Using tree-ring width and earlywood vessel features to study the decline of Quercus brantii Lindl in Zagros forests of Iran

Dieback has affected oak forests ecosystems of Iran in recent years. In order to understand this phenomenon, we selected 12 healthy and 16 dead trees of Quercus brantii Lindl in Dalab forest of Ilam. The responses of ring width chronologies to climatic factors were investigated in the living and dead trees. We also established the interrelationships between earlywood vessel features including vessel density, average vessel lumen area, the percentage of total vessel lumen area within the surface analyzed, the hydraulically weighted diameter, and ring width for 6 living and 6 dead trees. The results of crossdating showed that death occurred from 2008 onwards in most decayed trees. We found important differences in the effects of climatic factors on ring width chronologies between the living and dead trees, whereby water availability was the main factor controlling the radial growth of dead trees, which were more sensitive to drought. The analysis of earlywood vessels showed that the hydraulically weighted diameter and the vessel density are the anatomical features differing most strongly between the living and dead trees. From the difference of the hydraulically weighted diameter and the vessel density series between the living and dead trees, we concluded that dead individuals had the higher vulnerability to water stress. Moreover, by monitoring the annual variations of xylem anatomical features in the years before tree death, we propose the most critical timings of processes related to dieback.