Enhancing weeding strategy for Pinus thunbergii plantation in South Korea: Impacts of timing and frequency on the growth parameters

Weeds significantly affect the survival and growth of seedlings in the early stages of forest plantation. The present study investigated the effects of various schedules of weeding treatments on the survival ratio (SR), height (H), root collar diameter (RCD) growth, and growth modules of Pinus thunbergii seedlings over five years in Korea. The treatments consisted of eleven different weeding schedules employed from 2017 to 2021: control (no treatment), once-a-year weeding in June (W6), July (W7), August (W8), and September (W9), and twice-yearly weeding or combination of the above months with interval of 1, 2, and 3 months (e.g., W6/7, W7/8, W8/9, W6/8, W7/9, W6/9). In 2019 only, the SR of P. thunbergii was significantly higher in the once-yearly (84.7 %) and twice-yearly (83.8 %) weeding treatments, averaging 20.4 % more than the control (63.9 %), with no significant differences across treatments in other years. Similarly, the height for once-yearly (82.7 cm) and twice-yearly (82.3 cm) weeding treatments differed significantly from the control (94.6 cm) only in 2019. A notable increase in RCD over the years across all treatments, with an average annual increase of 55 % (p < 0.001) was observed from 2020. Generally, the annual growth modules of P. thunbergii seedlings were differed significantly over the years, while all weeding treatments had no significant effect. While weeding effects were not consistently significant across the study period, the accumulation of subtle growth differences over time suggests that early growth stages may play a critical role in the long-term structural stability of P. thunbergii seedlings. Therefore, long-term monitoring is required to better understand delayed weeding responses and refine plantation management.