Investigating the dynamic changes in forest communities is of substantial significance for understanding the conservation mechanisms, maintenance processes, and promotion of ecosystem restoration in forest ecosystems. This study draws upon a 15-year census dataset (2007, 2012, 2017, 2022) derived from a permanent 1-hectare monitoring plot located in a karst evergreen-deciduous broadleaved mixed forest within the Mulin National Nature Reserve. All woody plants with a diameter at breast height (DBH) ≥1 cm were surveyed, enabling a detailed examination of temporal trends in community composition, importance values, diameter-class structure, biodiversity indices, and population dynamics. The key findings are summarized as follow: (1) Over the 15-year period, the total number of stems declined from 3380 individuals (41 families, 81 genera, 99 species) to 2128 individuals (42 families, 79 genera, 98 species), indicating a reduction in density but relative stability in floristic composition. Concurrently, key biodiversity indices showed distinct trends: species richness, the Shannon-Wiener index, and the Simpson index all gradually decreased. In contrast, Pielou index initially increased before subsequently declining. (2) The size structure of the community shifted significantly. The abundance of small-diameter individuals progressively decreased, while the number of large-diameter individuals increased. An ecological pattern was evident where evergreen species dominated the small and medium diameter classes, and deciduous species were predominant in the large diameter class. Furthermore, indices of structural diversity-the coefficient of variation (CV) and the Gini coefficient of DBH-both exhibited a pattern of initial decline followed by a later increase. (3) Analysis of the top 14 species by importance value revealed that only Mallotus philippensis experienced a continuous decline, leading to its eventual disappearance from the plot. Comparing the latter five-year interval (2017-2022) with the preceding decade showed a notable shift in demographics for these dominant species: mortality rates decreased, while recruitment rates and the net population change rate increased. Dynamics varied strongly with tree size: small-diameter trees had a high mean annual mortality rate (3.18%) and a recruitment rate that rose from 0.36% to 2.14%. Medium-diameter trees showed a moderate annual mortality rate (1.44%), and large-diameter trees were highly stable, with an annual mortality rate of only 0.17%. The combined medium and large-diameter classes had an annual recruitment rate of 1.38%. These results clearly demonstrate that population stability strengthens with increasing tree size. In summary, this research elucidates the unique successional trajectory of karst forests under environmental stress. The findings on community reorganization and size-dependent demographic shifts provide a scientific basis for guiding biodiversity conservation and ecological restoration in subtropical karst ecosystems.