Historical human activities accelerated climate-driven desertification in China’s Mu Us Desert

Journal Pre-proofs Historical human activities accelerated climate-driven desertification in China’s Mu Us Desert Di Zhang, Hui Deng PII: DOI: Reference:

S0048-9697(19)34762-X https://doi.org/10.1016/j.scitotenv.2019.134771 STOTEN 134771

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Science of the Total Environment

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13 February 2019 28 August 2019 29 September 2019

Please cite this article as: D. Zhang, H. Deng, Historical human activities accelerated climate-driven desertification in China’s Mu Us Desert, Science of the Total Environment (2019), doi: https://doi.org/10.1016/j.scitotenv. 2019.134771

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1

Historical human activities accelerated climate-driven desertification in

2

China’s Mu Us Desert

3

Di Zhang *, Hui Deng **

4

College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

5

* Corresponding author.

6

** Corresponding author.

7

E-mail addresses: [email protected] (D. Zhang), [email protected] (H. Deng).

8 9

Abstract

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China’s Mu Us Desert, located in an energy-rich strategic base of the northwestern Loess

11

Plateau, has acted as a crucial agro-pastoral transition zone for thousands of years. However,

12

the area experienced notable climate and environmental change from 221 BC to AD 907

13

(1128 years), which may have profoundly affected its landscape evolution up to modern times.

14

To explore this process and associated driving mechanisms, we conducted a comprehensive

15

study based on a dataset of 882 human archaeological sites (HASs), historical documents,

16

and related environmental data of the Mu Us Desert and its surrounding area (MUDISA). We

17

found that the MUDISA experienced large-scale immigration on several occasions, as well as

18

an agricultural boom (790 HASs), during the Qin and Han dynasties (221 BC–AD 220). This

19

coincided with an ecologically favorable environment and may have potentially disturbed the

20

desert’s eco-environmental equilibrium. Coinciding with the deteriorating natural conditions,

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the MUDISA was dominated by animal husbandry with the HAS numbers declining sharply

22

and scattering in the desert during the era of disunity (AD 220–581, 33 HASs) and during the 1

23

Historical human activities accelerated climate-driven desertification in

24

China’s Mu Us Desert

25

Di Zhang *, Hui Deng **

26

College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

27

* Corresponding author.

28

** Corresponding author.

29

E-mail addresses: [email protected] (D. Zhang), [email protected] (H. Deng).

30 31

Abstract

32

China’s Mu Us Desert, located in an energy-rich strategic base of the northwestern Loess

33

Plateau, has acted as a crucial agro-pastoral transition zone for thousands of years. However,

34

the area experienced notable climate and environmental change from 221 BC to AD 907

35

(1128 years), which may have profoundly affected its landscape evolution up to modern times.

36

To explore this process and associated driving mechanisms, we conducted a comprehensive

37

study based on a dataset of 882 human archaeological sites (HASs), historical documents,

38

and related environmental data of the Mu Us Desert and its surrounding area (MUDISA). We

39

found that the MUDISA experienced large-scale immigration on several occasions, as well as

40

an agricultural boom (790 HASs), during the Qin and Han dynasties (221 BC–AD 220). This

41

coincided with an ecologically favorable environment and may have potentially disturbed the

42

desert’s eco-environmental equilibrium. Coinciding with the deteriorating natural conditions,

43

the MUDISA was dominated by animal husbandry with the HAS numbers declining sharply

44

and scattering in the desert during the era of disunity (AD 220–581, 33 HASs) and during the 2

45

Sui and Tang dynasties (AD 581–907, 59 HASs). Coupled with the dry climate and fragile

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geographical environment, both the increasingly large-scale human population and the

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presence of extensive livestock may have significantly accelerated the climate-driven

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desertification process from AD 220 to AD 907. This study highlights the long-term

49

human–nature relationship and their combined impact on historical desertification in the Mu

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Us Desert, and may shed new light on historical environmental change in arid and/or

51

semi-arid areas in northwest China and globally.

52 53

Keywords: Mu Us Desert; From 221 BC to AD 907; Comprehensive study; Human activities;

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Human–nature relationship; Historical desertification

55 56

1. Introduction

57

China’s Mu Us Desert is located at the northwest edge of the East Asian monsoon region and

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the junction of arid and semi-arid areas of the Eurasian continent (Liu, 2009; Fig. 1). This

59

area is a crucial agro-pastoral transition zone at the middle reaches of the Yellow River, with

60

pronounced fluctuations in population and land use types during its history (Wang, 2006).

61

Contrary to its importance and persistent status in China’s energy industry, the desert has

62

been ecologically fragile and highly sensitive to climatic change since ancient times (Dong et

63

al., 1989; Li et al., 2000; Li et al., 2005; Sun, 2000). It is therefore an ideal natural laboratory

64

for exploring global climate change, human activity, and environmental variation

65

relationships throughout history.

66 3

67

The desert’s situation from 221 BC to AD 907 deserves close attention because it presents a

68

critical link between the past and the present. Compared with the previous historical period,

69

the ability of the ancient Chinese people to transform the surface landscape during this period

70

was greatly enhanced by remarkable improvements in agricultural production (e.g., two-cattle

71

plowing and iron tools) and efficiency of social organization (e.g., landlord–tenant dynamics

72

and large-scale herding and/or nomadic tribes). The environmental changes and the

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human–land relationship during this early feudal society may have profoundly affected the

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evolution of the desert landscape up to modern times.

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Concerning this period, some researchers hold that the origin and evolution of the desert were

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mainly driven by climatic fluctuations (Dong et al., 1989; Wang et al., 2005). The region

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experienced climate-driven desertification in the late Tang Dynasty (Cui and Chang, 2013;

79

Huang et al., 2009). Other researchers have challenged this view. For example, Sun et al.

80

(2000), using evidence from sedimentary profiles, ancient city distribution, and historical

81

records, found that unreasonable human activities, especially cultivation, had greatly

82

accelerated sand reworking of the desert during the past 2300 years. Deng et al. (2001), using

83

aerial remote sensing images and historical documents, pointed out that local people’s

84

overuse of land resources played a leading role in ecological environmental deterioration

85

around the ancient Tongwan City (within the desert, Fig. 1B) from the beginning of the 5th

86

century to the late 10th century. The desertification in the Tang Dynasty may therefore be

87

closely related to intensified human activity (Guo et al., 2018; Li et al., 2019).

