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Article

Growth Motivation of Urban Agglomerations in Multiscale Spatial Structures from the Perspective of Synergy Theory

by
Lufeng Wu
1,
Yao Huang
2,3,* and
Qian Cheng
1
1
School of Public Affairs, Nanjing University of Science and Technology, Nanjing 210094, China
2
College of Management Science and Engineering, Dongbei University of Finance and Economics, Dalian 116023, China
3
School of Traffic and Transportation Engineering, Changsha University of Science and Technology, Changsha 410114, China
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(14), 6190; https://doi.org/10.3390/su16146190
Submission received: 4 June 2024 / Revised: 3 July 2024 / Accepted: 9 July 2024 / Published: 19 July 2024
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Abstract

:
The sustainability of urban agglomerations is crucial to regional development worldwide, and the growth motivation of multiscale spatial structures is a worthy scientific problem in urban agglomerations. This study takes the urban agglomeration in the Yangtze River Delta as a case study to explore the growth motivation of multiscale spatial structures based on synergy theory. The growth of urban agglomerations mainly involves four stages: central city, urban communities, metropolitan area and urban agglomeration, each experiencing fluctuations in development factors during input, aggregation, diffusion and upgrading. At the same time, the upgrading of spatial synergistic relationships with the growth of urban agglomerations can be categorised into four types: internal, point-to-point, circle-to-circle and multicircle synergies. The theoretical contribution of this study lies in identifying that the upgrading of spatial synergistic relationships and the changes in development factor fluctuations collectively drive the growth motivation for urban agglomerations. These findings will help advance the academic research on spatial structure and urban planning policy in practice.

1. Introduction

Regional economic and social development requires urban agglomerations [1,2,3]. An urban agglomeration is a highly urbanised spatial entity formed by the expansion of a single or multiple central cities and the exchange of material and information flows with surrounding small and medium-sized cities [4]. In contrast to the spatial structure of a single city, the differentiation of spatial functions and the aggregation of development factors in urban agglomerations are formed by the combination of multiple spatial units [5,6] that perform as a functional structure [7,8]. Organisational spatial synergetic and industrial division of labour are important conditions for the high-quality integrated development of urban agglomerations [9]. In comparison with traditional homogeneous competition between cities, the synergistic effect amongst cities, regions and development factors promotes the spatial structure of urban agglomerations [10]. Thus, recognising these synergistic relationships is necessary for the growth of urban agglomerations.
Urban agglomerations are a typical complex systems science problem, and optimisation and enhancement strategies for spatial synergy in such a problem can be achieved using systems science research methods [11,12]. Urban agglomerations are open and complex systems composed of internal subsystems that synergise with each other [13]. These subsystems generate functional differentiation and spatial regional division of labour, which forms a prerequisite for spatial synergy [14]. The differences and imbalances in development factors lead to the hierarchical, centralised and gradient characteristics of spatial structure [15]. The changes in development factors in urban areas drive the growth of new spatial scales and structures [16]. This study attempts to reveal the growth motivation of urban agglomerations, which helps discover the effects of development factors and spatial synergy during the growth process of multiscale spatial structures.

2. Research Progress of Urban Agglomeration

2.1. Growth Motivation of Urban Agglomerations

In his study ‘Megalopolis: The Urbanisation of the Northeastern Seaboard of the United States’, geographer Gottmann (1957) [17] argued that the development direction of urbanisation is towards the formation of megalopolises, with nearby urban areas gradually merging into larger urban entities. Related studies encompass many aspects of urban agglomeration in new statistical concepts [9], the causal relationship of population density and urban growth [14], spatial structures [18], definition of criteria [2], urban sprawling [8], driving factors [19] and superlinear growth [16]. Scholars have adopted qualitative methods in developing concepts [2] and quantitative methods in the growth model [10], linear regression [20], coupled co-ordination degree [11] and indicator evaluation [21] to study the scientific issues of urban agglomerations. Most research has used government statistical data of population, industries and GDP [16] and remotely sensed data [22]. Researchers have always endeavoured to find the scientific laws in spatial structure and the relationship between development factor clustering and urban agglomeration formation.
Given that scholars have begun to focus on the spatial clustering of cities, spatial evolution, development and spatial structure of urban forms are major research themes in urban agglomerations. The growth motivation of multiscale spatial structures, which contributes to the spatial evolution and development of urban agglomerations, has been investigated. Various factors drive the growth of urban expansion, including geographical and ecological aspects and socioeconomic dimensions [23]. Extant works have demonstrated the influences of human activities on urban expansion, which manifest through processes such as supportive policies [19], population, economic factors [24], urban parks, industrial structure, GDP [4], institutional transformations [1], degrees of dynamics [7] and population dynamics. However, urban expansion may grow into a sprawling rather than an ordered spatial structure. Elucidating the drivers operating in synergistic relationships is noteworthy, given that they exert great effects on the pace and evolution of urban expansion towards multiscale spatial structures.

