ARTICLE IN PRESS Habitat International 32 (2008) 471–484 www.elsevier.com/locate/habitatint Urban population densities and their policy implications in China Minghong Tana,, Xiubin Lia, Changhe Lua, Wei Luob, Xiangbin Kongc, Suhua Mad a Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences (CAS), Beijing 100101, PR China b Research Center for Eco-Environmental Sciences, CAS, Beijing 10085, PR China c College of Resources and Environment, China Agricultural University, Beijing 10094, PR China d China Land Surveying and Planning Institute, Beijing 100035, PR China Abstract In China, some people believe that urban land has a large potential for absorbing more of the urban population, while others think that urban population density is very high, and has already caused many urban problems. The population density of 135 major cities is studied by using the city population data (Shiren kou) from the Fifth Census of China in 2000. The data included the floating population who lived in cities for more than 6 months/year, so it could more closely reflect the real size of the urban population. Land-use data were obtained from a digital map interpreted from remotely sensed data collected in 2000. The results show that urban population density was fairly high in China and the average urban land per capita of these cities was only about 76 m2 in 2000, but that the corresponding value was 106 m2 if it was calculated with the non-agricultural population. Moreover, urban population density varied greatly between cities: from 4 103 to 22 103/km2. Regression results show the differences in urban population density were strongly related to six independent variables, including wage per capita, city size and shape index of urban land, etc. The policies derived from the results deserve more attention in the new land-use planning with the target year of 2020 in China. r 2008 Elsevier Ltd. All rights reserved. Keywords: China; Urban population density; Natural and socio-economic parameters; Policy implication Introduction It is not so easy to define urban population density, because of the definition of urban population is vague and the urban area boundary is unclear. However, urban population density continues to inspire researchers working in the fields of urban planning, geography and urban/regional economics (Kasanko et al., 2005), because it is closely related to land use, urban traffic, environment and living quality, etc. (Camagni, Gibelli, & Rigamonti, 2002; Li, Sato, & Zhu, 2003). Corresponding author. Tel.: +86 10 64889450 802; fax: +86 10 64854230. E-mail addresses: [email protected] (M. Tan), [email protected] (X. Li), [email protected] (C. Lu), [email protected] (W. Luo), [email protected] (X. Kong), [email protected] (S. Ma). 0197-3975/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.habitatint.2008.01.003 ARTICLE IN PRESS 472 M. Tan et al. / Habitat International 32 (2008) 471–484 In different countries/regions, there are large variations in urban population density. Generally speaking, urban densities in American and European countries are lower compared with densely populated cities in Asia, such as Hong Kong, Singapore, Tokyo, Seoul and Shanghai. In the Western context, high-density development is often considered to be over-crowded and cause excessive congestion, with potentially detrimental effects on society (Chan, Tang, & Wong, 2002; Oh et al., 2005; Williams, 1999), although more and more researchers are noticing the negative impacts of low-density development and believe that there are better modes of urban growth in the future, such as compact growth, growth management, etc. (Alig, Kline, & Lichtenstein, 2004; Frenkel, 2004; Nelson, 1999). On the contrary, in Asian countries, more people believe that dense and intense land use could be a solution used to satisfy rapidly increasing land demand for urban activities (Chen & Jia, 2005; Lau, Giridharan, & Ganesan, 2005). Urban population density has become one of the important concerns of policy makers and researchers (Fu, 2001), because China is a country with a huge population size of 1.3 billion and limited land resources (National Bureau of Statistics of China (NBSC), 2005). Furthermore, as one of the most dynamic countries, China’s sustained economic development and massive rural–urban migration have increased urban land demand. In the 1990s, built-up areas increased by 1.76 million ha, 80% of which was from arable land (Liu, Zhang, & Zhuang, 2003). At the same time, the arable land area per capita in China is only 0.094 ha, less than half of the world’s average (Land and Resources Minister of China, 2005; Lin & Ho, 2003). Hence, the Chinese government strictly controls the conversion of arable land to built-up land. In 1997 and 2004, the central government even stopped approving any application for non-agricultural occupation of arable land across the country. The discrepancy between urban land demand and arable land protection is becoming acute. In China, more intensive urban land use is often suggested as a solution to the discrepancy. One of the reasons is that most researchers prefer to use data on the urban non-agricultural population to calculate urban land per capita since the data are more continuous