88 4

89

The holistic picture of the desert’s environmental change and its driving forces during this

90

period still remains vague. Key to unraveling these controversial questions is the construction

91

of a human–nature-based model integrating quantitative and qualitative methods and data.

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Here, we collected and analyzed 882 human archaeological sites (HASs) in the Mu Us Desert

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and its surrounding area (MUDISA) for the historical period between 221 BC and AD 907 as

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provided by the Atlas of Chinese Cultural Relics, the Third National Cultural Relic Survey,

95

and relevant journal articles. Using these data, combined with related historical records,

96

stratigraphic evidence, historical rivers in the eastern MUDISA, and a map of the desert’s

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modern landscape, we established a database of human occupation and eco-environment in

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the MUDISA (see the section “Materials and methods” for the details).

99 100

Using this database, we explored the human–nature relationship and environmental changes

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in the desert over this continuous period and investigated potential driving forces. Evidence

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from the HASs, historical records, socio-economic characteristics, and quaternary geology

103

data (Jiang et al., 2010; Ma et al., 2011; Su et al., 2018; H. Zhao et al., 2016) produced

104

coherent scenarios for the three occupation phases in this 1128-year period: (a) Qin and Han

105

dynasties (221 BC–AD 220), (b) era of disunity (AD 220–581), and (c) Sui and Tang

106

dynasties (AD 581–907).

107

5

108 109

Fig. 1. The Mu Us Desert and its surrounding area (MUDISA) including historical sites,

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stratigraphic profile sites, and historical rivers mentioned in the text. (A) The MUDISA’s

111

location on Google Map images of China. The dashed green line indicates the approximate

112

line of modern 400 mm isoprecipitation. (B) Red dots with numbers indicate historical sites:

113

1, Yangqiaopan; 2, Tongwan/Xiazhou City; 3–7, five cities of the Liuhuzhou. Green dots with

114

numbers indicate stratigraphic profile sites: 1, Bayanchagan (Fig. 1A); 2, Dishaogouwan

115

(DSGW); 3, Jinjie. Details of the reconstruction of historical rivers are in “Materials and

116

methods”. m.a.s.l., meters above sea level.

117 118

2. Materials and methods

119

2.1. Study area

120

The Mu Us Desert belongs to the transitional zone of arid and semi-arid areas in northern

121

China. Located in the Hetao area bounded by the Yellow River to the west, north and east, the

122

desert consists of parts of 11 counties/cities, including the northern Yulin City, southern 6

123

Ordos City, and eastern Wuzhong City, with the total area of about 39,000 km2 (Fig. 1). The

124

regional mean annual temperature and hours of sunshine are ~8°C and ~2900 h, respectively;

125

its mean annual precipitation gradually ranges from ~450 mm/year in the southeast to < 200

126

mm/year in the northwest. The surface landscapes within the desert mainly include shifting

127

dunes, fixed/semi-fixed dunes, desert steppes and shrubs, and surface water. Based on spatial

128

connection and regional comparison, this study considers some aspects of the Mu Us Desert

129

and its surrounding area (MUDISA). The area of the MUDISA is ~95,000 km2—2.4 times

130

larger than that of the Mu Us Desert. This inclusion is aimed at making this desert research

131

more scientific and reliable.

132 133

It is worth noting that the desert is significantly different from the surrounding loess area in

134

terms of geological conditions and surface environment. At least since the historical period,

135

the Mu Us Desert has existed with sandy strata (Dong et al., 1989; Sun, 2000; Xu et al., 2015;

136

Zhou et al., 2009). The degree and range of its dunes’ fixation and activation in different

137

historical periods mainly have depended on the corresponding climatic conditions (Dong et

138

al., 1989; Li et al., 2000; Li et al., 2005; Li et al., 2019; Liu and Lai 2012) and/or human

139

activities (Guo et al., 2018; Li et al., 2019; Sun, 2000; Xiao et al., 2002; Zhou et al., 2002).

140

The desert’s eco-environment has thus always been more fragile and unstable than that of the

141

loess area surrounding it. These circumstances have formed the geographical basis for

142

potential regional desertification of the Mu Us Desert historically.

143 144

2.2. Human archaeological sites 7

145

Data on HASs in this study were drawn from the Atlas of Chinese Cultural Relics (Bureau of

146

National Cultural Relics, 1998, 2003, 2010), the Third National Cultural Relic Survey

147

(Leading group of the Inner Mongolia Autonomous Region for the Third National Cultural

148

Relics Survey, 2011; Leading group of Yanchi County for the Third National Cultural Relics

149

Survey [Unpublished internal materials]; Ordos Municipal Bureau of Culture and Leading

150

group of the Ordos for the Third National Cultural Relics Survey [Unpublished internal

151

materials]; Shaanxi Provincial Cultural Relics Bureau, 2012), and other relevant publications

152

(Guo, 1995; Kou et al., 2006; Li et al., 2012; Li et al., 2015; Qiao et al., 2011; Wang et al.,

153

2011; Yin et al., 2009). Sites containing iron tools during the Qin and Han dynasties

154

mentioned in the text also come from this database. The 882 HASs are composed of three

155

types: general human settlements (53.74%), ancient cities (7.03%), and burial areas

156

(39.23%).

157 158

Based on ArcGIS 10.0 spatial analysis, the HAS Point Density maps of different periods were

159

created after choosing the Neighborhood Method (Circle, Radius = 15 km) and the

160

Classification Method (Natural Breaks, Jenks). These Point Density maps provide spatial

161

information on the Minimum Enclosing Rectangles of relevant HASs before boundary

162

clipping. In this study, the spatial distribution of the HASs and Point Density is an important

163

way to characterize corresponding “human occupation,” not to represent spatial distribution

164

of the historical population or human activity intensity. According to historical geography

165

research (Deng et al., 2001, 2003) and the relevant historical documents (Ban, 2002; Fan,

166

2003; Liu, 2002; Sima, 1995; Song, 2003; Wei, 2002; Wei, 2003), human activities were not 8

167

limited to the vicinity of the HASs, and desert areas that do not contain HASs were not

168

necessarily lacking in historical human activities. It is often quite difficult for animal

169

husbandry activities, for example, to produce HASs, and such activities were common during

170

the era of disunity and the Sui and Tang dynasties.

