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As one of the three major five-leaved pines in the northern hemisphere,Pinus koraiensis is the most important dominant tree species in the natural mixed-broadleaved Korean pine forests. However, the regeneration ofP. koraiensisunder the canopy of secondary forest stands is poor because of the light limitation. This study was conducted to understand howP. koraiensisseedlings adapt to different light intensities and what would be the optimum light level for their establishment and growth. Three repetition plots with four light intensities (15%, 30%, 60% and 100% of the natural incident irradiances, achieved by suspending layers of black nylon net above and surrounding the plots) were set up under natural climate conditions in a montane region in eastern Liaoning Province, Northeast China. A total of 80P. koraiensis seedlings with similar height and root collar diameter were transplanted into four plots. After one year of acclimation to the specific light conditions, the seasonal variations of the photosynthetic variables and needle traits of the current and one-year-old needles, and the growth parameters were observed under four light intensities. The results indicated that: (1) The seedling at 60% treatment exhibited the greatest growth, which agreed with the response of the light-saturated photosynthetic rates (Amax) and the dark respiration rate (Rd) in the current and one-year-old needles, i.e.,Rd at 60% treatment was significantly lower than that at 100% treatment, but Amax did not differ between the seedlings at 100% and 60% treatments. (2) TheP. koraiensis seedlings have a certain photosynthetic plasticity to adapt the light conditions by adjusting their needle traits and regulatingthe physiological processes, becauseAmax,Rd, light saturation point and compensation point, the needle mass area, needle nitrogen and chlorophyll contents were significantly (p