Structural changes in forest stands considering the intensity of degradation in a montane ecosystem (Case study: Abr Forest, Semnan Province)

Background: The complexity of the vegetation structure is an important feature of natural ecosystems, often associated with higher ecological performance.Studies have shown that plant communities that are structurally more diverse are also more complex.However, the shape and overall nature of this relationship, as well as the involved mechanisms, are not well known.On the other hand, the assets and services of forest ecosystems have significantly declined due to destruction and deforestation in some parts of Iran, particularly in mountainous landscapes.This study aims to evaluate the structural changes in forest stands affected by varying degrees of degradation in a mountainous ecosystem.The research focuses on analyzing structural indicators, such as abundance, species composition, density, diameter at breast height (DBH), tree height, and the regeneration status of woody species.The primary goal is to achieve a better understanding of the human disturbance process and its effects on forest structure.Additionally, the study emphasizes providing recommendations and strategies for the sustainable management and restoration of degraded forest stands based on the findings.Methods: This study was conducted in three forest stands within the Abr Forest, located in Shahroud, Semnan Province, each characterized by different levels of disturbance as well as distinct species composition and structural features.In each stand, three sample plots (100 100 m) were established along transects following a southern-to-northern direction, corresponding to the general slope and increasing elevation gradient.The transects were aligned parallel to each other.All woody species with a diameter at breast height (DBH) greater than 7.5 cm were measured and identified within each plot.The collected DBH and basal area data were used to calculate several structural parameters, including tree density (N.ha -1 ), the proportion of largediameter trees, the Gini coefficient, the structural complexity index, and Doliucourt's diameter differentiation, to compare the stand structure among the different disturbance intensities.The forest structure triangle method was employed to further assess the forest structure based on disturbance intensity.For this purpose, trees were grouped into three DBH classes: small (< 30 cm) medium (30-50 cm), and large (> 50 cm) diameter classes .Regeneration of woody species was recorded within subplots measuring 2 2 meters (4 m) established inside each main plot.Regenerating individuals were categorized into three height classes; less than 0.3 m, 0.3 and 1.3 m, and greater than 1.3 m.Species diversity, richness, and evenness across stands were evaluated using the Shannon-Wiener diversity index.Species diversity, composition, distribution of number and basal area in diameter classes, and regeneration characteristics were compared using the one-way ANOVA test and Tukey's multiple comparisons at the 5% level according to the type of stand disturbance.Results: In total, 2638 individuals belonging to 11 woody species were observed in the study area.The highest and lowest tree densities were recorded in moderately disturbed (346 stems per hectare) and severely disturbed (189 stems per hectare) stands, respectively.Furthermore, the severely disturbed stands exhibited the greatest mean DBH (DBH = 37.46 cm) and mean tree height (11.86 m) among the three stands.The analysis of biodiversity indices showed the lowest species diversity (Shannon index H' = 2.31) and species evenness (0.63) in severely disturbed stands, while the highest values for both diversity (H' = 2.97) and evenness (0.87) were observed in the intact (undisturbed) stand.The distribution of species abundance varied among the