171 172

2.3. Historical river reconstruction

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Rivers located on the eastern MUDISA during this study period (Fig. 1B) were reconstructed

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on the basis of the historical records from the Shuijing Zhu (Notes on the Book of Waterways;

175

Li, 2009), Digital Elevation Model (DEM) extraction of ancient river channels, remote

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sensing images, and field surveys. The Shuijing Zhu, written in the late Northern Wei

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Dynasty (AD 386–534), describes the rivers’ sequence or order, geographical location, and

178

direction of flow. This book confirms that rivers of Kuye, Tuwei, and Wuding—the three

179

large tributaries of the Yellow River in the eastern MUDISA—already existed there during

180

the study period. The book also records several relatively large tributaries for each of these

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three rivers, and their approximate spatial distribution was determined. In addition, two other

182

small rivers are also recorded between the Kuye River and the Tuwei River and between the

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Tuwei River and the Wuding River, respectively (Fig. 1). Combining these records with

184

modern remote sensing images and field investigations, we mapped the general distribution

185

of these rivers.

186 187

Based on ArcGIS 10.0 software and DEM data, we applied the “surface runoff model” to

188

extract a vector map of the river channels in the eastern MUDISA (Tang and Yang, 2012). In 9

189

this model, we first obtained the water flow direction for each DEM grid on the basis of the

190

maximum slope gradient method of non-depression DEM. Using the grid data with water

191

flow direction, we calculated the total grid number accumulated in the water flow direction

192

for each grid; this is called the “flow accumulation”. Assuming that each grid carries one unit

193

of flow water, the flow accumulation represents the total water flow for each grid. When the

194

grid flow accumulation reaches a certain value, surface water flow will form and the potential

195

water flow paths consist of all of the grids whose flow accumulation exceeds the certain

196

value threshold. The river network is composed of all of the water flow paths.

197 198

ArcGIS 10.0 software provides a DEM data processing module called Hydrology in the

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Spatial Analyst Tools. This module was successfully applied to extract the vector map of the

200

river channels. After the ArcGIS operation steps for depression-DEM filling, flow direction

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calculation, flow accumulation calculation, river grid extraction, river vector formation, and

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river vector post-processing (Tang and Yang, 2012), we obtained the river network map.

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According to this vector map, the general map of the recorded rivers was adjusted, and we

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have treated these adjusted rivers as the historical rivers in our study (Hu, 2015). The modern

205

remote sensing images come from Google Earth data. The DEM data are from SRTM DEM

206

(90 m resolution) and ASTER GDEMV2 DEM (30 m resolution); these data were provided

207

by the Geospatial Data Cloud site, Computer Network Information Center, Chinese Academy

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of Sciences (http://www.gscloud.cn).

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2.4. Map of modern landscapes in the Mu Us Desert 10

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The modern landscape map of the Mu Us Desert was made on the basis of Landsat ETM+

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remote sensing images (July to September 2000). These data were provided by the Geospatial

213

Data Cloud site, Computer Network Information Center, Chinese Academy of Sciences

214

(http://www.gscloud.cn). After extensive data trials and comparisons/contrasts in ENVI 5.1,

215

we established the optimal image band combination of R7-G4-B1, which well distinguishes

216

different types of the desert’s major landscape features. Based on these, we completed the

217

supervised classification of these images combining a great deal of data pre- and post-

218

processing. We finally obtained the high-quality landscape map consisting of the six types:

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water, vegetation, shifting dune, fixed or semi-fixed dune, saline-alkali land, and sandy soil

220

or loess. The desert boundary is defined according to the Natural conditions and their

221

improvement and Utilization in the Mu Us Desert (Department of Geography in Peking

222

University et al., 1983).

223 224

2.5. Historical records, historical population reconstruction

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Historical records containing important information on the relevant eco-environment and/or

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human activities mentioned in this study are listed in Texts S1–S26 in the Supplementary

227

Information (SI). Based on relevant available historical documents (Ban, 2002; Fan, 2003;

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Liu, 2002; Sima, 1995; Song, 2003; Wei, 2002; Wei, 2003) and the Historical Atlas of China

229

(Tan, 1982), we reconstructed the historical populations of six periods for the Mu Us Desert,

230

as well as for the ancient Tongwan City and its surrounding area. Due to the lack of exact

231

statistical years for each of the population data, we have limited the historical time sections to

232

periods as short as possible on the basis of relevant historical materials. 11

233 234

For historical population documents using people as the unit, we used these population data

235

directly. For historical population documents that used households as the unit, we converted

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the household-based population data using the common formula that one household equals

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five people; this formula has frequently been used to convert household-based to

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person-based historical demographic data in Chinese history and historical geography (Deng

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et al., 2001). The relevant military populations have already been included in the

240

demographic data. More details about the specific historical documents and population

241

calculations can be seen in the SI. Generally speaking, the desert’s population was mainly

242

concentrated in ancient cities and their surrounding areas.

243 244

3. Results and discussion

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3.1. Agricultural boom during the Qin and Han dynasties (221 BC–AD 220)

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During the Qin and Han dynasties, humans dwelt in nearly the entire MUDISA (Fig. 2A).

247

There were 790 HASs with a Minimum Enclosing Rectangle of 136, 898 km2; both of the

248

two numbers are the maximum values for all three periods (Fig. 2 and Table 1). The

249

proportion of high slope (>12°) HASs is as high as 13.9% (Table S1). The HAS high-density

250

areas (>225 per ha) are mainly distributed in the regions of southern hilly ravine and eastern

251

river valley plain (Fig. 2A and D). Within the Mu Us Desert, compared with the other two

252

periods, there are numerous HASs (231 sites) and high-density areas (>113 per ha), mainly

253

distributed in the eastern and southern edges, although their percentage in the total 790 HASs

254

is not high (29.2%, Fig. 2A and D, Tables 1 and 2). Most of these sites tended to be 12

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distributed near present-day fertile zones, with 102 sites (44.2%) on modern vegetation areas

256

(Fig. 3A, Tables S2 and S3).

