Forest and Wood Products

Forest and Wood Products

Dynamics of the slenderness coefficient in the Hyrcanian habitat of the National Botanical Garden of Iran: Evidence of structural duality

Articles in Press

Document Type : Research Paper

Authors
Parisa Panahi 1
Abolfazl Jaafari 2
Maryam Hasaninejad 1
1 Botany Research Division, Research Institute of Forests and Rangelands, Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.
2 Forest Research Division, Research Institute of Forests and Rangelands, Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran.
10.22059/jfwp.2026.424317.1416
Abstract
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.
Keywords
Ex-situ collection
Hyrcanian habitat
Mechanical stability
Slenderness ratio
Stand density
Structural duality
Subjects
[1] Watt, M.S. & Moore, J.R. (2023). Modeling spatial variation in radiata pine slenderness (height/diameter ratio) and vulnerability to wind damage under current and future climate in New Zealand. Frontiers in Forests and Global Change, 6, 1188094. https://doi.org/10.3389/ffgc.2023.1188094
[2] Bošeľa, M., Konôpka, B., Šebeň, V., Vladovič, J. & Tobin, B. (2014). Modelling height to diameter ratio – an opportunity to increase Norway spruce stand stability in the Western Carpathians. Central European Forestry Journal, 60(2), 71-80. https://doi.org/10.2478/forj-2014-0008
[3] Burschel, P. & Huss, J. (1997). Grundriss des Waldbaus. Parey Publication, Berlin.
[4] Skrzyszewski, J. & Pach, M. (2020). The use of the slenderness coefficient in diagnosing wind damage risks. Acta Silvestria, LVII: 7-24. https://doi.org/10.15576/actasilvestria/2020.lvii.7
[5] Bošeľa, M., Konôpka, B., Šebeň, V., Vladovič, J. & Tobin, B. (2016). Modelling individual tree height to diameter ratio for Norway spruce and European beech in Czech Republic. Trees - Structure and Function, 30(6), 1969-1982. https://doi.org/10.1007/s00468-016-1424-3
[6] Panahi, P., Jamzad, Z., Jalili, A., Sagheb Talebi, K. & Pourhashemi, M. (2021). The role of the National Botanical Garden of Iran in ex situ conservation of Buxus hyrcana Pojark.; An endangered species. Urban Forestry & Urban Greening, 57, 126951. https://doi.org/10.1016/j.ufug.2020.126951
[7] Panahi, P., Jamzad, Z., Pourhashemi, M., Hosseininejad, M. & Ehsani, M. (2008). Quantitative and qualitative study of the Hyrcanian section of the National Botanical Garden of Iran for its optimum management. Final Report of Research Project, Research Institute of Forests and Rangelands, 81p. (In Persian)
[8] Mirabdollahi, M., Bonyad, A.E., Torkaman, J. & Bakhshandeh, B. (2011). Study on tree form of Oriental Beech (Fagus orientalis Lipsky) in different growth stages (Case study: Lomir forest). Iranian Journal of Forest, 3(3), 177-187. (In Persian)
[9] Mohajer, N. & Mirkazemi, S.Z. (2012). Investigation on qualitative and quantitative characteristics and succession of oriental beech natural stands in Vatan forest (Golestan Province). Journal of Wood and Forest Science and Technology, 19(2), 43-58. (In Persian) https://doi.org/20.1001.1.23222077.1391.19.2.3.6
[10] Bozorgi, K. & Sheykholeslami, A. (2016). Survey slenderness coefficient of beech mixed trees in Hajikola Tirankoli area- Sari. Journal of Renewable Natural Resources Research, 7(1), 1-9. (In Persian)
[11] Charkazi, A., Amiri, M., Ravanbakhsh, H. & Moghadasi, D. (2022). A comparison of the stability of Italian cypress (Cupressus sempervirens var. horizontalis) and Brutia Pine (Pinus brutia Ten) plantations in Ramian, Golestan Province of Iran. Journal of Environmental Sciences and Technology, 23(11): 247-258. (In Persian) https://doi.org/10.30495/jest.2018.21819.3087
[12] Abedi R. & Ostad Hashemi R. (2023). The stability of trees species in terms of height-diameter ratio and the relationship of stand quantitative characteristics in natural forest stands (Case study: Arasbaran Forest). Ecology of Iranian Forests, 11(22), 48-61. (In Persian) https://doi.org/10.61186/ifej.11.22.44
[13] Đodan, M. & Perić, S. (2019). Windthrow resistance of Norway spruce (Picea abies /L./ Karst.) forest cultures - preliminary results. South-East European Forestry, 10(1), 77-88. https://doi.org/10.15177/ seefor.19-09
[14] Tian, D., Jiang, L. & Wang, S. (2023). The influence of climate, soil physicochemical properties and tree size inequality on tree slenderness in mixed forests of Northeastern China. Forest Ecology and Management, 529, 120719. https://doi.org/10.1016/j.foreco.2022.120719
[15] Cysneiros, V.C., Souza, E.L., Pinho, L.C., Vuolo, A.F. & Pereira, I.R.S. (2025). Tree slenderness and stability of Brazilian pine in a secondary forest. Floresta e Ambiente, 32(3), e20240040. https://doi.org/10.1590/2179-8087-FLORAM-2024-0040
[16] Sasanifar, S., Namiranian, M. & Zargham, N. (2015). Evaluation of variation trend in qualitative measures in natural even-aged stand in Kheyroud Forest. Journal of Forest and Wood Products (Iranian Journal of Natural Resources), 68(3), 655-665. (In Persian) https://doi.org/10.22059/jfwp.2015.55596
[17] Sharma, R.P., Vacek, Z., Vacek, S. & Kučera, M. (2019). Modelling individual tree height–diameter relationships for multi-layered and multi-species forests in central Europe. Trees - Structure and Function, 33(1), 103-119. https://doi.org/10.1007/s00468-018-1762-4 
[18] Vatani, L., Rafiei, A.A. & Shamsi, S.S. (2021). Determination of the slenderness ratio of Acer velutinum plantation for tending operations (Pahneh-Kola Forestry Plan – Sari County). Sustainable Management of Hyrcanian Forests, 3(1), 45-50
[19] Wonn, H.T. & O'Hara, K.L. (2001). Height:diameter ratios and stability relationships for four Northern Rocky Mountain tree species. Western Journal of Applied Forestry, 16(2), 87-94. https://doi.org/10.1093/wjaf/16.2.87
[20] Cacciatori, C., Gazda, A., Bodziarczyk, J., Foremnik, K., Madalcho, A.B., Maciejewski, Z., Pielech, R., Tomski, A., Zięba, A., Zwijacz-Kozica, T. & Szwagrzyk, J. (2025). Deer browsing increases stem slenderness and crown irregularity and modifies the effects of light gradients on architecture of forest tree saplings. Ecology and Evolution, 15, e70837. https://doi.org/10.1002/ece3.70837
[21] Díaz-Yáñez, O., Mola-Yudego, B. & González-Olabarria, J.R. (2019). Modelling damage occurrence by snow and wind in forest ecosystem. Ecological Modelling, 408, 108741. https://doi.org/10.1016/j.ecolmodel.2019.108741
Forest and Wood Products

Articles in Press, Accepted Manuscript
Available Online from 08 November 2026