To respond to the accelerating biodiversity loss in forest ecosystems caused by climate change and forest disturbance, it is crucial to examine the long-term changes in understory vegetation, which is fundamental to the maintenance of ecosystem functions. For over a decade, we analyzed changes in understory species composition and the environmental factors influencing them in the Pyeongchang Gariwangsan Long-Term Ecological Research (LTER) site. We conducted vegetation and tree surveys in 2014, 2019, and 2024 across 100 subplots (10 × 10 m) established within a 1-ha permanent plot. Importance values, species-area curves, partial canonical correspondence analysis (partial CCA), partial redundancy analysis (partial RDA), non-metric multidimensional scaling (NMDS), and multi-response permutation procedures (MRPP) were used to evaluate the changes. As of 2024, we identified a total of 141 taxa in the understory vegetation. There was a greater variation in species richness in the herb layer than in the shrub layer. In all the survey periods, there was a continuous decline in stand density as mortality exceeded recruitment. In the importance value analysis, the shade-tolerant species had high importance values in both the layers. The interpretation could be that the species were associated with changes in light conditions caused by canopy closure and the formation of canopy gaps in the overstory. Ordination analyses revealed that both environmental and spatial factors affected understory species composition. Temporal changes in species composition were more pronounced in the herb layer than in the shrub layer. The environmental conditions driven by changes in overstory stand structure was changing the understory vegetation of Gariwangsan toward a dominant, shade-tolerant species. Our findings suggest that the current Gariwangsan stand is in the mid successional stage and highlight the need to integrate long-term monitoring in the future.