experienced water stress; seedling death was recorded at (psi) < -5.0 MPa and RWC <50%. P. densiflora has advantages in being able to survive in its native habitat by adaptation to withstand a long season (November–June) of little precipitation and to take quick advantage of improved growing conditions following spring rains.