Background The stem exclusion stage is a stage of forest development that is important for understanding the subsequent understory reinitiation stage and maturation stage during which horizontal heterogeneity is formed. Over the past 11 years (2009–2019), we observed a deciduous broad-leaved forest in the Albongbunji Basin in Ulleungdo, South Korea in its stem exclusion stage, where Fagus engleriana (Engler’s beech) is the dominant species, thereby analyzing the changes in the structure (density and size distributions), function (biomass and species richness), and demographics. Results The mean stem density data presented a bell-shaped curve with initially increasing, peaking, and subsequently decreasing trends in stem density over time, and the mean biomass data showed a sigmoidal pattern indicating that the rate of biomass accumulation slowed over time. Changes in the density and biomass of Fagus engleriana showed a similar trend to the changes in density and biomass at the community level, which is indicative of the strong influence of this species on the changing patterns of forest structure and function. Around 2015, a shift between recruitment and mortality rates was observed. Deterministic processes were the predominant cause of tree mortality in our study; however, soil deposition that began in 2017 in some of the quadrats resulted in an increase in the contribution of stochastic processes (15% in 2019) to tree mortality. The development of horizontal heterogeneity was observed in forest gaps. Conclusions Our observations showed a dramatic shift between the recruitment and mortality rates in the stem exclusion stage, and that disturbance increases the uncertainty in forest development increases. The minor changes in species composition are likely linked to regional species pool and the limited role of the life-history strategy of species such as shade tolerance and habitat affinity. Our midterm records of ecological succession exhibited detailed demographic dynamics and contributed to the improvement of an ecological perspective in the stem exclusion stage.

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.