Long-term tree growth-climate relationships of Fagus orientalis Lipsky at high-elevation forest edges in the Alborz Mountains, northern Iran

Rapid warming at high elevations may increase the sensitivity of tree growth to climatic signals. While several studies have examined tree growth in the Hyrcanian forests of the Alborz Mountains (Iran), the impact of climatic factors on Fagus orientalis Lipsky at its upper growth limit remains unclear. This study investigates the relationship between the annual tree-ring width of Fagus orientalis and climate over more than a century at its elevational growth limit. We constructed two tree-ring width chronologies from cores collected at elevations of 2400–2550 m a.s.l. at two contrasting sites in the Hyrcanian Forest, which differ in soil physical conditions. We correlated the tree-ring data with monthly mean temperature, precipitation sums, and the six-month standardized precipitation-evapotranspiration index. Our results show that moisture deficit, particularly during the growing season (June to November), is a critical factor affecting growth. Precipitation has a more pronounced influence on growth at high elevations compared to lower elevations in the Hyrcanian Forests, suggesting that Fagus orientalis is more dependent on precipitation and soil moisture during summer and autumn drought stress. The distance from the Caspian Sea and the constellation of local climatic factors such as low precipitation and high irradiation are likely to contribute to reduced moisture availability at higher elevations. Divergent results from contrasting sites highlight the important role of soil physical conditions in shaping growth-climate relationships. This study provides valuable insights into the radial growth-climate relationships of Fagus orientalis at elevated sites, and enhances our understanding of the sensitivity of the species in response to future climate change.