Tree-ring cellulose oxygen isotope reveals summer hydroclimate variability across the East European Plain

Hydroclimate variability across the East European Plain plays a critical role in regional ecosystems and climate dynamics, yet moisture-sensitive tree-ring proxies remain scarce in this region, particularly at high latitudes where tree-ring width (TRW) and density are primarily controlled by temperature. Tree-ring cellulose oxygen isotopes ( δ 18 O TRC ) have the potential to serve as a useful hydroclimate proxy, but their climatic signal in this region remains poorly understood. Here, we developed three δ 18 O TRC chronologies of Scots pine ( Pinus sylvestris L.) from the northern, central, and southern East European Plain to assess their climate signals. The δ 18 O TRC chronologies from the three sites show similar responses to summer moisture conditions. The northern δ 18 O TRC shows the strongest relationship with vapor pressure deficit (VPD), the central δ 18 O TRC records both hydroclimate variability and precipitation oxygen isotopes ( δ 18 O p ) signals, and the southern δ 18 O TRC mainly reflects the Standardized Precipitation Evapotranspiration Index (SPEI). In contrast, TRW shows weak hydroclimate sensitivity at the northern and central sites, whereas at the southern site it mainly reflects soil moisture. Overall, compared with TRW, δ 18 O TRC better captures regional summer hydroclimate signals, particularly capturing signals related to atmospheric drought.