Drought sensitivity analysis of Persian oak based on stable carbon isotopes from the Zagros Mountains, western Iran

Abstract Drought-resilient native tree species are becoming increasingly vulnerable to the intensifying impacts of climate change. In this study, we conducted the first tree-ring stable carbon isotope and intrinsic water-use efficiency (iWUE) study of annually separated tree rings of Persian oak ( Quercus brantii Lindl.) covering the period 1953–2022. The study sites are located at three different elevations in the Zagros Mountains. δ 13 C chronologies at all sites showed higher (less negative) δ 13 C values during the major drought events of 1964, 1984, and 2007–2008, followed by corresponding increases in their respective iWUE values. The highest iWUE values were observed at the high-elevation site. iWUE at the mid and high-elevation sites showed a pronounced increase from 2008 to 2022, suggesting a common physiological response of trees to increasing water limitation. Correlations with climate variables revealed significant negative relationships between winter and spring precipitation and δ 13 C, at mid and high-elevation sites. All three sites exhibited negative correlations with the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation-Evapotranspiration Index (SPEI) at accumulation periods ranging from 1 to 12 months, confirming the high sensitivity of Persian oak to hydroclimatic conditions during both the previous and current growing seasons, with particularly strong correlations observed at the mid and high-elevation sites. These patterns highlight the dominant role of drought in regulating stomatal conductance and carbon uptake, providing physiological evidence relevant to the observed oak dieback phenomena in the Zagros Mountains. Overall, δ 13 C variations in Persian oak are primarily driven by hydroclimatic factors (precipitation, relative humidity, and vapor pressure deficit), with timing and strength of these parameters varying in space and time among all studied sites. Our findings reveal that Persian oak exhibits different adaptive physiological strategies across the altitudinal transect, shifting from conservative water-use behaviour in more arid environments (low and high-elevations) to more flexible gas-exchange regulation under mesic conditions (mid elevations).