and steady compared with other urban population data. In the country, the urban non-agricultural population only includes the permanent residents in urban areas, who engage in the secondary and tertiary industries and people they feed (National Bureau of Statistics of China (NBSC), 1991–2001). These data neglect the floating population who actually work and live in urban areas. This leads to considerable distortions of urban population in some cities with a large inflow of migrant workers (Zhou & Yu, 2004). In most cases, urban population is under-estimated. Thus, it is easy to draw the conclusion that urban land has a big potential for carrying an increased urban population in the country. On the contrary, others believe that the urban population density is very high in China, which has caused many problems. Firstly, over-development and over-concentration of urban areas results in various environmental problems, such as air and water pollution, and water shortage (Chen & Lotspeich, 1998). Secondly, a high urban population density leads to serious traffic congestion, especially in large cities, such as Beijing and Shanghai. Thirdly, in part due to the high urban population density, housing prices in cities are continuously increasing. For instance, the average housing price in China rose by 14.4% in 2004 (Liang & Ma, 2004; XinhuaEnglish, 2005). Now, the new land-use planning (Tudiliyong Zongtiguihua Xiubian) is under way in China with the target year of 2020. The planning will continue to place special emphasis on arable land protection through controlling urban land expansion and intensively using existing urban land. In this context, it is necessary to study urban population density with more reliable data, which will be discussed in the Data section. The rest of the paper is organized as follows. In the Method section, a set of simple indicators to interpret urban population density is introduced. Results section examines the differences of population density for 135 selected cities across the country and the factors causing the differences. Discussion section presents several policy implications arising from the results in the Results section. Finally, short conclusions are offered in the Conclusions section. We hope that this study will help planners to strike a better balance between arable land protection and urban land growth, and allow more reasonable planning for future land use in China. Data Urban population data In China, there are three levels of cities, namely municipalities, prefecture-level and county-level cities. The selected 135 cities in this paper are all above (and not including) the county level (Fig. 1). In the country, these ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 473 Fig. 1. One hundred and thirty-five cities and their population densities (1000 persons/km2) in China in 2000. Note: The black line goes along Huai River and Qinling Mountanin, which divide China into two regions, namely, South China and North China. cities often administer a large area, comprising city proper, extensive exurban districts and rural counties (Fig. 2). Population in the administration area of a city was divided into three parts in the Fifth National Census of China in 2000: urban population, town population and rural population (National Bureau of Statistics of China (NBSC), 2002). Fig. 2 shows the population composition of municipalities or prefecture-level cities in China. In the Census, the city population is defined as the urban population in the city proper and exurban districts, and does not include the town population or rural population (Shi Renkou). In addition, in the Census, floating people were recorded at their current abode rather than at their place of registration if they lived at the abode for more than 6 months/year. So, the city population, including some of the floating population, reflects more closely the real size of the urban population, when compared with other population data in China (Zhou & Yu, 2002). The city population data are used to study urban population density in this paper. Urban land-use data The basic urban land-use data were extracted from the 1:100,000 digital land-use map of 2000 which was obtained from the Landsat Thematic Mapper images. These data are provided by the Resources and Environment Data Center, CAS. The field survey and random sample check (covering a survey line of 70,000 km and 13,300 patches) testified that the average interpretation accuracy for land-use/land-cover was better than 92% (Liu, Liu, & Zhuang, 2003). The data have been successfully employed to study land-use changes in China (Liu, Liu et al., 2003). The original map consists of six classes of land-use types, i.e., arable land, forest, grassland, residential areas and land for stand-alone industrial and mining sites, water, and unused land. Here, urban land, one of the subclasses of residential areas and land for stand-alone industrial and mining sites, is selected to study urban population density. Urban land includes urban built-up areas in urban districts (city proper, and exurban districts) in this study. For example, in Beijing in 2000, urban land included urban built-up areas (not including built-up areas for rural residents) in