Tree-ring density reveals precipitation evolution patterns in the agro-pastoral ecotone and the northward shift of the 400 mm precipitation line

Global warming has intensified the climatic fluctuations in the monsoon-southwest wind convergence zone in northern China, which is a key boundary for agricultural and pastoral areas.Previous reconstruction studies rarely employed multi-species MXD indicators, and failed to comprehensively analyze the evolution of precipitation by integrating the migration of the 400 mm precipitation line and the carbon emission effect. Therefore, this study based on the maximum latewood density (MXD) of larch, birch, spruce and Chinese pine, we developed 213-351 year MXD chronologies and established quadratic regression models to reconstruct long-term precipitation at four sites across the agro-pastoral ecotone of northern China. The region experienced three common wet episodes (1855-1882, 1908-1952, 2015-2024) and two widespread drought intervals (1951-1979, 1985-2012), with precipitation periodicity modulated by latitude and large-scale atmospheric circulation. High-latitude precipitation series showed good synchrony, while spatial divergence at the same latitude was controlled by topography and circulation regimes. Cross-wavelet results indicated clear latitudinal gradients in the impacts of PDO, AMO and ENSO, governed by the competing influences of the westerlies and East Asian summer monsoon. Combined with the 400 mm precipitation isoline shift and 1960-2024 water vapor flux and divergence fields, we clarified the north–south differentiation of moisture transport and improved the credibility of tree-ring reconstruction. The correlation between precipitation and CO₂emissions followed a spatial pattern of Genhe>Aershan>Chifeng>Yuxian, with significant coupling periods migrating from higher latitudes in early centuries to mid-latitudes in recent decades, consistent with accelerated global CO₂ rise. This study reveals the centennial precipitation evolution across the Greater Khingan Mountains and its southern margin, and provides a reference for long-term climate reconstruction in comparable monsoon marginal regions.