Abstract: Relative humidity (RH) plays a crucial role in maintaining both forest and human health. However, the historical characteristics and drivers of RH in the southeastern Chinese Loess Plateau (SECLP) remain poorly understood. Here, a high-resolution tree-ring-width record from the SECLP reveals that the radial growth of Pinus tabuliformis is limited by RH from April to mid-August (RH_c10–23), which coincides with the period from the Chinese solar term Pure Brightness to End of Heat. The 180-year RH_c10–23 reconstruction explains 45.40% of instrumental variance (1956–2020 CE) and reveals a long-term drying trend since 1844 CE, especially evident since 1956 CE. Three iconic Northern China droughts—the ~ 1900, late-1920s, and 1940–1943 events—are clearly resolved, together with an unprecedented 1997–2023 drought. Conversely, wet periods occurred during 1850–1890, 1910–1916, and 1951–1966 CE. The reconstruction mirrors broad Chinese Loess Plateau hydroclimatic patterns while retaining local signals. Over the past two centuries, SECLP hydroclimatic variations have been anti-correlated with regional temperature and modulated by the Asian summer monsoon, Atlantic Multidecadal Oscillation/Atlantic Multidecadal Variability, El Niño-Southern Oscillation, and Indian Ocean Dipole. This study enhances our understanding of regional hydroclimatic evolution, highlights human impacts on natural systems, and provides scientific support for climate change adaptation and water resource management.

Key words: Tree rings, Relative humidity, Extreme climate, Drivers, Human activity