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Extended Abstract Introduction and Objective: Obtaining information and identifying the changes and dynamics of forest ecosystems in different developmental stages is necessary for optimal management, restoration and improvement of forests. The aim of This research was to investigate the effect of different development stages of the stand on tree regeneration and understory species in the mixed hornbeam stand in spring and summer (due to the beginning of vegetative activities until the optimal vegetative stage of herbaceous plants) in the forestry plan of Gorgan Shasat-Kalate district two was done. Material and Methods: In order to investigate development stages, after forest surveying, three rectangular half-hectare sample plots with dimensions of 50 x 100 meter for each development stage based on the characteristics provided by Leibundgut and Korpel in the field of the structure pattern of development stages and phases in natural forests were used. In order to study and investigate the vegetation cover, five small sample plots (20x20 meters), one of them in the center and four other in the four corners. In each sample plot, the percentage of herbaceous species cover and the tree regeneration frequency were recorded. To investigate and compare diversity in different development stages, use the diversity indices of Simpson and Shannon-Wiener, and to calculate the richness, use the richness indices of Margalf and Manhinik, and for evenness, use the evenness indices of Smith-Wilson and Evenness in PAST and R software (package Adiv software and specieseve function( were used. Results: The results showed that the number of plant species in the degradation stage is more than the initial and optimal stages. Diversity indices of understory species and tree regeneration species showed that the diversity of Shannon and Simpson in the two seasons of spring and summer in the optimal stage was lower than the initial and degradation stages. Also, Smith-Wilson and Evenness indices had the highest values in both spring and summer in the optimal stage. The highest value of the Menhinic richness index of understory species was determined in the optimal stage in the spring season, while no significant difference was observed between the different stages in the summer season. The highest value of Margalef richness index of tree species regeneration in both seasons was also observed in the degradation stage. Conclusion: Based on the results of the present research, it can be stated that different development stages have an effect on the diversity of herbaceous species and forest regeneration. Therefore, there are significant implications of sustainable management on the vegetation layer, which should be considered in future of forest management strategies.