257

258 259

Fig. 2. Human archaeological sites (HASs) and HAS Point Density maps of three historical

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periods in the Mu Us Desert and its surrounding area (MUDISA). In HAS maps of the Qin

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and Han dynasties (221 BC–AD 220) (A), the era of disunity (AD 220–581) (B), and the Sui

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and Tang dynasties (AD 581–907) (C), HASs indicating major human occupation areas

263

consist of historical general settlement sites (GSSs), city sites (CSs), and burial area sites

264

(BASs). HAS Minimum Enclosing Rectangles from (D) to (F) are 136,898, 96,348, and

265

75,690 km2, respectively. m.a.s.l., meters above sea level. (D), (E), and (F) are the Point

266

Density maps of the three periods, respectively. The Point Density unit in the legends is “sites 13

267

per 10,000 km2” (see “Materials and methods” for the method details of these Point Density

268

maps).

269 270

Table 1

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Statistics for human archaeological sites (three types) in three historical periods in the Mu Us

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Desert and its surrounding area (MUDISA), as well as in the Mu Us Desert. Archaeological site MUDISA General settlement site City site Burial area site Total (N)

Qin and Han n1 n1/N (%) 454 57.5 41 5.2 295 37.3 790 100

era of disunity n2 n2/N (%) 9 27.3 5 15.1 19 57.6 33 100

Sui and Tang n3 n3/N (%) 11 18.7 16 27.1 32 54.2 59 100

Mu Us Desert General settlement site City site Burial area site Total (N)

n1 139 11 81 231

n2 5 2 10 17

n3 3 9 20 32

n1/N (%) 60.2 4.7 35.1 100

n2/N (%) 29.4 11.8 58.8 100

n3/N (%) 9.4 28.1 62.5 100

273 274

Table 2

275

Nearest distance statistics between human archaeological sites for three historical periods in

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the Mu Us Desert and its surrounding area (MUDISA), and in the Mu Us Desert. Distance

277

unit: km; SD: standard deviation. Historical period Qin and Han era of disunity Sui and Tang

N 790 33 59

MUDISA Range Mean±SD 0.2–33.5 3.9±4.1 0.7–64.2 18.6±19.9 0.5–65.0 13.2±12.3

278

14

n 231 17 32

Mu Us Desert n/N (%) Range 29.2 0.3–36.8 51.5 0.7–53.7 54.2 0.5–77.7

Mean±SD 4.7±5.1 9.8±14.2 16.6±18.8

279 280

Fig. 3. Human archaeological sites (HASs) during three historical periods on a modern

281

landscape map of the Mu Us Desert. In the HAS maps of the Qin and Han dynasties (221

282

BC–AD 220) (A), the era of disunity (AD 220–581) (B), and the Sui and Tang dynasties (AD 15

283

581–907) (C), HASs indicating major human occupation areas consist of historical general

284

settlement sites (GSSs), city sites (CSs), and burial area sites (BASs). Details concerning the

285

map of the desert’s modern landscape can be found in “Materials and methods” (see Tables

286

S2 and S3 for the spatial relationship between the HASs and different landscape types).

287 288

This boom situation was directly linked to the robust military control and large-scale

289

immigration in this area implemented by centralized governments. The First Emperor of Qin

290

sent General Meng Tian leading an army of 300,000 people to defeat nomadic tribes and

291

reclaim vast areas of land in the Hetao area (including the MUDISA, Texts S1 and S2).

292

Afterward, Qinzhidao—the first ancient “highway” in human history—was built, passing

293

south-to-north through the desert (Text S3; Tan, 1982). Based on historical records (Texts

294

S4–S7), large-scale populations were migrated into the MUDISA and its adjacent regions

295

from southern farming areas on several occasions. Newcomers totaled over 700,000 in 120

296

BC, for example (Text S5). Hundreds of thousands of border guards joined the ranks of

297

agricultural reclamation (Text S7). The MUDISA’s farming population increased sharply,

298

coinciding with the large number of HASs in this period. The total population of the four

299

counties of Sheyan, Baitu, Qiuci, and Gaowang within the desert was as high as about

300

105,504 in the late Western Han Dynasty (Fig. 4), which is the highest value during the six

301

historical periods considered, although the population once dropped significantly to less than

302

10,000 in the late Eastern Han Dynasty due to social unrest and farmer withdrawal.

303

16

304 305

Fig. 4. Historical populations of different historical periods for the Mu Us Desert, as well as

306

for the ancient Tongwan City and its surrounding area. The unit of numbers on the ordinate is

307

“1,000 people”. P1–P6 represent the periods AD 1–5, AD 136–141, AD 391–427, AD

308

533–536, AD 605–618, and AD 742–756, respectively. The six historical time sections are in

309

the late Western Han Dynasty (202 BC–AD 8), the late Eastern Han Dynasty (AD 25–220),

310

the late Sixteen Kingdoms period (AD 304–439), the late Northern Dynasties (AD 386–581),

311

the Sui Dynasty (AD 581–618), and the middle of the Tang Dynasty (AD 618–907),

312

respectively. Details of the historical population reconstruction can be found in “Materials

313

and methods” and Supplementary Information (SI).

314 315

Meanwhile, agricultural production techniques and tools were greatly improved compared

316

with prehistoric times. Tomb murals from the Eastern Han Dynasty (AD 25–220) within the

317

Mu Us Desert vividly reproduce scenes of agricultural hoeing and two-cattle plowing (Fig. 5).

318

This suggests that farmers in this area attached great importance to field management and

319

mastered the then-advanced two-cattle plowing technique. Canal irrigation was also 17

320

introduced here (Texts S8 and S9). Notably, throughout the study area, iron tools (including

321

iron farm tools, Fig. 6) contributed significantly to the efficiency and intensity of land use.

