Analysis of species diversity and regeneration density in the canopy of old-growth trees in the Hyrcanian Forests of Iran (Case study: Baharbon district, Kheyrud Forest)

Document Type : Research article

Authors

1 M.Sc., Department of Forestry and Forest economic, Natural Resource Faculty, University of Tehran, Karaj, Iran

2 Assistant Prof., Kermanshah Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education and Extension Organization, Kermanshah, Iran

3 Associated Prof., Department of Forestry and Forest economic, Natural Resource Faculty, University of Tehran, Karaj, Iran

4 Assistant Prof., Department of Statistics and Mathematics, Gonbad-Kavoos University. Gonbad-Kavoos, Iran

Abstract

Background and Objectives: The Hyrcanian forests, as dynamic and highly diverse ecosystems, play a key role in maintaining ecological stability and conserving biodiversity. The sustainability of these forests critically depends on natural regeneration processes that determine species composition, age structure, and the forest’s capacity for renewal. Although numerous studies have examined regeneration patterns in northern Iranian forests, the specific role of old-growth trees in shaping species diversity and regeneration density within their understory has remained largely unexplored. This study aimed to analyze species diversity and regeneration density under the canopy of old-growth trees across different forest types in the Baharbon district of the Kheyrud Forest, thereby clarifying the contribution of these legacy trees to natural regeneration dynamics.
Methodology: The study was conducted in the Baharbon district of the Kheyrud Educational and Research Forest within an elevation range of 1,000–2,200 m a.s.l. Sampling was conducted during the summer season. Old-growth trees were identified using species-specific minimum diameter at breast height (DBH) thresholds, forest-type maps, and a completely randomized sampling method. A total of 47 one-hectare plots comprising 188 micro-samples and 60 old-growth trees from five species, including Acer velutinum Boiss., Fagus caspica Denk & G.W. Grimm, Alnus subcordata C.A. Mey., Carpinus betulus L., and Quercus castaneifolia C.A. Mey., were studied. For each old-growth tree, four 6 × 6 m subplots were installed beneath the canopy to assess regeneration characteristics, including seedling density, species composition, health status, and height and diameter class distributions. In addition, herbaceous cover percentage and litter depth were measured. Species diversity was quantified using the Shannon–Wiener and Simpson indices, and differences in regeneration density among forest types were examined using independent t-tests. Data analyses were performed using Excel, SPSS, and PAST software packages.
Results: Findings indicated that in both the pure Caspian beech type and the other forest types, Acer velutinum dominated regeneration with a relative abundance exceeding 70%, whereas Fagus caspica, Carpinus betulus, and Acer cappadocicum Gled. contributed minimally. The highest regeneration density occurred within the smallest diameter class (<2.5 mm) and the lowest height class (<30 cm), reflecting the predominance of early-stage seedlings. Herbaceous cover was generally below 50%, and litter depth was greater in the pure Caspian beech type. Shannon (0.39) and Simpson (0.08) diversity indices were lower in the pure Caspian beech type compared to other types (0.47 and 1.03, respectively), indicating the dominance of a few species and an uneven distribution of individuals. More than 90% of the seedlings were healthy, and no significant difference in regeneration density was observed between forest types (P= 0/854, ns).
Conclusion: Overall, natural regeneration in the Baharbon region exhibits satisfactory density and seedling vitality; however, species diversity is low, and the regeneration layer is strongly dominated by Acer velutinum. If this pattern persists, it may lead to structural homogenization and reduced ecological resilience of the forest. Given the lack of significant differences in regeneration density among forest types, factors associated with microsite conditions, seed production, and interspecific competition appear to exert stronger influences than forest-type identity. Thus, although the regenerative potential of these forests is high, active management interventions aimed at enhancing species diversity within the regeneration layer are essential to ensure the long-term ecological sustainability of these valuable Hyrcanian forests.
 
 
 

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References

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