Population migration with improved productivity caused a heterogeneity pattern of Holocene vegetation succession in typical areas of the lower Yangtze region

Abstract

Climate fluctuations and human activities shaped vegetation dynamics during the Holocene, yet the underlying mechanisms remain elusive. Our investigation utilizes the MAT-REVEALS approach to analyze 17 fossil pollen records spanning the past 8500 years collected at three typical areas of the lower Yangtze: South Zhejiang Mountains (SZJM), Taihu Basin (THB), and Yaojiang Valley (YJV). The reconstructed vegetation in SZJM revealed a high proportion of arboreal content during 8500–2000 cal yr BP, and the vegetation succession generally followed the variation of mean annual temperature and precipitation, indicating the Asian monsoon climate dominated the vegetation change. Of note, the minor decline of the broadleaved forest at 6000–5000 cal yr BP coincided with the drop in winter temperature, indicating its role as a limiting factor for the broadleaved arboreal. A marked increase of herbs (15.6 % to 36.5 %) occurred in the coastal region of YJV at 7500–5000 cal yr BP, which was not registered in SZJM, was coeval with a surging population and the start of extensive and intensive rice farming on the newly formed alluvial plain, indicating this noticeable landscape openness was induced by anthropogenic factors. Conversely, significant anthropogenic modifications of vegetation only emerged after 5000 cal yr BP in the THB, likely linked to large immigrations of population and the establishment of the Liangzhu hierarchical state that consumed abundant arboreal resources for large-scale constructions. Since 3000–2000 cal yr BP, a widespread decline of arboreal proportion took place in all three areas, which was coeval with a sharp decline of winter temperature as well as the introduction of bronze and later iron to this region during the Spring and Autumn to Three Kingdoms period. Notably, the decline in forest cover in the SZJM region was relatively late. However, it accelerated in the recent millennium, matching the timing of new immigrants from northern China during the Tang-Song dynasties. Overall, natural vegetation changes mainly followed the pattern of monsoon rainfall and winter temperature variations. Nevertheless, the heterogeneity pattern of vegetation changes on the coast underscored the pivotal role of the spatial-temporal distribution of population and advancing technology. This study highlights the regional and local impact of climatic and anthropogenic forces on vegetation composition, providing new insights for interpreting the timing and scale of human impact on past land cover and land use changes.