322

323 324

Fig. 5. Tomb murals of farming activity from the Eastern Han Dynasty (AD 25–220) within

325

the Mu Us Desert. Unearthed from the Yangqiaopan site in Jingbian County, northern

326

Shaanxi Province (Yin et al., 2009; see Fig. 1 for its location), (A) vividly reproduces the

327

scene of agricultural hoeing, and (B) depicts a two-cattle plowing scene, suggesting that this

328

farming technology was introduced there no later than the Eastern Han Dynasty.

329

18

330 331

Fig. 6. Spatial distribution of the iron sites (ISs, unrecognizable type) and iron farm tool sites

332

(IFTSs) from the Han Dynasty (202 BC–AD 220) in the Mu Us Desert and its surrounding

333

area (MUDISA). These sites come from the same database as the human archaeological sites

334

(HASs) in “Materials and methods.” The considerable number of the ISs (31 sites) and IFTSs

335

(11 sites) to some extent indicates the scope and intensity of agricultural activities at that time.

336

m.a.s.l., meters above sea level.

337 338

All of the above, combined with the innovative landlord ownership of the land, greatly

339

promoted the development of agriculture in the region. The Hetao area became a place where

340

“fertile land and surplus grains are ubiquitous” (Text S9; Wang et al., 2011). The farming

341

culture brought here by these immigrants from the Central Plains was prevalent. Some animal

342

husbandry and hunting still existed (Text S9; Lv and Zhang, 2004; Shaanxi Provincial

343

Institute of Archaeology and Cultural Relics Management Committee of Yulin City, 2001),

344

complementing the dominant agricultural economy. 19

345 346

This farming prosperity in the populous MUDISA indicated favorable climatic and

347

eco-environmental conditions there during this period (Fig. 7). Similarly, the desert’s

348

ecological situation was probably much better than it is today, and there was a relative

349

abundance of water sources in perennial rivers and lakes, especially in the east and south

350

fringes (Texts S2 and S9–S11). While a certain proportion of scattered bare areas (Guo et al.,

351

2018; Shaanxi Provincial Institute of Archaeology and Cultural Relics Management

352

Committee of Yulin City, 2001), the dunes of this period were generally fixed within the

353

desert. High-intensity agricultural activities may, however, have exceeded the local ecological

354

carrying capacity (Text S10; Sun, 2000; Xiao et al., 2002). After all, the desert’s strata during

355

the historical period were still sandy and ecologically fragile as they were inherited from

356

previous geological periods (Dong et al., 1989; Xu et al., 2015; Zhou et al., 2009); the area

357

was massively reclaimed as cultivated land. The potential human impact on the desert’s

358

eco-environment is discussed below.

359

360 20

361

Fig. 7. Climatic records and stratigraphic profiles from two sites in the Mu Us Desert. (A)

362

Based on climate reconstruction of the Bayanchagan Lake sediment core, curves a and b

363

represent mean annual precipitation (Pann) and temperature (Tann), respectively (Jiang et al.,

364

2010). H1–H3 represent the Qin and Han dynasties (221 BC–AD 220), the era of disunity

365

(AD 220–581), and the Sui and Tang dynasties (AD 581–907), respectively. (B) Stratigraphic

366

profile with the optical stimulated luminescence (OSL) dating at the Dishaogouwan (DSGW)

367

site (H. Zhao et al., 2016). (C) Stratigraphic profile with the OSL dating at the Jinjie site (Ma

368

et al., 2011). See Fig. 1 for locations of the three sites. Climatic change trends of (A) can

369

largely be applicable to the Mu Us Desert and its surrounding area (MUDISA), since the two

370

places are not far from each other and located on the same marginal zone of the East Asian

371

Monsoon region.

372 373

3.2. Regional desertification and the rise of animal husbandry during the era of disunity (AD

374

220–581)

375

Good times do not last forever. The number of HASs in the MUDISA during the era of

376

disunity decreased sharply to the lowest point (33 sites) for all three periods. Minimum

377

Enclosing Rectangle of these HASs was reduced by 30% in area and shrank towards the

378

middle compared with the previous period (Fig. 2 and Table 1). Compared with the Qin and

379

Han dynasties, both the mean values of HAS nearest distances in the MUDISA and in the

380

desert increased to 18.6 km and 9.8 km, respectively (Table 2). The desert’s 17 HASs

381

accounted for ~51.5% of the 33 total sites (Table 1) and were distributed closer to the rivers

382

(Fig. 2E and Table 3). The only relative high-density HAS area (>56 per ha) was near the 21

383

ancient Tongwan City along the Wuding River (Fig. 1B and E). The ancient Tongwan City, a

384

famous Xiongnu capital city site, was founded in the early 5th century AD by the Xiongnu

385

leader Helian Bobo. Consistent with the climatic shift towards dry and cold (Fig. 7A), this

386

HAS exodus implied deteriorating natural conditions after the Han Dynasty (Liu and Lai,

387

2012). Although lower than during the late Western Han Dynasty, the desert’s populations

388

reached ~45,000 and ~56,000 for the late Sixteen Kingdoms period and the late Northern

389

Dynasties, respectively. For the latter two periods, populations in the ancient Tongwan City

390

and its surrounding area were ~40,000 (88.9% of the 45,000) and ~28,000 (50% of the

391

56,000), respectively (Fig. 4). Obviously, the average population of the desert’s settlements

392

(consisting of city and general settlement sites) during this period was generally much higher

393

than during the Qin and Han dynasties.

