Investigation and Comparison of Slenderness Coefficient of Old Persian Oak and Wild Pistachio Trees in Different Site Conditions in the Middle Zagros

Slenderness coefficient is one of the indicators for studying the sustainability of forest trees and the results of its study on old trees have great importance in terms of ecology and silviculture. In this research, the Slenderness coefficient of Old Persian oak and wild pistachio trees in different site conditions in the forests of Ilam province was investigated and compared.

For this, after identifying 26 old trees belonging to the Persian oak species and 47 old trees belonging to the Wild pistachio species, the quantitative characteristics of them including dbh and height were measured. Also, the status of their locations in the forest in terms of slope, aspect and altitude were examined.

The results showed that for both oak and wild pistachio species there are significant relationships between dbh and slenderness coefficient which were decreasing in both of them and the best model for cloud points of both species was obtained exponential model. The regression explanation coefficient for oak and wild pistachio was 57% and 35%, respectively. The identified old trees of oak (Slc= 7.82) and wild pistachio (Slc= 5.72) had a low slenderness coefficient; however, more old wild pistachio trees had a lower slenderness coefficient than oak. So that 62% of old wild pistachio trees had a slenderness coefficient between 5 and 7, but 65% of old oak trees had a slenderness coefficient of more than 7. The slenderness coefficient of old oak trees was significantly related to altitude (Sig= 0.049) and its value was higher at altitudes of 900-1300 m above sea level (Slc=9.45). The slenderness coefficient of wild pistachio was significantly related to altitude (Sig= 0.000) and aspect (Sig= 0.001) and its value was higher at altitude class of 900-1300 m above sea level (Slc= 10.14) and smoothness areas without aspect (Slc= 10.14).

Based on the obtained results, it was concluded that the habitats of old oak trees located at altitudes of 900-1300 m above sea level and also the habitats of old wild Pistachio trees located at altitude 900-1300 m above sea level and areas without aspect where had the highest slenderness coefficient, have more potential for the growth and establishment of trees. Also, the habitats of old oak trees located in the altitude classes of 1300-1700 m above sea level (Slc= 7.52) and also the habitats of old wild Pistachio trees located in the altitude classes of 1300-1700 m above sea level (Slc= 5.24) and in east (Slc= 3.52) and north (Slc= 5.11) aspects, where had the lowest slenderness coefficient, have a better desirability for the stability of the mentioned old trees. Therefore, it can be said that the low slenderness coefficient of Old Persian oak and wild Pistachio trees is one of the indicators of their stability and durability in the studied forests.