Introduction: Variation in species composition and tree distribution leads to highly complex characteristics in forest ecosystems, arising from interactions among smaller‐scale spatial structures. Forest stand structure encompasses three major aspects: spatial pattern diversity, species diversity, and tree size diversity. The present study aimed to determine the minimum plot size required to assess structural characteristics of Juniperus excelsa M. Bieb. stands and to evaluate the structural diversity of these stands in the Aminabad forest reserve of Firuzkuh, located within the Irano–Turanian phytogeographical region.

Materials and Methods: Three 3-ha sample plots (three stands) were randomly chosen., each consisting of six nested square subplots with areas of 0.5, 0.75, 1, 1.5, 2, and 3 ha. For all trees and shrubs within each subplot, species identity, spatial coordinates, height, and crown variables (maximum crown diameter and its perpendicular diameter) were measured. Tree spatial coordinates were recorded using a multi-frequency GPS. Stand structure was quantified using the uniform angle index, nearest-neighbor distance, mingling index, tree size differentiation, and size dominance indices. To determine the most appropriate plot size, species–area curves and statistical comparisons of variables across different plot sizes (analysis of variance and Kruskal–Wallis test) were applied.

Results: A total of 13 tree and shrub species were recorded in the study area. Analysis of species number variation with increasing plot size showed that species richness increased up to 1.5 ha, after which the curve reached a plateau and species number remained constant; however, with further increase to 3 ha, species richness increased again. The first increase represented the species within the target plant association, whereas the second increase resulted from inclusion of areas outside the association boundary. Mean crown diameter of Juniperus trees did not differ significantly among plot sizes. Mean tree height also showed no significant difference among plots ranging from 1 to 3 ha, although in the first station significant differences were observed compared with smaller plots. No significant differences were detected in shrub size attributes among different plot sizes; however, results indicated that plots smaller than 1 ha may not include a sufficient number of shrub individuals for reliable analysis. The uniform angle index indicated a random pattern tending toward clustered in the first and second stations and a random pattern in the third station, with no significant differences in spatial distribution patterns of trees and shrubs across plot sizes. In none of the stations did mingling, size diversity, or composite structural diversity indices show significant differences when plot size increased from 1 to 3 ha; however, significant differences were observed in smaller plots, particularly in the first and third stations.

Conclusion: Based on the results, a plot size of 1 ha was identified as appropriate for structural studies of Juniperus forests in Alborz. In stands with higher density (e.g., the second stand), satisfactory results can also be obtained using 0.75-ha plots, whereas in lower-density stands (e.g., the first stand), plots of up to 1.5 ha may be required. To record tree and shrub species within a Juniperus association, an area of approximately 1.5 ha is necessary, while larger areas (up to 3 ha) are required to capture the complete tree and shrub flora of Juniperus habitats.