Rare species impacts on structural complexity index (SCI) in the Hyrcanian beech forests

One of the most important goals of the ecological management of forests is to enhance the complexity of the structure of the forest stands. Also, despite the recommendation to increase mixing, the consequence of the presence of rare tree species on the forest ecosystem functions has not been well clarified. This research was conducted to analyze the presence and abundance of rare tree species on the structural complexity index in beech stands. the number of 12 one-hectare study areas with the presence of rare tree species was selected and after measuring the structural features, the multivariate complexity index was determined in each of the plots. We employed ten variables including stem per hectare, mean diameter at breast height, coefficient of variation of dbh, diameter Gini coefficient, number of trees with a diameter larger than 100 cm at breast height, the ratio of the number of trees in different stories, the faction of canopy gaps, amount and volume of dead woods and tree size variation to the calculation of this index. Comparing the correlation between the relative importance value of beech, Maple, and other rare tree species with the structural complexity index revealed the differences in complexity variation toward the rising in the abundance of tree species. Accordingly, beech and hornbeam, as the two dominant tree species with the highest relative value, are weakly correlated with this index. While the maple and other rare tree species including alder, elm, and lime positively correlated with increasing complexity in the structure. According to the positive effect of the presence of rare tree species on the complexity of the structure of forest stands, it is recommended to the retention of rare tree species in stands to increase the complexity of the forest stands structure.