Climatic signals in stable carbon isotope ratios of Juniper and Oak tree rings from northern Iran

Stable isotope ratios in tree rings are increasingly used as palaeoclimatic archive and ecophysiological indicator. We used cross-dated tree-ring series from different tree functional types, i.e. the evergreen conifer Juniperus polycarpus and the deciduous broadleaved Quercus macranthera. The samples were collected from Chaharbagh Gorgon forest in northern Iran, where oaks and junipers grow on north-facing and south-facing slopes, respectively. We extracted α-cellulose from the whole wood of annually separated tree rings and evaluated their potential for palaeoclimate reconstructions by examining the relationships between variations of δ13C in tree-ring cellulose and climate parameters. Based on tree-ring δ13C, we calculated intrinsic water use efficiency (iWUE) and evaluated changes in water availability at the study site over the past 50 years. We found significant negative relationships between tree-ring δ13C values in oak and juniper and precipitation in April and spring (only in junipers), while no significant correlations of tree-ring δ13C with temperature occurred. A strong negative relationship between tree-ring δ13C and summer Palmer Drought Severity Index (PDSI) was found at the drier site, indicating that juniper growing on south-facing slopes is a better indicator of drought stress in semiarid environments. A continuous increase in iWUE in both species was observed. δ13C variations in both species reflect the combined influence of climate and local site conditions. The interplay between climatic conditions and species behavior determines the inter-annual δ13C patterns of oak and juniper trees. Due to its more enriched mean δ13C (i.e., less negative average δ13C) and its higher climatic sensitivity, juniper is a more suitable proxy for paleoclimatic reconstructions in northern Iran.