Mid- to late Holocene vegetation change recorded at a Neolithic site in the Yangtze coastal plain, East China

Abstract

There has been an ongoing debate about human impacts on the evolution of the vegetation cover on the Yangtze delta plain dating back to the Neolithic period. In this study, we carried out pollen identification and grain size analysis on two sediment profiles obtained from the Neolithic Guangfulin site in the Yangtze delta. Together with published results of radiocarbon ages, organic elemental chemistry and archaeological findings, we reconstructed the palaeoecological evolution and human activities at the site during the mid- to late Holocene and distinguished between the impacts on vegetation induced by human and hydrological processes. The results show three events of significant reduction in the abundance of arboreal pollen, which occurred at c. 4635, 2000, and 800 cal yr BP, respectively. The first two events were accompanied by an obvious increase in Poaceae pollen, whereas the last one occurred with a sharp increase in the abundance of Brassicaceae pollen. These findings suggest that the reductions in arboreal pollen resulted from deforestation for the expansion of rice cultivation at c. 4635 and 2000 cal yr BP, and expansion of Brassicaceous oil crops c. 800 years ago. The change in cultivation pattern c. 800 years ago was consistent with the increase in population migration from northern China caused by war at that time. The pollen of aquatic plants increased sharply at c. 4500 cal yr BP, which reflected the change in hydrological environment related to sea-level rise at the Yangtze River mouth.

Introduction

Palynological studies show that vegetation change was affected by both climate change and human activities in the Holocene (Xiao et al., 2004; Chen et al., 2009; Innes et al., 2009, 2014; Shu et al., 2010). For example, Innes et al. (2014) reported the increase in abundance of Pinus in the Taihu plain, near the Yangtze coast at c. 4200 cal yr BP and linked it to the climatic cooling at that time. Other studies of the pollen records indicate that obvious human disturbance occurred, particularly at archaeological sites during the Neolithic time (Zong et al., 2007, 2012; Innes et al., 2009; Shu et al., 2010). At the Kuahuqiao site, south of Hangzhou Bay, deforestation for human settlement and rice farming was inferred from the significant decline in tree flora at 7700 cal yr BP (Zong et al., 2007; Innes et al., 2009). In the sedimentary record of Lake Chaohu in the lower Yangtze valley, Chen et al. (2009) reported a decline in the arboreal pollen at c. 5000 cal yr BP, which was suggested to be the result of deforestation by the Neolithic people. Further, Zong et al. (2012) suggested that human disturbance of the vegetation had increased significantly in the Yangtze delta plain during the late Neolithic period.

However, the nature of human disturbance of the vegetation during the late Neolithic period was debated for a long time because archaeological evidence showed that the prehistorical human population mainly utilized the swamp/marsh on the Yangtze coast that was easy to reclaim and manage (Zheng et al., 2009, 2012; Shu et al., 2010; Li et al., 2012; Wen et al., 2014; Wang et al., 2017). It was therefore proposed that deforestation by the prehistorical population should be limited to within a small area of the settlement sites even during the period of the highly developed Liangzhu culture (Shu et al., 2010; Li et al., 2012). Some other pollen studies in the Yangtze delta (Fig. 1a) also reported that human impacts on vegetation occurred mainly during the historical period of the late Holocene (Liu and Qiu, 1994; Yi et al., 2003, 2006).

The hydrological environment is also one of the critical factors controlling the vegetation in coastal plains (Hayden et al., 1995; Röderstein et al., 2014) and it is directly affected by changes in sea level and freshwater supply (Chen et al., 2018). Previous studies of pollen indicate that the rising water table extensively occurred at the last stage of the Liangzhu culture in the Taihu plain (Zong et al., 2011, 2012). Our latest study on the coastal plain south of Hangzhou Bay revealed a rapid sea-level rise at c. 4510–4390 cal yr BP (Fig. 1b; Wang et al., 2018). It remains unclear whether the previously reported increase in water level inferred from the pollen records (Zong et al., 2011, 2012) is the response of vegetation linked to this sea-level rise on the Yangtze coast.

Furthermore, we examined those published profiles from the Yangtze coast and found limited age constraints from the late stage of the Neolithic culture. We suggest that the lack of chronology will restrict full understanding of the complicated processes of vegetation change resulting from both natural and human's impact. Hence, in this study we used two profiles obtained during the excavation of the Neolithic Guangfulin site on the east Taihu plain (Fig. 1) and analysed pollen records and grain size. Combined with published results of organic chemistry and AMS ¹⁴C dating (Wang et al., 2014), we investigated the changes in vegetation at this site from the late Neolithic period and reconstructed human activities and hydrological environmental change influenced by sea level.