the city proper and exurban districts of Shunyi, Changping, Fangshan and Tongzhou (Fig. 3). In the city proper, all population who live here for more than 6 months/year, belongs to the city population, so it is easy to extract the urban built-up area. In an exurban district, only the members of the population, who live in the central area where the district government is located, belong to the city population. A city often has several exurban districts. As a result, it is possible to miss urban built-up areas in some exurban districts. In order to ensure that no exurban districts are missed, we have examined each exurban district for all the cities selected in this paper according to administrative maps at different levels. Thus, the spatial scope of the city population is consistent with that of urban land. ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 474 Rural population, town population Counties Exurban districts City proper Urban population, town population, rural population Urban population Fig. 2. Division of administrative areas of municipalities or prefecture-level cities in China. Note: The population in a county includes town population and rural population, and that in exurban district includes urban population, town population and rural population. In a city proper, there is only urban population. According to the Fifth National Census, city population includes two parts, that is, urban population in city proper and exurban districts (National Bureau of Statistics of China (NBSC), 2002). Fig. 3. Urban built-up areas and administrative division of Beijing in 2000. Note: Urban land in this paper refers to urban built-up areas in the city proper and exurban districts. In addition, socio-economic data are sourced from Urban Statistical Yearbook of China (National Bureau of Statistics of China (NBSC), 1991–2001). Their meanings are shown in Table 1. Method Urban population density is influenced by various factors: natural resources endowments, economies of scale in production and consumption, usage of cars, etc. (Alig et al., 2004; Fujita, 1989; Kasanko et al., 2005; Oh et al., 2005). Through studying 15 European urban areas, Kasanko et al. (2005) present a more comprehensive understanding for the differences of population density and think that geographical surroundings, historical onset of the urbanization process, policy factors etc., lead to the differences of urban population density among different parts of Europe, namely, compact cities in southern Europe, cities with looser structures in eastern Europe, and central and western cities midway between the extremes. ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 475 Table 1 The independent variables and their meanings Variable group Variable Code of variable Ncityi Cities in North China Geographical surroundings Regional dummy variables Dummy variables of city types PLcityi MFcityi Cities on the plains Cities at the foot of mountains Urban shape Shape index of urban land Shapei City size Salary level Sizei Wageij 1000 persons Yuan/person Economic structure Investment in fixed assets Sgdp2i Percent (%) Fixij Yuan/km2 Arable land per capita Arablei ha/person Economic factors Social factors Unit Meaning City size of city i in terms of city population Average wage of fully employed staff and workers of city i in year j Share of secondary industry in GDP of city i Investment in fixed assets per km2 of city i in year j Arable land per capita in the scope of urban districts of city i Note: GDP means gross domestic product. Urban population density Geographical surroundings Urban shape Economic factors Social factors Fig. 4. Factors affecting urban population density. However, it is unfortunate that, to date, only a few researchers have established a set of indicators to interpret differences of urban population density. In this study, a simply explanatory framework is established with a set of indicators in order to explain the differences of urban population density among the selected cities (Fig. 4 and Table 1). Selection of independent variables mainly depends on the following principles in this study. Firstly, data should be available. Secondly, effects of an independent variable on urban population density should be easily measured. For example, some social factors, such as historical tradition and cultural background, may affect urban population density, but the effects are difficult to estimate. Thirdly, serious multicollinearity problems should be avoided among independent variables. Selection of independent variables will be discussed in depth as follows. Geographical surroundings Geographical surroundings can largely express the characteristics of the natural conditions and socioeconomic development. This paper will consider the effects of geographical surroundings on urban population density from two aspects: regional location and landforms around a city. Firstly, in China, regional features vary greatly due to an area of 9.6 million km2. To express the regional differences, mainland China is traditionally either divided into three large regions: the East, the Middle and the West, or divided into two: the North and the South, by the geographic boundary of the Huai River and Qinling