Abstract ID: 3.11531 | Accepted as Talk | Talk/Oral | TBA | TBA

Mountain ecosystems in Iran are increasingly affected by hydroclimatic extremes, such as severe droughts, flash floods, and enhanced atmospheric moisture deficits intensified by ongoing climate change, low resilience, and high vulnerability. To better understand long-term hydrological variability and its impact on tree growth, we developed a 499-year tree-ring width (TRW) chronology (1523–2021) and a 200-year stable oxygen isotope (δ¹⁸O) record (1821–2020) from Juniperus polycarpos in the high-elevation Hezar Masjed Mountains of northeastern Iran. Using these proxies, we reconstructed the Standardized Precipitation-Evapotranspiration Index (SPEI) and vapor pressure deficit (VPD) for the growing season (March–September) from 1821 to 2020 CE. Our results showed the highest frequency of extreme and moderate soil moisture stress (SPEI) events over the past five decades. An increasing number of dry growing seasons marked by both atmospheric and soil moisture deficits emphasize a shift toward drier conditions. Spatial correlation analysis demonstrates the regional representativeness of the reconstructed series across Iran and neighboring regions, with strong negative teleconnections between reconstructed VPD and gridded March–September vapor pressure across Iran and Central Asia. Superposed Epoch Analysis (SEA) revealed that soil moisture stress (SPEI) is the dominant driver of reduced tree growth, while high VPD alone does not necessarily lead to significant growth reductions. In fact, tree-ring width showed a slight but significant increase in high-VPD years, likely due to compensatory soil moisture conditions or species-specific adaptations. However, when high VPD coincided with low SPEI, growth declined sharply, emphasizing the compounding stress effect of simultaneous atmospheric and soil moisture droughts. Our findings revealed the critical role of soil moisture availability in tree growth resilience and suggest that while trees can tolerate short-term atmospheric droughts, their growth is highly sensitive to prolonged soil moisture deficits. This study underscores the increasing hydroclimatic stress on mountain forest ecosystems in Iran and provides valuable insights for climate adaptation strategies in water-limited environments.