Reconstructing the rapid transitions of ecosystems during the mid-late Holocene: A pollen record from Haixing wetland in Bohai Bay, North China

Vegetation in coastal regions is sensitive to climate and sea-level changes. However, currently there is still a lack of understanding about the stability of the Holocene coastal ecosystem and the rapid conversion processes of different coastal wetland ecosystems. In this study, the high-resolution vegetation succession history of the southwest coast of Bohai Bay during the mid-late Holocene was revealed, based on pollen analysis and REVEALS model, and the rate of vegetation change was estimated. Furthermore, the characteristics and mechanisms of different ecosystem changes were explored. The results indicate that there was a prolonged transition from shallow sea to lagoon before the formation of the Haixing wetland during the interval of 7500–4100 cal yr BP. The rate of watershed vegetation change increase significantly under the common influence of the 4.2 ka event and human activities, causing a regime shift in the mountain forest ecosystems and a decrease in landscape resilience. It was characterized by a substantial reduction in broad-leaved forests (from>30% to<5%), especially for Quercus (∼3%), which have not recovered to pre-event levels since the end of the 4.2 ka event. Since 3500 cal yr BP, the Haixing area was completely detached from the influence of the Bohai Sea, forming freshwater wetlands. The local vegetation rapidly shifted from alkali-tolerant communities that followed the sea retreat to freshwater marsh plant communities. During 2000–1100 cal yr BP, under the strong impact of human activities, the succession of forest and grassland communities in the basin became more frequent. The area of coastal salt marshes expanded, with salt-tolerant plant communities taking the absolute advantage. The wetland vegetation landscape became closer to the modern appearance. Overall, our study provides new evidence for understanding the rapid evolution of coastal ecosystems in East Asia influenced by a combination of climate, hydrology and humans. It will help guide the coastal regions in facing the challenges of future global climate changes.