Mountains. Since Fig. 1 clearly shows the difference in urban population density between the ARTICLE IN PRESS 476 M. Tan et al. / Habitat International 32 (2008) 471–484 Fig. 5. Landform and spatial distribution of 135 cities in China. Note: Plain city refers to cities on the plains, and Foot and Mountain cities are those at the foot of mountains and in mountain areas, respectively. North and the South, the regional dummy variable of the northern cities (Ncityi) is set here. Ncityi is a binary variable. If a city is in North China, its value equals one, and zero otherwise. Secondly, landforms around a city may affect its population density. According to the landform characteristics around cities, they are divided into three groups: cities on the plains (PLcityi), cities at the foot of mountains (MFcityi) and cities in mountain areas. This division is realized by overlaying China’s landform map and urban map (Fig. 5). Thus, two dummy variables of PLcityi and MFcityi are set and they are also binary variables. Urban shape The shape index of the landscape has been used in many studies (Chust, Ducrot, & Pretus, 2004). Given that the urban land of a city is often composed of many land patches, the shape index of city i (Indexi) is defined by the following formula: Pn j¼1 Pij q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Indexi ¼ , Pn 2 p j¼1 Aij where Pij and Aij are the perimeter and area of land patch j of city i, respectively. Indexi ¼ 1 when the urban land of a city is composed of a single circular patch. With the number of patches increasing or as the patch shape becomes more irregular, the value of Indexi will be higher. A higher value means a longer perimeter per unit area of urban land. A longer perimeter means a longer rural–urban interface, often characterized by low buildings, less building form and more natural surroundings. So, a city with a higher value of Indexi may be less compact. Economic factors For a city, economic factors mainly include city size, economic development level and economic structure. Firstly, in urban areas, many firms are located close together in order to reduce transport costs and increase the opportunities for face-to-face communication with other firms and customers (Fujita, 1989). Hence, the urban population density may be higher because more human activities bid for scarce land resources in a ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 477 bigger city. For example, after examining 88 US cities in 1960, Hoch (1972) found that urban density increases with city size. Secondly, urban economic development level can be mirrored by income per capita, GDP per capita, etc. Because, many studies prove that land consumption per capita grows with an increase of income (Alig et al., 2004; Camagni et al., 2002; Zhang, 2001), wage per capita is used to interpret urban population density in this study. However, wage per capita cannot affect urban land demand immediately. For instance, an urban family needs some time to save up to buy a house. Hence, the sum of average wage per capita of fully employed P P staff and workers in the last 5 years (1996–2000) ( Wageij) is used as an independent variable (Table 1). Wageij is calculated at the constant price of 2000. Economic structure may have some effects on urban population density. Secondary industry, in particular heavy industry, may need more urban land and less labor relative to tertiary industry, because manufacturing processes of the former are capital intensive, and its input and output requires significant amounts of land (Goldberg & Chinloy, 1984). Since there is an obvious collinearity relationship between shares of secondary and tertiary industries in GDP, only the former (the variable of Sgdp2i) is employed as one of the independent variables (Table 1). In addition, higher investment in fixed assets per km2 (Fixij) can improve building conditions and even offset some of the disadvantages of the natural environment, for instance, construction of transport infrastructure and drainage. Moreover, by building some industrial and service establishments, higher investment can create more social and economic activities, which will present more job opportunities. Hence, Fixij is selected as one of the independent variables. Because it is a stockP variable, the sum of investment in fixed assets during the last P 5 years (1996–2000) ( Fixij) is employed here. Fixij is calculated at the constant price of 2000. Social factors It is very difficult to estimate the effects of social factors on urban population density, because social group covers a wide range of factors, including cultural background, historic tradition, policy, land ownership and social fairness, etc. (Ding, 2007). Of all the social factors, the policy factor may play an important role. For example, adopting a growth-management policy could contribute to the prevention of urban sprawl and the preservation of open space, which can effectively control the urban de-concentration trend (Alig et al., 2004; Frenkel, 2004; Nelson, 1999). In China, of all the policies related to urban population density, the policy of dynamic balance of arable land plays a central role. It emphasizes that the government at provincial level must