Diversity, Composition and Occurrence Probability of Tree-Related Microhabitats in the Decay Developmental Stage of Hyrcanian Intact Beech Forests (Sika and Shafarood Regions)

Background and Objectives: In recent years, the structural characteristics of tree microhabitats (TreMs) have garnered significant attention due to their importance for forest biodiversity. In the context of planning nature-based forest management policies, quantitative assessment of tree microhabitats plays a vital role in preserving the ecological functions of forest ecosystems. Tree microhabitats are morphological features found on living trees or standing deadwood, and their occurrence is not uniform across trees but is utilized by specific species or groups of species. These microhabitats are formed through natural processes such as fungal invasion, woodpecker activity, storm damage, and other physical disturbances, creating the structural complexity required by wildlife. As a result, tree microhabitats can serve as indicators of biodiversity and ecological integrity, with much of forest biodiversity relying on such elements for survival. Therefore, the conservation of habitat trees is a critical consideration in forest management. In this regard, the diversity, composition, and likelihood of occurrence of tree microhabitats during the degradation stage of developmental phases in beech forests of northern Iran were examined.
Methodology: This study was conducted in two plots located in the degradation stage of developmental phases in the Shafarood (Guilan) and Sika (Mazandaran) regions. The examined plots, each covering one hectare, are supervised by the Research Institute of Forests and Rangelands and are situated within intact and unmanaged beech stands. Slope and aspect in each plot were recorded using a Suunto clinometer and compass. The species and diameter at breast height (DBH) of all trees in the plots were recorded. Tree microhabitats were sampled using a standardized tree microhabitat guide, which categorizes microhabitats into seven main forms, 15 groups, and 47 types. Accordingly, all living trees and standing deadwood were systematically inspected from all sides, beginning at the lower trunk (buttress) and progressing upward to the canopy. Observations from the lower trunk to the middle trunk were conducted with the naked eye, while the upper trunk, canopy, and secondary branches exposed to sunlight but less accessible were examined using binoculars. All observed microhabitats on each tree species were recorded with a specific code on the corresponding sample plot form. Logistic regression was used to evaluate the Occurrence Probability of various microhabitats. Tree microhabitats were compared using Hill numbers. Rarefaction-Extrapolation curves were generated after standardizing the sample size based on the number of recorded microhabitats.
Results: The oriental beech species accounted for 86.5% and 88.8% of the total trees in the degradation stage stands in the Sika and Shafarood regions, respectively. The average DBH of beech trees between the two regions was significantly different. However, no significant difference was observed in the total number of TreMs per beech tree between the two regions. The total number of TreMs on beech trees in Sika and Shafarood were 455 and 442, respectively, with a microhabitat rate of 2.4 in Sika and 2.1 in Shafarood per beech tree. The results showed that the most common form of TreM in both regions was buttress cavities. The study also revealed that, in terms of TreMs type, the highest frequency in both regions was related to epiphytic bryophytes, trunk cavities, and buttress cavities, with totals of 174, 146, and 118, respectively. Additionally, the occurrence probability of epiphytic bryophytes was significantly higher in Shafarood than in Sika. A comparison of the diversity of TreMs between the two regions using Hill numbers showed that Sika consistently exhibited higher richness in terms of Hill numbers, species richness (q=0), exponential Shannon entropy index (q=1), and the inverse Simpson index (q=2) compared to Shafarood. However, no significant difference was found between the two regions. The q=1 index, representing the exponential Shannon entropy, indicated that the diversity of TreMs in Sika was nearly twice that of Shafarood. Furthermore, based on the q=2 index, which reflects the diversity of TreMs with low dominance, the abundance of TreMs in Sika was approximately three times greater than in Shafarood.
Conclusion: This study highlights the critical role of tree microhabitats in preserving biodiversity within the Hyrcanian forests. Understanding the relationship between forest structure and tree microhabitats can facilitate the development of more effective conservation and management strategies. Based on the findings of this research, Sika's potential as a model for biodiversity conservation warrants further attention. It is ultimately recommended that long-term studies on tree microhabitats be conducted to gain a comprehensive understanding of ecological processes and their impacts on biodiversity. Additionally, examining the relationships between environmental conditions, forest structure, and tree microhabitats is essential to provide a more effective understanding.