Effects of Some Environmental Factors on Dieback Severity of Trees in Middle Zagros forests of Iran (Case Study: strait Daalaab, Ilam Province)

In recent years, one of the most important occurred events in Zagros is dieback of forest trees; which every day become more critical. To estimate the dieback severity of trees and determination of the relation between physiographic factors including: slope, direction and elevation from sea level with the dieback severity of forest trees, an area of 400 ha were selected and surveyed in the Ilam forests. Inventory was done by a systematic random sampling, and 80 rectangular sample plots with area of 0.1 ha (40 m × 25 m) were measured. Evaluated parameters into each sample plot were including: species name, diameter at breast height (DBH), severity of dieback, and topographic factors. The results showed that there is a significant relation between severity of dieback and slope (p = 0.008) and direction (p = 0.00001). Dieback of trees was increased with increasing of slope gradients. Severity of dieback was observed in west and south directions more than others. On the base of results, dieback percent of coppice trees was more than high trees. That's means; coppice trees relative to their abundance were damaged more than high trees. Also, there was a significant relation between DBH and severity of dieback (P=0.042) and dieback of trees was increased with increasing of DBH. The most number of withered trees were observed in diametric classes of equal & higher than 40 cm (DBH ≥ 40 cm). About 50% of the total trees were wizen or have had a signs of dieback.