The relationship of some tree characteristics, forest stands and physiographic factors with the rate of dieback

In recent years, Zagros, as the largest forest ecosystem in the country, have been affected by the phenomenon of oak decline. Oak decline can occur simultaneously in different geographic regions by completely different or identical stressors and leading to the death of oak trees. Tree mortality has a negative effect on the ecosystem performance by decreasing the density of the stand, changing the abundance of tree species, reducing the canopy cover and changing the structure of the forests. Therefore, this study was carried out with the aim of determining the relationship between some tree characteristics, forest stand and physiographic factors with the rate of dieback.

In 50 hectare of this forest by a random systematic grid (100*100) 42 circular sample plot with 15 (R) area was s elected. The layers of slope in two categories, aspect, and altitude in two categories were prepared. In each sample plot the structural characteristics of stand such as; diameter at breast height, Crown area, Crown density (open, medium, closed), tree height, number of sprouts, trees origin, tree position, and dieback rate (Using four classes of crown dieback (healthy: less than 5%, poor crown dieback: 33- 5%, average crown dieback: 66-34%, severe crown dieback: more than 66%), were measured and recorded.

The results showed that the relationship between dieback rate with trees origin, tree height, and stand density was significant. However, there was no significant relationship between dieback rate and the diameter at breast height (DBH), the number of sprouts and Crown area (canopy). The trees located on aggregation group had the higher rate of dieback than individual trees. The relationship between physiographic factors with dieback rate showed that there was no significant relationship between slope and dieback. The rate of dieback in South aspect and in higher elevations were higher than other classes.

Most of the diebacks belong to the 4th and 1st classes. If the current trend along with frequent droughts continues, the trees are transported from lower classes to higher classes. In such a situation it is impossible to restore the forest to the natural state, but if escalating factors are managed and human intervention is controlled in nature, the current situation can be kept constant, or even given the evidence of the presence of sprout shoots, it was hoping to return and restore the forest.