394 395

Table 3

396

Nearest distance statistics for human archaeological sites (HASs) during three historical

397

periods in the eastern valley plain (EVP) and the Mu Us Desert to the eastern historical rivers

398

in the Mu Us Desert and its surrounding area (MUDISA). Distance unit: km; the EVP

399

consists of the counties of Shenmu, Yulin, Hengshan, and Jiaxian (northern part, Fig. 1B); SD:

400

standard deviation; n1, n2, and n3 represent the numbers of relevant HASs with the nearest

401

distances of no more than 1 km, 5 km, and 10 km to the rivers, respectively. Region and period Qin and Han EVP era of disunity Sui and Tang Mu Us Qin and Han Desert era of disunity

N 238 9 16 231 17

Range <0.1–23.8 0.1–22.5 0.1–22.5 <0.1–83.8 0.3–30.7

Mean±SD 5.5±5.1 7.0±8.1 5.1±6.4 19.6±22.8 9.9±10.2 22

n1/N (%) 22.3 44.4 31.3 10.8 23.5

n2/N (%) 54.2 55.6 68.8 33.8 41.2

n3/N (%) 83.6 66.7 81.3 54.5 70.6

Sui and Tang

32

0.2–80.4

17.9±23.3

12.5

34.4

56.3

402 403

During this period, historical documents suggest that the study area experienced a

404

desertification process of sand dune activation. On the western and northern parts of the

405

ancient Tongwan City, there were ubiquitous areas of sandy land, large sand piles, and hills of

406

sand (Texts S12 and S13). The west and north of the Mu Us Desert had deep sand, roads

407

mostly covered by deep sand, and shifting dunes (Text S14). This desertification process was

408

also recorded in stratigraphic profiles from the desert’s eastern river valley plain (Fig. 7B and

409

C). Although some ecologically favored refuges were indicated by the HASs (Fig. 3B) and

410

recorded in the Shuijing Zhu (Text S13), the desert may have suffered the consequences of

411

human disturbance of the ecological equilibrium during the Qin and Han dynasties (Text S10;

412

S.H. Li et al., 2012; Sun, 2000). Dunes and quicksand originated primarily from the local

413

sandy strata (Sun, 2000; Xu et al., 2015), correlating with the weak East Asian monsoon (Nie

414

et al., 2015; Wen et al., 2016; S. Zhao et al., 2016; Zhou et al., 2009). Compounding the

415

effects of the sandy strata and dry climate (Fig. 7A), the desert’s considerable population,

416

especially near the city sites, may have contributed to this desertification process.

417 418

Why did Helian Bobo establish his capital at the ancient Tongwan City within the desert? The

419

then relatively attractive eco-environment in the region (Texts S15 and S16; Deng et al., 2001,

420

2003; Hou, 1973) was limited to a small area that did not represent the whole situation of the

421

desert. Indeed, the city’s location was more a matter of a vital political and military strategy

422

to resist the powerful enemy in the north called Beiwei (Wang and Dong, 2001). The ancient

423

Tongwan City maintained its status as a capital for only 19 years: from AD 413, at the 23

424

beginning of its construction, to AD 431, when Helianxia (the regime established by Helian

425

Bobo) was conquered by Beiwei.

426 427

Despite some farming activity near rivers, lakes, and other ecological refuges (Deng et al.,

428

2001; Wang and Dong, 2001), animal husbandry became the basic subsistence economy in

429

the region (Text S17). Animals, such as sheep, goats, horses, and cattle, could be bred in large

430

numbers here (Text S17). As a stage for fierce competition among northern ethnic minorities

431

(e.g., Hun, Xianbei, Jie, Qiang, and Di), the desert witnessed the exchange and integration of

432

different cultures (Deng et al., 2003), among which Buddhism began to be universally

433

respected and welcomed. Its prevalence was more or less related to the deterioration of

434

natural conditions and social unrest at that time.

435 436

3.3. Continued population growth and desertification during the Sui and Tang dynasties (AD

437

581–907)

438

Environmental degradation seemed to continue. The number of HASs in the MUDISA during

439

the Sui and Tang dynasties remained small (59 sites, Fig. 2C and Table 1), with the HAS

440

Minimum Enclosing Rectangle reducing to its minimum of 75,690 km2 for all three periods

441

(Fig. 2F). These HASs generally retreated to the southern regions of the desert and loess (Fig.

442

2). Meanwhile, HASs still formed a relatively high-density area (>99 per ha) around the

443

ancient Xiazhou City (namely the ancient Tongwan City); people also moved closer to rivers

444

in the eastern valley plain (Fig. 2F and Table 3). All of these situations can be regarded as

445

humans’ response to the dry climate (Fig. 7; Huang et al., 2009). Compared with the era of 24

446

disunity, both the number of desert HASs and their percentage in the total MUDISA HAS

447

count increased slightly (32 sites and ~54.2%, Table 1). The desert’s populations reached

448

~77,180 and ~84,089 for the Sui Dynasty and the middle of the Tang Dynasty, respectively.

449

Meanwhile, populations in the ancient Xiazhou City and its surrounding area were ~19,455

450

(25.2% of the 77,180) and ~25,638 (30.5% of the 84,089), respectively (Fig. 4). Similarly, the

451

mean population of the desert’s settlements (consisting of city and general settlement sites)

452

during the Sui and Tang dynasties still remained at a relatively high level.

453 454

That the Mu Us Desert was continuing the rapid desertification from the previous period is

455

strongly supported by evidence from stratigraphic profiles (Fig. 7B and C) and historical

456

records (Texts S18–S22). Most of the desert was filled with quicksand (Text S20), and the

457

ancient Xiazhou City and vast areas of the desert west of the ancient Xiazhou City suffered

458

from raging sandstorms (Texts S18 and S21). The desert’s hinterland was full of sand seas

459

(Texts S19 and S22). Ubiquitous shifting dunes may have spread over large areas, with water

460

resources from seasonal rivers and lakes becoming even scarcer. Consequently, the desert

461

may have witnessed further fragmentation of surface ecological landscapes during this

462

period.

463 464

According to the travel records of a geographer called Jia Dan (AD 730–805) who lived

465

during the Tang Dynasty, we calculated the average distance (15.9 km) between the oases

466

mentioned within the desert (Text S23). This value is quite consistent with the mean value

467

(16.6 km) of the nearest distances of the desert HASs (Table 2), confirming the rationality of 25

468

applying HAS spatial distribution to characterize ecological landscape fragmentation. Some

469

relatively large scattering oases were occupied by ancient cities such as the ancient Xiazhou

470

City and the five cities of the Liuhuzhou (Fig. 1B). The development of these cities was

471

related to administrative policies of the Tang government to settle nomadic people (Texts S24

472

and S25), and these ecological refuges supported the desert’s HASs, which accounted for a

473

relatively high proportion of the total number of HASs in the MUDISA during the regional

474

desertification. As previously analyzed, the desert’s increasing population during this period,

475

especially high-intensity human activities near the city sites located in oasis zones, may have

476

further promoted the desertification controlled by the dry climate.

