All phenomena within an ecosystem occur at a slow pace and unfold over extended periods of time. Therefore, long-term ecological research is essential for understanding and studying these phenomena. In Korea, Pinus densiflora is currently the most widely distributed single tree species, but its distribution is expected to change due to various factors, including climate change and the succession to shade-tolerant species. Monitoring the process of change is crucial for understanding ecosystem dynamics and managing forest stands, so we conducted diameter measurements and analyzed changes in a long-term ecological monitoring plot. The study was conducted at a 1 ㏊ (100 m × 100 m) plot in the P. densiflora forest of Mt. Jiri, established as part of the National Long-Term Ecological Research. tree diameter measurements were conducted four times biennially (2017, 2019, 2021, and 2023). Based on the collected measurement data, analyses were performed to examine density, basal area, importance value, diameter distribution, stem vitality, mortality rate, and recruitment rate. The density of the P. densiflora population decreased by approximately 6.8%, from 292 trees/㏊ to 272 trees/㏊, over the six-year period. Notably, the population of Lindera erythrocarpa showed a sharp decline of about 71.4%, dropping from 161 trees/㏊ to 46 trees/㏊ during the same period. The basal area per hectare increased for all species except L. erythrocarpa. As a result, only the importance value of L. erythrocarpa decreased, while the importance values of other species either increased or remained constant. Diameter class distribution showed an inverted J-shape, with the highest number of individuals in the <10 ㎝ diameter class for all species combined, and a normal distribution with the highest number of individuals in the 30-40 ㎝ diameter class for P. densiflora. In terms of P. densiflora stem vitality, the proportion of trees classified as AS(Alive Standing) was highest at 76.1% (252 trees/㏊) in 2017 but decreased by 12.8% (42 trees/㏊) to 63.4% (210 trees/㏊) by 2023, due to an increase in the AL(Alive Leaning) and DF(Dead Fallen) categories. The average annual mortality rate of P. densiflora over the six-year period was 1.18%, while no recruitment was observed due to the absence of seedling establishment. However, a notable finding was that the average annual mortality rate for L. erythrocarpa was significantly high at 19.75%. Although the density of the P. densiflora population in the Mt. Jiri forest decreased, the basal area and importance value remained stable, indicating a generally healthy condition. However, no recruitment was observed, likely due to the species' characteristics as a shade-intolerant tree. Further research is needed to investigate the population dynamics of P. densiflora, L. erythrocarpa, Cryptomeria japonica, and Quercus variabilis, focusing on canopy competition and changes in stand structure.