Tree-Ring Growth Response of Juniperus excelsa to Climate and Extreme Pointer Years in the Northwest of Iran

Abstract

Northwestern Iran represents a significant data gap in the dendroclimatic network of West Asia, hindering a comprehensive understanding of regional hydroclimatic variability. This study aims to bridge this gap by constructing a robust tree-ring width chronology for Juniperus excelsa and evaluating its response to climatic extremes in the Hashtjin mountains of northwestern Iran. The authors analyzed 19 increment cores from 15 old-growth trees to develop a 110-year chronology (1898–2007). Statistical quality indicators confirmed a robust climatic signal. Analysis revealed that radial growth was primarily constrained by moisture availability, showing significant positive correlations with April and June precipitation and a significant negative correlation with April temperature, highlighting spring drought stress. Temporal instability in these climate-growth relationships was detected using moving window correlation. Four independent methods (IT, RGC, NW, zChron) identified major pointer years, with extreme droughts in 1907–1908 and 2001 consistently flagged. These extreme years show strong spatiotemporal coherence with historical drought records across West Asia and the Eastern Mediterranean, validating the chronology’s climatic sensitivity and underscoring regional synchronicity in major hydroclimatic events. The findings underscore the vulnerability of juniper ecosystems to warming-induced drought and provide a crucial proxy for filling paleoclimatic gaps in the region.