477 478

Sporadically scattered on the remaining oases (Texts S25 and S26), agricultural development

479

was greatly restricted. The dominant economy was still animal husbandry (Texts S18 and

480

S20), feeding nomads such as the Tujue, Dangxiang, and Sute. The then desert was also part

481

of an important supply base for horses. Located on the transportation hub of the extended Silk

482

Road, multi-cultural beliefs (e.g., Buddhism, Zoroastrianism, and Shamanism) were

483

accompanied by multi-ethnic immigration and mixed residence. The long-term maintenance

484

and conflict of this cultural diversity can therefore be viewed as an ideological reflection of

485

uninterrupted struggles among different groups for limited living resources and space

486

(D’Odorico et al., 2013).

487 488

3.4. Impact of human activities on the regional eco-environment

489

In this study, human beings primarily affected the regional ecological environment through 26

490

agricultural and animal husbandry activities (Guo et al., 2018; Zhou et al., 2002). High

491

population density and associated high-intensity cultivation and grazing directly reduced

492

surface vegetation (Roskin et al., 2013), and this maintained a living economy and supplied

493

the demands for civil engineering, fuel, and tool making. Fine soil particles in the

494

human-induced exposed fields (e.g., farmland and grassland) that were constantly being

495

disturbed were more easily blown away by wind, leaving only sand and stones (Deng et al.,

496

2001; Sun, 2000). It is noteworthy that agricultural irrigation and the nomadism that

497

propagated and spread widely under favorable conditions largely consumed the already

498

limited water resources. Not only did this process increase the degree of regional

499

desertification but also potentially caused considerable soil organic matter to be leached out

500

and washed away, given that most places attracting these anthropogenic activities were

501

located in relatively fertile areas.

502 503

The impact of the abovementioned anthropogenic factors cannot be underestimated for the

504

desertification of the Mu Us Desert in this study. This view is not only consistent with other

505

studies focusing on this desert’s historical desertification (Guo et al., 2018; Li et al., 2019;

506

Sun, 2000; Xiao et al., 2002; Zhou et al., 2002) but also supported by research on modern

507

desertification involving this (Liang and Yang, 2016; Xu et al., 2009; Zhou et al., 2015) and

508

other deserts worldwide (D’Odorico et al., 2013; Kouba et al., 2018; Lamqadem et al., 2018;

509

Varghese and Singh, 2016; Zhou et al., 2013). The Net Primary Productivity (NPP) model (Q.

510

Li et al., 2016; Xu et al., 2009; Xu et al., 2011; Zhou et al., 2015) can quantitatively assess

511

the relative roles of climate change and human activities in modern desertification processes. 27

512

The relative contributions of these two factors to desertification can be revealed according to

513

changes in potential NPP and the difference between potential and actual NPP. Using this

514

model, Xu et al. (2009) revealed that the relative contributions of climate change and human

515

activities in the desertification of the Ordos area from 1981 to 2000 were 54.87% and 45.13%,

516

respectively. Similar studies were conducted on the modern desertification of northwest

517

China (Zhou et al., 2015) and the Qinghai-Tibet Plateau (Q. Li et al., 2016), with

518

anthropogenic contributions accounting for 70.3% and 58.6%, respectively.

519 520

During the Qin and Han dynasties, the large number of settlements and the rise and fall of

521

over-farming triggered by political and/or military factor(s) potentially disturbed the desert’s

522

already fragile ecosystem, leading to land degradation (Text S10). From AD 220 to 907, the

523

large-scale human population and the presence of extensive livestock contributed to the

524

desert’s ecological deterioration and desertification. After all, compared with agriculture, the

525

animal husbandry activities were spread over a much wider area within the desert, not just

526

near the HASs (section “2.2. Human archaeological sites”). As a pivotal center of politics,

527

military, and economy in the MUDISA, the ancient Tongwan/Xiazhou City and its

528

surrounding area had maintained a population of tens of thousands (Fig. 4); the intensified

529

human activity (mainly grazing and cultivation) (Texts S17 and S26; Deng et al., 2001) may

530

have profoundly affected the area’s sandy landscape evolution up to modern times (Fig. 3).

531 532

However, we emphasize that natural conditions represented by the climate change and

533

geographical environment were the dominant factors for the historical desertification in our 28

534

study; this explains why the desertification occurred during the era of disunity and the Sui

535

and Tang dynasties, while the numbers for both the HASs and populations were generally

536

lower than during the Qin and Han dynasties. This view is partly supported by other scholars.

537

Li et al. (2019), for example, revealed that a cold and dry climate led to strong dune activity

538

in the Mu Us Desert from AD 500 to AD 600; Liu and Lai (2012) found a paleo-lake’s

539

ultimate desiccation between 1.8 and 1.0 thousand years before AD 1950 in the southern Mu

540

Us Desert, which may indicate the desert’s obvious desertification then; and Wang et al.

541

(2005) confirmed that desertification during the past several decades were mainly related to

542

climatic fluctuations. When coupled with natural factors, anthropogenic activities

543

significantly contributed to historical desertification; this contribution rate could be amplified

544

under dry climate conditions. We call this mechanism the “human superimposed effect,”

545

which has also been confirmed by modern studies (Huang et al., 2016; Huang et al., 2017;

546

J.C. Li et al., 2016; Liu, 2009; Villagra et al., 2009). This kind of eco-environmental

547

exacerbation around ancient cities and/or in oasis areas with high demographic pressure was

548

a crucial way in which human beings may have promoted regional desertification (Texts S11,

549

S13, S15, S23, S25, and S26; Shen et al., 2018; Zhang et al., 2017).

