Late Holocene vegetation and environment at Cauduntou, west of Yangtze Delta, SW Jiangsu Province, East China

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Abstract

This study presents vegetation and environmental history of the Subboreal to Subatlantic periods for Cauduntou, located ca. 200km to the west of the southern Yangtze Delta, Southwest (SW) Jiangsu Province, East China. Relative and absolute pollen analyses and accelerator mass spectrometry (AMS) 14C dating were performed. The Subboreal (prior to 2500 BP) vegetation was a well-diversified deciduous forest of Ulmus/Zelkova, Quercus (Lepidobalanus and Cyclobalanopsis), Celtis/Aphananthe, Sapindus, etc. The Subatlantic (following 2500 BP) vegetation was deciduous and evergreen broad-leaved mixed oak (Cyclobalanopsis) forest associated with Lepidobalanus and Liquidambar. No distinct influence of human activities is observed in the earliest Subatlantic palynoflora, showing that the mixed oak forest represents the natural vegetation of the Cauduntou area. Intense deforestation and spread of agriculture are recognised in the mid- to late Subatlantic period. Pollen of Artemisia, Poaceae, Brassicaceae, Tubuliflorae, Polygonum and Fagopyrum increases indicating the onset of rapeseed and buckwheat cultivation. Reduction of total pollen concentration results from increased erosion and sediment supply to the study site, as a consequence of loss of vegetation cover from the surrounding hills.

Introduction

The flat, low-lying deltaic regions in East China are densely populated and are sensitive to climate changes. The southern Yangtze Delta plain, which consists of the Tai Lake basin and eastern strand plain (Zheng et al., 1999), is one of the economic centres of East China and will be severely damaged by a few metres of sea level rise. Formation and evolution of the delta plain related to past sea level changes have been studied for the last decades (Wang and Wang, 1980; Fang, 1991; Hong, 1991; Chang and Liu, 1996; Yu et al., 1999). The results revealed that large-scale migration of shoreline occurred in the southern Yangtze Delta plain during the last 10,000 years. Tai Lake (Taihu), one of the biggest fresh-water lakes in China, was formed as late as 4000 BP. In a larger scale, Pleistocene environments of the lower Yangtze region have been affected by glacial eustasy, which almost drained the East China Sea at the Last Glacial Maximum (Zhao et al., 1979; Peng et al., 1984; Qin and Zhao, 1991). Hundreds of Neolithic cultural sites have been developed around the Yangtze Delta since 7000 BP (e.g. Zheng et al., 1994). Most sites are located near the past shorelines, suggesting that the Neolithic human settlements were subject to sea level changes during the mid- to late Holocene. Archaeological studies also suggested that the lower Yangtze valley is one of the cultural origins for growing paddy rice in East Asia (e.g. Wu, 1983).

The lower Yangtze region also provides important themes for vegetation science. The long, intense human settlements during the last millennia have altered landscapes, and the natural vegetation of lower Yangtze valley is merely imagined from small patches of native forest surviving on surrounding hills. A large amount of pollen records has been accumulated in and around the Yangtze Delta to supplement knowledge of paleovegetation and paleoenvironments (Wang et al., 1984; Liu et al., 1992; Sun and Huang, 1993; Liu and Chang, 1996; Xu et al., 1996; Han et al., 2000; Yu et al., 2000). The southern Yangtze Delta plain has thick Quaternary deposits (ca. 200–300m) of alluvial, lacustrine and marine origins (Zhu et al., 1987; Shao et al., 1991), and is suitable for such Holocene (and Pleistocene) palynological studies. Although these studies have provided many pollen data sets for the Yangtze Delta and Tai Lake, the image of natural vegetation of the lower Yangtze valley is still dimmed beyond artificially altered palynofloras with abundant pine and herbs.

