Annual changes of oak mortality in different crown-dimensions in the forests of Kurdistan province (Case study: Baneh County)

Oak mortality is one of the most important management problems in the Zagros forests and the first signs of oak mortality appear in the canopy of trees. The aim of this study was to investigate the trend of oak mortality in trees with different crown-dimensions in the forests of Baneh County (northwest of Kurdistan province). For this study, four sites in the northern slopes (Saraki and Mirhesam sites) and southern (Belveh and Gandman sites) in Baneh county were selected. At each site, floating sample plots (with 100 trees) were selected and the trees were numbered and measured (two large and small diameters of tree crowns). The crowns of the trees were divided into eight classes. At the end of summer of every year (2019 and 2020), the status of trees in the five oak mortality class was reviewed and recorded. The weighted average of decay codes was used to compare the rate of decline in two consecutive years and Wilcoxon non-parametric test was used to compare the decline codes in the crown area classes in two consecutive years. The results of Wilcoxon test showed that in the four sites studied, there was a significant difference between the decline codes from 2019 to 2020. At Sarki and Belveh sites (oak mortality site), the most oak mortality was observed in the crown and 11 and 17 m2 crown classes. In control sites (Mirhesam and Gandman), the highest increase of oak mortality trees was observed in the 5 and 11 m2 crown classes. The results of oak mortality weight of four stands, showed that the highest increase in oak mortality was observed in the 11 m2 crown classes of Sarki site (with 0.67 increase) and 5 m2 classes in Belveh (with 0.33 increase, respectively) and the highest oak mortality in the southern slope. In order to manage forest stands in the study area, it is suggested to focus the breeding operation and remove the dried branches on small to medium-sized trees (trees with a canopy up to 25 m2) and the decay trend of oak trees in the study area be monitored.