550 551

4. Conclusions

552

We revealed the holistic and continuous process of human–nature interaction and

553

corresponding driving mechanisms for desertification on a regional scale during a 1128-year

554

historical period. The MUDISA experienced large-scale immigration and agricultural

555

reclamation implemented by the centralized governments during the ecologically favorable 29

556

period of the Qin and Han dynasties. However, pressures from large-scale HASs, population,

557

and cultivation potentially disturbed the eco-environmental equilibrium that may have been

558

closely related to subsequent land degradation within the Mu Us Desert. Coinciding with the

559

deteriorating natural environment from AD 220 to 907, the MUDISA was dominated by

560

animal husbandry, with the number of HASs sharply reduced and scattered in the desert.

561

Coupled with the sandy strata and an increasingly dry climate (Fig. 7A), both the increasingly

562

large-scale human population and extensive livestock presence accelerated the climate-driven

563

desertification and ecological landscape fragmentation. Although the historical desertification

564

was mainly driven by natural conditions, the human superimposed effect explained above

565

could promote desertification on a regional scale, especially by exacerbating local

566

eco-environment weaknesses in densely populated areas located around ancient cities and/or

567

in oases.

568 569

This multi-disciplinary research circumvents the locality and low temporal resolution of

570

single stratigraphic section data, as well as probable inaccurate interpretation of some

571

historical records and/or some ancient city distribution. Our approach integrates quantitative

572

analysis and qualitative description, and may shed new light on regional studies of historical

573

environmental changes in arid and/or semi-arid areas of northwest China and globally.

574

Regarding the current construction of the ecological environment and sustainable

575

development in the Mu Us Desert, we should fully respect the laws of natural environment

576

evolution while protecting local vegetation and water resources. More importantly, the scope

577

and scale of farming and grazing in the desert must be strictly limited to within reasonable 30

578

thresholds to avoid potential land degradation during dry years.

579 580

Acknowledgements

581

This research was financially supported by the National Natural Science Foundation of China

582

(Grant Nos. 41330748 and 41230634). We thank the Ordos Municipal Administration of

583

Cultural Heritage (Inner Mongolia) and Yanchi Municipal Administration of Cultural

584

Heritage (Ningxia) for providing access to the latest archaeological discoveries, as well as the

585

Loess Plateau Data Center, National Earth System Science Data Sharing Infrastructure,

586

National Science & Technology Infrastructure of China (http://loess.geodata.cn) for sharing

587

basic geographic data. Sincere thanks are extended to the three anonymous reviewers and to

588

Dr. Ralf Ludwig (the editor) for their kindness and helpful comments to improve the

589

manuscript.

590 591

Appendix A. Supplementary data and materials

592

Supplementary data and materials to this article, including Supporting Texts S1–S27 and

593

Tables S1–S3, can be found online at ------.

594 595 596 597 598 599 600 601 602 603 604

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796

797

36

798

37

799

38

800

801 39

802

803 804

Table 1

805

Statistics for human archaeological sites (three types) in three historical periods in the Mu Us 40

806

Desert and its surrounding area (MUDISA), as well as in the Mu Us Desert. Archaeological site MUDISA General settlement site City site Burial area site Total (N)

Qin and Han n1 n1/N (%) 454 57.5 41 5.2 295 37.3 790 100

era of disunity n2 n2/N (%) 9 27.3 5 15.1 19 57.6 33 100

Sui and Tang n3 n3/N (%) 11 18.7 16 27.1 32 54.2 59 100

Mu Us Desert General settlement site City site Burial area site Total (N)

n1 139 11 81 231

n2 5 2 10 17

n3 3 9 20 32

n1/N (%) 60.2 4.7 35.1 100

n2/N (%) 29.4 11.8 58.8 100

n3/N (%) 9.4 28.1 62.5 100

807 808 809

Table 2

810

Nearest distance statistics between human archaeological sites for three historical periods in

811

the Mu Us Desert and its surrounding area (MUDISA), and in the Mu Us Desert. Distance

812

unit: km; SD: standard deviation. Historical period Qin and Han era of disunity Sui and Tang

N 790 33 59

MUDISA Range Mean±SD 0.2–33.5 3.9±4.1 0.7–64.2 18.6±19.9 0.5–65.0 13.2±12.3

n 231 17 32

Mu Us Desert n/N (%) Range 29.2 0.3–36.8 51.5 0.7–53.7 54.2 0.5–77.7

Mean±SD 4.7±5.1 9.8±14.2 16.6±18.8

813 814 815

Table 3

816

Nearest distance statistics for human archaeological sites (HASs) during three historical

817

periods in the eastern valley plain (EVP) and the Mu Us Desert to the eastern historical rivers

818

in the Mu Us Desert and its surrounding area (MUDISA). Distance unit: km; the EVP

819

consists of the counties of Shenmu, Yulin, Hengshan, and Jiaxian (northern part, Fig. 1B); SD: 41

820

standard deviation; n1, n2, and n3 represent the numbers of relevant HASs with the nearest

821

distances of no more than 1 km, 5 km, and 10 km to the rivers, respectively. Region and period Qin and Han EVP era of disunity Sui and Tang Mu Us Qin and Han Desert era of disunity Sui and Tang

N 238 9 16 231 17 32

Range <0.1–23.8 0.1–22.5 0.1–22.5 <0.1–83.8 0.3–30.7 0.2–80.4

Mean±SD 5.5±5.1 7.0±8.1 5.1±6.4 19.6±22.8 9.9±10.2 17.9±23.3

n1/N (%) 22.3 44.4 31.3 10.8 23.5 12.5

n2/N (%) 54.2 55.6 68.8 33.8 41.2 34.4

822 823 824

Graphical Abstract:

825 826 827

Highlights:

828

• We revealed human–nature interactions during a 1128-year historical period.

829

• Qin and Han: farming boom in an ecologically favorable environment.

830

• Era of disunity: HAS exodus and grazing increase in regional desertification.

831

• Sui and Tang: increasing population and livestock with continuing land degradation.

832

• Human activities accelerated climate-driven desertification in the Mu Us Desert.

833

42

n3/N (%) 83.6 66.7 81.3 54.5 70.6 56.3