Processes driving understory community dynamics in Ulleungdo Island broadleaved forest, South Korea

The development stage of forest trees has considerable impact on the understory plant species; however, little research has been dedicated to in-depth investigation of the changes in the ecological processes (deterministic and stochastic) over a mid-term observational period (≥10 years). We analyzed the forest floor environment, including bare mineral soil, tree communities (density and biomass), understory plant species composition, diversity, cover, and changes in co-occurring species in the Albongbunji Basin on Ulleungdo Island over an 11-year period (2009–2019). The forest floor environment exhibited a broad spectrum of changes, beginning in the middle of the observation period and after heavy rainfall, and heavy sediment deposition events were observed. The overstory tree density showed patterns of a general increase and subsequent decline, and the overstory tree biomass increased and then remained steady. The heavy rainfall and sediment deposition as disturbance events also coincided with changes in species composition and the turnover rate of the understory plant communities, causing changes in major ecological processes. The general pattern of species co-occurrence (zeta diversity) was fitted to an exponential model, indicating the dominance of stochastic processes. The results show a shift in relative importance from deterministic to stochastic processes in the successional stages of understory plant community assembly over a mid-term observational period.