Analysis of the Structure of Iran's Forests: A Review of Research and Findings in Various Forest Ecosystems of Iran

Extended Abstract
Background and objectives
Evaluating forest structure is considered one of the most effective tools in modern forest management. Forest structure represents the distribution of tree characteristics in forest ecosystems and includes factors such as age, size, and species type. Examining forest structure is essential for proper management and conservation of biodiversity, as it aids in selecting appropriate methods for forest sustainability. Additionally, the structure of forest stands is regarded as an indicator for assessing forest quality, wildlife habitat, and environmental values. Various methods exist for measuring forest structure, one of which is the nearest neighbor index, which analyzes patterns of distribution and dimensional diversity of trees. Numerous studies have been conducted on the structure of forest stands in various regions of the country, and the aim of this research is to collect, compile, evaluate, and summarize all studies conducted in this field with no time restrictions up to 2024.
Methodology
This paper reviews articles published in the field of examining the structure of natural stands in various ecological regions of Iran. Articles published in domestic and international journals related to forest structure, addressing topics such as the effects of physiographic factors (elevation, slope, aspect) and the impact of management on stand structure, quantification of forest structure, species diversity and forest structure, species mixing, and spatial patterns of forest stands were searched in national and international databases including Scopus, Magiran, Web of Science Core Collection, SciELO Citation Index, Sid, Google Scholar, Science Direct, and Biological Abstracts with no time restrictions up to 2024. A total of 96 articles on the structure of forest stands were reviewed, of which 46 pertained to studies conducted in the northern forests of the country (Hyrcanian), four related to the Arasbaran forests, 40 related to the Zagros forests, and six related to the Khalij-Omani forests. These articles were compiled, summarized, and organized into separate tables based on vertical and horizontal structure indices, mixing and homogeneity status, the impact of physiographic factors, and the influence of management and human interventions on the non-spatial characteristics of the structure in various ecological regions of Iran.
Findings
The results of the reviewed studies, which employed nearest-neighbor-based methods, revealed that the spatial patterns in Hyrcanian and Zagros forest stands dominated by beech (Fagus orientalis) and oak (Quercus spp.), respectively, were dissimilar; however, a clustered pattern was observed in most cases. In contrast, in Hyrcanian forest stands dominated by species other than beech, the spatial patterns were either random or intermediate (between random and clustered). Furthermore, spatial patterns analysis using alternative structural assessment methods (e.g., Ripley's K-function, O-ring statistic, distance/quadrat methods) in Hyrcanian and Zagros forest stands predominantly revealed a clustered pattern. The vertical structure exhibited divergent trends among Hyrcanian, Zagros, and Khalij-Omani forest stands, with the highest and lowest number of canopy layers observed in Hyrcanian and Khalij-Omani stands, respectively. Additionally, most reviewed studies employing nearest-neighbor-based methods in Hyrcanian, Zagros, Arasbaran, and Khalij-Omani forests indicated low species mingling and a homogeneous forest structure. The results regarding the influence of physiographic factors (slope, aspect, elevation) on non-spatial structural characteristics of trees (DBH, height, basal area, crown area, tree density) demonstrated that in most studies, the mean values of the examined indices were higher on northern and eastern slopes, whereas they drcreased with increasing slope steepness and elevation. Furthermore, in most studies, forest management and human interventions were found to have a negative impact on the non-spatial structural attributes of the studied forest stands (except for tree density).
Conclusion
The findings of the conducted studies reveal that the horizontal structure of forest stands in Iran's different phytogeographical regions is influenced by both natural and anthropogenic factors. Analysis of the Hyrcanian (beech) and Zagros (oak) forests indicates that the dominant horizontal dispersion pattern is clustered, primarily shaped by ecological factors such as seed weight and patchy regeneration. The low mingling index and uniformity in tree dimensions across the studied regions suggest intense competition or ecosystem degradation. A more complex vertical structure in Hyrcanian forests and the heterogeneous influence of physiographic factors on structural characteristics are additional key findings. While forest management positively impacts stand density, its adverse effects on non-spatial indices highlight the necessity for adopting selective logging, preserving patchy structures, and developing region-specific conservation guidelines (particularly for sensitive areas like Zagros). This study, limited by temporal data gaps, calls for future research to integrate modeling for structural trend analysis and meta-analyses of stand structure studies.