The Mutual Relationship between Earlywood Vessel Features of Persian oak (Quercus brantii Lindl.) and Tree Mortality

The oak mortality in the west of Iran has been studied in various aspects. However, the physiological mechanisms underlying susceptibility or resistance of tree to mortality are poorly understood. To find out whether the features of xylem, such as tree ring-width, size, and the number of vessels affect the susceptibility to disease, these features were compared between healthy and dead trees in Dalab forest of Ilam province. By cross-dating the time series of the ring widths, it was determined that the most of the dried trees dies in 2008. However, data analysis showed that the symptoms of the disease appear in xylem, several years before the dying. Since the early 1990s, the trend of annual variation in the vascular features of dead trees were different compared with the healthy ones, and at the same time, the ring width of these trees has decreased. Successive shrinking of earlywood vessels and narrowing ring width reduces the tree's hydraulic conductivity. However, during this decade, dead trees actively tried to cope with reduction by increasing the number of their vessels (the incubation period of disease). Nonetheless, from the early 2000s till the death of sick trees, trees were not able to produce more vessels (the emergence period of disease). Overall, it can be concluded that oaks with smaller vessels are more susceptible to drying, and after infection, the decrease in diameter growth and successive shrinking of earlywood vessels aggravate the tree's condition leading to its death