Biogeographically, the plain of the mid-lower reaches of the Yangtze River lies between the temperate deciduous forest of North China and the subtropical evergreen forest of South China (e.g. Li et al., 1995). The plain consists of a sort of transitional vegetation zone between the two major vegetation regimes of China. This means that the vegetation of lower Yangtze valley is sensitive to a few degrees of temperature variation. The existing pollen and sea level records have indicated that the climate of the Climatic Optimum (ca. 6000 BP) was 2–4°C warmer than at present (e.g. Wu, 1983; Zheng et al., 1999). The natural vegetation distribution and migration of the lower Yangtze valley could be an analogue of vegetation responses to the coming global warming.

In this study, we present a pollen record from Cauduntou, about 200km to the west of the Yangtze River estuary (Fig. 1). As most Neolithic cultural sites are located around the Yangtze Delta (and Tai Lake), this inland area is expected to have been less affected by human settlements, and preserved natural landscapes of the lower Yangtze valley until recent epochs. Both relative and absolute pollen analyses are performed for sediment samples from trench sections. Four AMS 14C ages are provided for two trench sections.

Section snippets

Study location, modern vegetation, and climate

Cauduntou (31°19′N,118°59′E,<10m a.s.l.) is a village located ca. 6km to the east of Kucheng Lake and ca. 90km to the south–southeast of Nanjing (Fig. 1). It is located in an alluvial plain of the lower reaches of the Yangtze River, which is low lying and very flat. However, Mesozoic basement rocks expose to the east and form highlands of hundreds of metres high. They divide the Cauduntou area hydrologically from the southern Yangtze Delta plain. The Yangtze River is meandering north on the

Materials and methods

Field survey and sampling were performed in December 1999 with collaboration from the Institute of Food Crops, Jiangsu Academy of Agricultural Sciences, Nanjing. Three trenches (CDT-1, 2 and 3) 2m long and 1m wide were opened in the midst of modern rapeseed fields. Trench CDT-3 was approximately 100m apart from CDT-1 and CDT-2 and was a few metres lower in altitude. CDT-1 was a 100cm deep trench, and two radiocarbon samples were collected. Trench CDT-2 consisted of completely massive

Pollen

Results of relative and absolute pollen analyses are shown in Fig. 2, Fig. 3. Seven local pollen zones (CDT-3 I–VII) are established from the base upward. Zones CDT-3 II, III, IV, VI and VII are based on the variations of percentages, while zones CDT-3 I and V are defined by significantly low pollen concentrations. As pollen counts were insufficient, data from CDT-3 I and V are presented in the concentration diagram (Fig. 3) only. In the relative diagram (Fig. 2) these zones are represented as

Vegetation history

A general feature of the CDT-3 pollen record is low frequencies of evergreen taxa, with the exception of abundant Cyclobalanopsis. Castanopsis, one of the main components of the subtropical evergreen forest of South China (Zheng and Lei, 1999; Xiao et al., 2000; Zheng, 2000), is rare in the diagram. In contrast, there is a wide variety of deciduous palynoflora consisting of Ulmus/Zelkova, Lepidobalanus, Celtis/Aphananthe, Acer, Betula, Carpinus, Sapindus, etc. Conifers (Pinus, etc.) are almost

Conclusions

A pollen analysis at Cauduntou, SW Jiangsu Province, East China detects deciduous and evergreen mixed oak (Cyclobalanopsis) forest associated with Lepidobalanus and Liquidambar, in the earliest Subatlantic period (<2500 BP). This palynoflora is more subtropical than available pollen data from the southern Yangtze Delta plain, but is consistent with native forest preserved near Nanjing. The pollen also indicates that human disturbance has not been significant around Cauduntou until the mid- to

Acknowledgements

We appreciate Ms. K. Moritomo's arrangement of radiocarbon dating in US. We give special thanks to Prof. Y. Saito of the Geological Survey of Japan for assisting in collecting references. The work was funded by the Yangtze River Civilization Programme (YRCP) organised by the International Research Centre for Japanese Studies, Kyoto, Japan.

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