Herbaceous Layer Response to Overstory Vegetation Changes in Quercus mongolica Fisch. ex Ledeb. Forests in Korea

The development of overstory vegetation can considerably influence the composition and dynamics of herbaceous layer vegetation. However, the type of ecological processes underlying these changes remain poorly understood. We aimed to analyze changes in herbaceous layer species composition in Quercus mongolica Fisch. ex Ledeb. forests in central South Korea and identify the ecological processes driving succession, using zeta diversity and species turnover. We also sought to address regional bias in existing long-term monitoring data. Permanent 1 ha survey plots were established according to International Long Term Ecological Research Network guidelines, divided into 100 subplots. Data on species composition, crown openness, transmitted light, and structural variables were collected through four surveys (2014, 2015, 2017, and 2020) between 2014 and 2020. Zeta diversity and turnover metrics were used to evaluate succession dynamics. Species richness, cover, and turnover in the herbaceous layer were significantly correlated with overstory structure and rock cover. Crown openness and transmitted light declined but did not correlate with species turnover. Zeta diversity shifted from a power function model (2014–2017) to an exponential model (2020), indicating a shift from deterministic to stochastic processes. Successional changes in herbaceous vegetation may indicate a potential shift in forest structure in Q. mongolica stands—from stable, deterministic patterns to more variable, stochastic processes—highlighting the need for long-term monitoring in dynamic forest ecosystems.