Vegetation and climate dynamics in Southwestern China during the Han and Jin Dynasties (202 BC-AD 420): Insights from the Spore-pollen and hydrological records from the Chengba site

The Han and Jin Dynasties (202 BC-AD 420) were marked by frequent social upheavals and climate fluctuations in Chinese history. Abundant evidence suggests that the central government effectively managed and controlled the southwestern region during this period. Nevertheless, in-depth exploration is necessary to delineate the dynamic interplay between environmental changes and social development. This study conducted chronological, grain-size, and spore-pollen analyses on sediment from the T8SW section at the Chengba site in southwestern China, aiming to investigate the hydrology, vegetation, and climate changes during the Han and Jin Dynasties (202 BC-AD 420). The WAPLS (Weighted Average Partial Least Squares Regression) method was also employed to reconstruct the paleoclimate quantitatively. The results showed that in the early and middle phases of the Western Han Dynasty (202-48 BC), this area was characterized by a subtropical-warm temperate mixed forest and grassland landscape. The Jinguan relic is nestled within a serene riverine bay and wetland expanse, boasting lush vegetation. During the Eastern Han Dynasty to the Jin Dynasty (AD 25–420), with the cooling of the climate and the drying of the sedimentary environment, this area transformed into a warm temperate forest and grassland landscape. The position of the Jinguan relic gradually emerged out of water, evolving into high floodplains and terraces with sparse vegetation. Upon entering the Eastern Han Dynasty, a substantial alteration occurred in pollen concentration and composition, which might have been associated with intricate variations in hydrological environments, climate change, and human activities. Quantitative reconstruction approaches have reconstituted the annual precipitation (Pann) and mean temperature of the coldest month (MTco). These data reveal that the climate in the area experienced a ‘warm and humid - cold and dry - warm and humid' cycle from the Han to the Jin Dynasty. Notably, a significant dry climate event occurred around the transition between the Western and Eastern Han Dynasties, approximately AD 8–25. Archaeological and documentary data suggest that changes in human activities within the area were closely linked to variations in vegetation and climate. The shift in fluvial hydrological regimes under the cooling and aridification trend during the Eastern Han Dynasty likely served as the primary catalyst for both the architectural transition from stilt-based to ground-level construction methods, as well as the significant rise in both the quantity and depth of water wells. Simultaneously, the large-scale urban construction and the boom in the mining and smelting industry during the middle and later periods of the Eastern Han Dynasty likely led to selective logging. This activity could have resulted in a significant reduction in the tree population in the area.