Many factors affect forest structure and species composition suchas climate change and human activities that cause loss of forest area andbiodiversity. The study of forest dynamics is an importantstrategy for managing forest areas. Therefore, this studyaims to clarify forest dynamics over twelve-year periodsin mixed deciduous forests (MDF) at QueenSirikit Botanic Garden (QSBG). Apermanent 100 × 100 m² plot was established in 2012, with tree monitoringconducted during the establishment year and subsequently in 2018 and 2024. Trees (at diameter at breast height (DBH) at 1.3 m ≥ 4.5 cm) and saplings (2cm ≤ DBH < 4.5 cm) weremeasured, tagging number and identified to species. Theresults revealed that 84 species from 34 families were found in MDF. The dominant species in the plot, based on the importancevalue index (IVI), were Xyliaxylocarpa (Roxb.) W.Theob. var. xylocarpa, Schimawallichii (DC.) Korth., Castanopsis acuminatissima (Blume) A. DC. and Dipterocarpusobtusifolius Teijsm. ex Miq. TheShannon-Weiner index (H´), which indicates tree diversity, increased slightly from 3.57 to 3.68 in 2018 but decreased to 3.57 in 2024. The basal area (BA) was increased during twelve-year periods. The recruitment (R) rates(6.74%/year) was higher than the mortality (M) rate (3.78%/year). Moreover, the R and M rate in thesecond period were higher than in the fi rst period. Themean relative growth rate (RGR) was 1.99% per year. Theaggregated or clumped pattern was the main pattern of tree distribution basedon Morisita index (Iδ). This long-termecological research showed that the forest became covered by colonizing bamboo. This important information was used to design conservationstrategies for MDF in QSBG, focusing on bamboo control, native speciesrestoration and biodiversity conservation.