Seasonal relative humidity recorded in tree-ring earlywood and latewood δ18O in the West Tianmu Mountains, southeastern China

The West Tianmu Mountains, situated in the southeastern region of China, is profoundly influenced by the East Asian monsoon. Total ring oxygen isotope (δ¹⁸O) has been used previously to reconstruct hydroclimatic signals during the growing season in this region, but high-resolution analyses, i.e., at the intra-annual scale, are scarce. Here, we selected four suitable Pinus taiwanensis Hayata tree cores, measured each ring's α-cellulose δ¹⁸O values in both earlywood and latewood, and established earlywood and latewood δ¹⁸O_cell chronologies from 1951 to 2019, overlapping the period of the observed meteorological data. The mass-weighted mean values of earlywood and latewood δ¹⁸O_cell were calculated to build the total ring δ¹⁸O_cell chronology. The total ring δ¹⁸O_cell chronology is revealed to respond primarily to hydroclimate during the growing season, more specifically relative humidity (RH) during the period May to October. Earlywood δ¹⁸O_cell chronology exhibits the highest correlation with May to June RH, while latewood δ¹⁸O_cell chronology correlates significantly with August to October RH. This observation indicates that the extraction of different seasonal hydroclimatic signals, such as the early or late stages of the growing season, is feasible. Furthermore, we show that tree-ring δ¹⁸O_cell is positively correlated with central and eastern Pacific sea surface temperatures and, accordingly, exhibiting a close connection with El Niño-Southern Oscillation (ENSO). Earlywood δ¹⁸O_cell contains stronger ENSO signals than latewood for two reasons. Firstly, earlywood δ¹⁸O_cell generally retains more precipitation δ¹⁸O signals, and secondly, May to June precipitation δ¹⁸O, which is more contained in earlywood than in latewood δ¹⁸O_cell, has stronger connections with ENSO than other months.