ensure that arable land quantity is in a state of dynamic balance in the jurisdictions (Yang & Li, 2000). This means that governments in some regions with less arable land resources are forced to adopt a more compact urban land-use policy. Here, arable land per capita in 1995 is used as an independent variable (Rural Survey Organization of National Bureau of Statistics of China, 1996), for two reasons. First, for 135 cities, only the arable land data for 1995 and 1999 can be obtained. Here, the arable land of a city only refers to that in its urban districts (including the city proper and exurban districts) (Fig. 2). Second, impacts of arable land per capita on urban population density cannot be reflected immediately. Finally, as linearity is necessary in the multiple regression analysis, this paper uses logarithms of the dependant and independent variables. Since Ncityi, PLcityi and MFcityi are binary variables, which equal one or zero, they are not transformed into logarithms. Results The results show that urban land per capita of the 135 cities was about 76 m2 in 2000. However, the corresponding value would approach 106 m2 if it was calculated with the non-agricultural population data. The latter is about 47% higher than the former, especially in large cities with city populations more than 5 million (Fig. 6). ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 478 Urban land per capita (m2/person) 164 144 124 104 84 64 Land 1 Land 2 44 24 4 >5 million 1-5 million 0.5-1 million City classification <0.5 million Urban land per capita (m2/person) Fig. 6. Urban lands per capita of four classifications of cities in China in 2000. Note: In this figure, 135 cities are divided into four classifications according to city population and non-agricultural population, respectively. Lands 1 and 2 are urban lands per capita calculated with city population and non-agricultural population, respectively. Urban land data are from the Resources and Environment Data Center of CAS. Land 1 120 Land 2 110 100 90 80 70 60 50 40 Cities in North China Cities in South China Average City classification Fig. 7. Urban lands per capita of two types of cities in China in 2000. Note: Lands 1 and 2 are urban lands per capita calculated with city population and non-agricultural population, respectively. Urban land data are the same as Fig. 6. Urban population density varied from 4 103/km2 in Kalamyi city in Xinjiang Autonomous Region to more than 20 103/km2 in Fuzhou city in Fujian Province in 2000 (Fig. 1). Of all the 135 cities, 120 had urban population densities exceeding 10 103/km2. In North China, urban land per capita was 84 m2, much higher than 69 m2 in South China (Figs. 1 and 7). 7 of the total 8 cities with an urban population density higher than 20,000 persons/km2 were situated in the Yangtze River Delta in South China, such as Hangzhou, and Ningbo (Fig. 1). However, it is notable that the urban population density in South China would be lower than that in North China if it was calculated with the non-agricultural population, and urban land areas per capita in South China and North China were 110 and 102 m2 in 2000, respectively (Fig. 7). To shed light on the differences in urban population density, the variables in Table 1 are entered as a block in a single step in the linear regression model of the SPSS software package. Table 2 lists the results. Overall, our results are consistent with the discussion in the Method section above. Of all the independent variables, six of them exhibited close relationships with the dependent variable at the 5% statistically significant level, ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 479 Table 2 Variable coefficients in the linear regression model Model Unstandardized coefficients Standardized coefficients B Std. error Beta t Sig. Collinearity statistics Tolerance VIF Geographical surroundings (Constant) Ncityi PLcityi MFcityi 7.735 0.298 0.097 0.050 0.848 0.044 0.045 0.044 0.489 0.143 0.077 9.116 6.852 2.166 1.153 0.000 0.000 0.032 0.251 0.643 0.748 0.737 1.554 1.337 1.356 Urban shape Shapei 0.157 0.050 0.239 3.157 0.002 0.570 1.753 Economic factors Sizei P Wageij P Fixij Sgdp2i 0.181 0.748 0.272 0.162 0.027 0.095 0.039 0.093 0.513 0.632 0.492 0.110 6.648 7.865 7.040 1.743 0.000 0.000 0.000 0.084 0.551 0.506 0.672 0.819 1.816 1.975 1.489 1.221 Social factors Arablei 0.034 0.024 0.092 1.449 0.150 0.809 1.236 Note: Dependent variable is urban population density. And the adjusted R2 is 0.560. P including Salaryij, Sizei, Fixi, Ncityi, Shapei and PLcityi. Among them, the variables of Sizei and Fixij had positive relationships with urban population density, while the remaining four variables all exhibited negative relationships with the dependent variable (Table 2). The relationship between Sgdp2i and urban population density was not as apparent as we expected. One of the important reasons is that most cities in China are in the process of industrialization, and their shares of secondary industry in the GDP were very high. The shares of 129 of 135 cities were higher than 40% in 2000, which weakened the explanatory power of the differences of urban population density between cities. Similarly, because of the implementation of the strict farmland