Introduction: The slenderness coefficient (height-to-diameter ratio, h/d), a key indicator of the mechanical stability of trees, is influenced by factors such as tree age, stand density, light competition, and species characteristics. Ex-situ collections, as genetic reservoirs of forest species, require continuous monitoring of their structural stability. This study aimed to assess the structural stability of the Hyrcanian habitat of the National Botanical Garden of Iran using the slenderness ratio over two inventory periods (2006 and 2025).
Method: In 2006, 43 permanent circular sample plots, each covering an area of 200 m², were established within the 8-ha Hyrcanian habitat using a systematic-random sampling method. During the initial inventory, both quantitative and qualitative variables of woody species were recorded. These plots were subsequently re-inventoried in 2025, following a 19-year interval. The slenderness Coefficient was calculated for each individual and categorized into four stability classes: highly unstable, unstable, stable, and highly stable. Statistical analyses were conducted in R using specialized packages.
Results: Stand density increased from 990 stems ha⁻¹ in the initial inventory to 1,693 stems ha⁻¹ in the second (+71%), primarily driven by the massive recruitment of Cornus australis C.A.Mey., which rose from 152 to 579 stems ha⁻¹. Although the mean slenderness coefficient of the entire stand increased slightly from 89.5 to 93.5, this change was not statistically significant. However, the proportion of unstable stems (slenderness coefficient > 80) rose from 59% to 64%. At the species level, significant variations were observed: the slenderness coefficient of Diospyros lotus L., Fraxinus excelsior L., Acer velutinum Boiss., and Parrotia persica (DC.) C.A.Mey. decreased significantly, indicating improved stability, whereas those of Ulmus carpinifolia G.Suckow and Morus alba L. increased significantly, denoting reduced stability.
Conclusion: The Hyrcanian habitat exhibits a distinct structural duality: while certain species (e.g., D. lotus and F. excelsior) demonstrate enhanced stability as they reach maturity, others (e.g., U. carpinifolia and M. alba) become increasingly slender and unstable due to accelerated vertical growth. This divergence is primarily driven by the sharp increase in stand density and intensified light competition. Our findings suggest that relying exclusively on the stand's mean slenderness coefficient is inadequate for a robust stability assessment; rather, species-specific responses must be prioritized. Consequently, density management through species-targeted thinning is recommended as a primary strategy to mitigate light competition and enhance the overall mechanical stability of the stand.