Stable oxygen isotopes in juniper and oak tree rings from northern Iran as indicators for site-specific and season-specific moisture variations

Variations of stable isotopes ratios in tree rings are indicators of climate fluctuations. The current study examines the relationships between precipitation and temperature variations and stable oxygen isotopes ratios (δ18O) of the last 50 years in cellulose extracted from growth rings of Juniperus polycarpus and Quercus macranthera in northern Iran. We sampled at high-elevation sites in the Alborz Mountains, where J. polycarpus covers south-facing slopes and Q. macranthera covers north-facing slopes, respectively. δ18O values of tree-ring cellulose in oak and juniper show site-specific differences in their mean and standard deviation, but temporal variations are significantly correlated, indicating a common forcing. Correlation functions of tree-ring δ18O with local and modelled climate data reveal positive effects of temperature on δ18O values in J. polycarpus during January–June, with significant correlations in January and May. Moreover, juniper δ18O is influenced by winter precipitation, probably due to an influence of snow melt on the isotopic composition of soil water during spring. Negative correlations were found between juniper δ18O and early vegetation period precipitation, when new xylem cells are formed. In contrast, δ18O of oak contains a late summer (September) precipitation signal. Besides, δ18O values in oak are positively affected by temperatures during summer and winter, with strongest connections with temperatures of September during the previous and current growing seasons. By combining stable isotope signals of species growing under different site conditions, there is a potential to reconstruct moisture variations during different seasons in the mountain areas of north Iran.