protection policy across the country, the variable of Arablei had a very weak relationship with urban population density. MFcityi had no obvious relationship with urban population density, which meant the urban population density of cities in mountain areas was approximately equal to that of cities at the foot of mountains (Table 2). In the model, the adjusted R square is 0.560. Collinearity statistics results show that no obvious multicollinearity exists among independent variables since the values of variance inflation factor (VIF) are all lower than two (Table 2). In general, if the VIF value is more than five, there will be significant multicollinearity between independent variables (Zhu, 2004). Discussion Compared with the western countries (Demograhia, 2005; Kasanko et al., 2005; Kline, 2000; Nelson, 1999), the urban population density in China is very high. Population densities of some cities selected in this study are even higher than those of other densely populated countries, such as Japan, Israel and India (Demograhia, 2005; Frenkel, 2004). Of the 20 urban areas with the highest population density in the world, eight are located in mainland China (Demograhia, 2005). In addition, Hong Kong (China HKSAR) and Taipei (ChinaTaiwan) rank in first and fifth place, respectively. In the future, China’s government will continue to implement the policy of strict farmland protection and control urban land expansion. In order to mitigate the discrepancy between urban land growth and arable land protection, the following countermeasures are put forward based on the results of this study. To more exactly assess urban land demand It is strongly necessary to assess urban land demand, in order to strike a balance between urban land expansion and arable land protection. At present, two aspects deserve more attention. ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 480 10000 RMB (Yuan) 8000 6000 4000 2003 2001 1999 1997 1995 1993 1991 1989 1987 0 1985 2000 Year Fig. 8. Changes of per capita annual income of urban households, 1985–2004. Note: Data are from (National Bureau of Statistics of China (NBSC), 2005), and income per capita is computed at constant price for 2004. RMB (Yuan) is China’s money (now about 7.5 Yuan/ US dollar). Firstly, China is a developing country with a low urbanization rate (41.2% in 2004) (National Bureau of Statistics of China (NBSC), 2005), so many factors causing urban agglomeration still work. As a result, the urban population of nearly all cities is increasing, because of massive migration into cities from rural areas. With the step-by-step cancellation of the ‘‘urban household registration system’’, more floating people will live in the urban areas and enjoy the same urban welfare and equal job opportunities as urban residents, which can cause urban land growth. Secondly, urban land-use planning is often based on the assumption that urban land demand per capita is constant over time in the country. However, the regression model shows that wage per capita had a negative relationship with urban population density. In China, per capita annual income of urban households is increasing rapidly. From 1985 to 2004, it increased by 339% (Fig. 8). With income increasing, land consumption per capita may increase (Mak et al., 2007). In China, economic growth and massive migration from the rural population are exacting an unprecedented demand on urban land, so it is important to assess urban land demand for both maintaining urban economy growth at a stable and relatively fast rate, and for avoiding excessive expansion of urban land and enclosure of land. To establish discrete criteria for quantifying urban land per capita for different types of cities China has 660 cities, with diverse characteristics of urban development. So, it is necessary to establish discrete criteria for quantifying the urban land per capita for different types of cities. Based on the results of this study, several types of cities should be taken into account. Firstly, South China has a higher urban population density than North China, due to better natural conditions in South China, such as warmer weather, and richer water resources. But, in many studies, urban population densities are often seriously underestimated in some cities of South China. So, in South China, we should appropriately increase land supply to meet urban land demand in order to maintain sustainable urban growth. Secondly, city size had a positive relationship with urban population density (Table 2). The standardized coefficient was 0.513. In fact, this relationship has already been mentioned in the planning criteria of urban land per capita promulgated by the Minister of Construction of China in 1991. However, the criteria emphasize calculation of urban population density with the non-agricultural population. This hides the real relationship between urban population density and city size, because the non-agricultural population seriously underestimates city size in China, especially in large cities with city populations over 5 million (Fig. 6). ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 481 Thirdly, in the regression model, cities in plain areas were negatively related to urban population density. This implies that urban population densities of these cities were lower than those of cities at the foot of mountains. The main reason may be that cities in plain areas can expand in every direction with fewer natural barriers. Owing to the importance of the plain in grain production in China, a more strict policy should be implemented to control urban land expansion of cities in plain areas. To efficiently use urban investment in fixed assets P In the regression model, the variable Fixij had a positive relationship with urban population density. In China, urban investment in fixed assets per km2 varies greatly from region to region. From 1996 to 2000, cities with the sum of urban investment in fixed assets more than 30,000 Yuan/km2 were mainly located in east China, especially in the provinces of Hebei, Jiangsu, Zhejiang and Guagndong. Cities with values less than 20,000 Yuan/km2, were mostly in Northeast China and West China, including Liaoning, Shanxi, Inner Mongolia and Guangxi (Figs. 9 and 10). Most of these belong to mining cities, such as the cities of Liaoyang, Fushun, and Anshan in Liaoning province, Panzhihua city in Sichuan Province, and Hegang city in Heilongjiang province. Now most of the mining cities are facing resources exhaustion, and construction conditions are destroyed due to mining activities. Thus, these cities develop very slowly and cannot attract more migrants. So, the cities need more urban investment for urban development. By contrast, in some cities in developed regions, urban investment in fixed assets still increases quickly although the amount is already very large. It is known that unscientific investment can cause serious economic loss and even has negative effects on urban development and urban environment. For instance, in Hangzhou in Zhejiang Province, a 22-story building on the bank of the famous West Lake was pulled down in 2007. The building served a mere 13 years, although it cost 20 million Yuan (about US$2.7 million) and was designed to last 100 years (Wu, 2007). Actually, in China, many high buildings and some basic infrastructures in developed regions suffer similar fates, due to random decision making. So, how to efficiently use urban investment is very important for China’s urban development, since high urban investment in fixed assets cannot only improve the urban environment, but also help transform the economic structure and create job opportunities to attract the urban population, especially for mining cities. To properly increase population density in cities with high shape index The regression model shows that the shape index of urban land had a negative relationship with urban population density. In a city with a high shape index, its infrastructure cannot be efficiently used due to its looser urban land-use structure. For instance, in Zibo city in Shandong Province, the shape index of urban land was high and close to 6.61 in 2000. In this city, it is difficult for urban residents who live in Place A to use Fig. 9. Urban investment in fixed assets (Yuan/km2) of 135 major cities in China from 1996 to 2000. ARTICLE IN PRESS 482 M. Tan et al. / Habitat International 32 (2008) 471–484 Fig. 10. China’s administration map at provincial level. Fig. 11. Urban land-use map of Zibo city in Shandong Province in 2000. the public infrastructure in Place B (Fig. 11). Hence, in Zibo, the population density was only 7.37 103 people/km2 (135 m2 person) although it was a large city with a 1.76 million city population in 2000. In some cities composed of several separate sub-centers, the urban population scale of each center is not very large, which can mitigate urban problems (e.g., traffic over-crowding) to some extent. So we should properly increase urban population density of these cities by improving the public infrastructures. In the world, there are many cities with high shape index, which have very high urban population densities, such as Singapore, and Hong Kong (Liang, 1973). Conclusions In China, the urban population density is often underestimated, because it is calculated with the nonagricultural population, which does not include the massive migration from rural areas. As a result, most ARTICLE IN PRESS M. Tan et al. / Habitat International 32 (2008) 471–484 483 people believe that urban land has a big potential for burdening urban population and that it is necessary to strictly control urban land growth in order to protect arable land. However, if taking into account the floating population, population density in most cities is very high. In 2000, the average urban land per capita was only 76 m2. Moreover, urban land per capita varied greatly among these cities. Now, rapid economic growth and massive rural–urban migration are exacting unprecedented demands on urban land. It is very important to assess urban land demand for different types of cities with reliable data. To analyze the differences in urban population density between cities, a simple explanatory framework is established. However, a city is a very complicated system, and its formation and growth are affected by many factors. Due to lack of data, some explanatory variables were not considered in the framework. 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