The Stability of Trees Species in Terms of Height-Diameter Ratio and the Relationship of Stand Quantitative Characteristics in Natural Forest Stands (Case Study: Arasbaran Forest)

Correct performance to reduce the sensitivity of forest stands to environmental hazards requires knowledge and proper understanding of its effects on the trees. The tree height-diameter ratio is a tool that is known as an index of tree resistance and stand stability. The present study evaluated the tree's height-diameter ratio in a mixed species natural forest stand in a protected forest in Arasbaran to obtain basic information about the stand stability.

500 sample trees (DBH ≥ 7.5 cm) were randomly selected from the most abundant species present in the region to include all types of diameter classes from the lowest to the highest that are present in the stand. Each tree is one sample in this method. The diameter at breast height and the tree height were measured to calculate the height-diameter ratio. The height-diameter ratio of the stand and every tree species was calculated by dividing the tree's height by the diameter.

The results of this research showed that the value of the height-diameter ratio was 54.31 at the stand level. In addition, this ratio value was 54.93 for maple, 55.04 for hornbeam, 54.63 for ash, and 52.64 for oak at the species level. The result showed that the trees were in the stable category in terms of this ratio. The changes in the height-diameter ratio in different diameter classes showed that the value of this ratio decreased with the increase of the diameter classes. The correlation analysis showed a high, positive, and significant correlation with the tree’s height. Oak and hornbeam species had the highest significant correlation between height and height-diameter ratio (0.718 and 0.729, respectively).

Computing the value of the height-diameter ratio is considered necessary in quantifying the resistance against adverse environmental factors (snow, storm, and wind) and is considered a management issue. Estimating the value of this coefficient determines the diameter class in which instability occurs for any tree species, and it is valuable and practical in future silvicultural and protection planning because of the tree's proper spatial arrangement in the dbh classes in the stands, the density of the stand, and the critical values of the heigh-diameter coefficients are regulated such that the stability of the stand was not threatened.