2.2. Synergy Theory in Urban Agglomeration Systems

Bertalanffy (1969) introduced the concept of general systems, which reveals the relationships and logical rules between wholes and parts and elucidates the basic knowledge of the factor composition, structural form and functional attributes of general systems [25]. To develop precise theoretical content in systems theory and effectively solve practical systems problems, Prigogine (1978) proposed dissipative structure theory [26]. This theory suggests that a continuous exchange of matter and energy with the outside world in an open system far from equilibrium leads to nonequilibrium phase transitions under branching points, fluctuations and sudden changes. This driving force leads the system to gradually grow into a spatially and functionally ordered structural form. Moreover, to further explain the laws of system growth and factor structure operations, Haken (1997) found that the key to restoring a new system to equilibrium is the synergistic effect between subsystems [27]. He argued that the continuous upgrading of the synergistic relationship is key to becoming a higher-order structure. He proposed concepts such as order parameters, branching points and fluctuations, which further clarify the operating mechanism of the system towards stability and purpose.
Synergistics can explain the spatial synergy between subsystems in the growth process of urban agglomerations, where various synergistic relationships are key to promoting spatial structural growth [28]. The continuous exchange of population, industry [19,22] and natural resources between urban agglomerations and the outside world and the differences in development factors between subsystems have led to changes in the trajectory of fluctuations. Under the influence of the external environment, the urban agglomeration system may evolve into a higher-level, orderly spatial structure. Specifically, the nonlinear mechanism amplifies the development factors at the critical point as huge fluctuations, which cause sudden changes in the system. New synergistic relationships between subsystems promote a new equilibrium state, and urban agglomerations further extend and expand into a new spatial structure characterised by ‘multilevel, multicentre, multiscale and multifactor’ dimensions. The fluctuations in urban agglomeration systems can be used to examine the spatiotemporal changes in development factors, such as population, GDP and industry [24,29]. Key development factors dominate and change the system state and phase transition as order parameters. The interaction between new order parameter fluctuations and new spatial synergistic relationships promotes the spatial structure to grow into a more orderly and advanced equilibrium state.
The growth of urban agglomerations is a structured and functionalised process of systemic self-organisation. Over time, cities and metropolises collaborate with each other to form a multiscale spatial structure. Moreover, the changes in development factor fluctuations are synchronous with the spatial extension. Therefore, synergy theory is suitable to reveal the growth motivation behind the generation of spatial structures in urban agglomerations. The remainder of this paper is organised as follows. Section 3 introduces the methodology. Section 4 presents a case study of the urban agglomeration in the Yangtze River Delta. Section 5 explores the growth motivations of multiscale spatial structures of urban agglomerations. Section 6 provides policy implications and Section 7 elaborates the conclusions of the study.

3. Methodology

3.1. Case Study Design

Yin (2009) proposed that the case study method is preferred when a ‘what’ or ‘how’ academic question is being raised [30]. The present study aims to develop a general theory to explain a phenomenon prevalent in the study and planning of urban agglomerations across various contexts. On the basis of the abovementioned findings and views related to urban agglomerations and synergy theory, the following research questions are formulated to discuss the issues of the formation of multiscale spatial structure in urban agglomerations:
(1)
How do development factors influence the formation of urban agglomerations?
(2)
How does spatial synergy form the multiscale spatial structure?
(3)
What is the growth motivation for urban agglomerations?
The synergy effect is the driving force for urban expansion to become a multiscale spatial structure. Scale is used to describe the spatial scope, including cities, central cities, metropolitan areas and urban agglomerations. It encompasses the development factors within the scale, which can be used to classify subsystems in urban agglomerations. Given sufficient spatiotemporal data and geographic convenience, the urban agglomeration in the Yangtze River Delta was used as a research case to develop a theory of growth motivation and derive policy implications for the development of the urban agglomeration system (Figure 1).

3.2. Data Source

This study relied on gray literature sources to analyse the changes in development factors and the upgrading of spatial synegy in the urban agglomeration in the Yangtze River Delta. On the basis of these data, this study aims to generate insights into the growth motivation of urban agglomerations. Most of the data were collected from government documents, the Internet and the literature (Table 1).

4. Spatiotemporal Evolution of the Spatial Structure of the Urban Agglomeration in the Yangtze River Delta

4.1. Spatial Structure Form of Urban Agglomerations

The urban agglomeration in the Yangtze River Delta is located in the eastern coastal region of China and consists of three provinces (Jiangsu, Zhejiang and Anhui) and one city (Shanghai). It consists of 26 prefecture-level cities, 104 counties (cities) and thousands of towns. The urban agglomeration in the Yangtze River Delta centres around Shanghai, Nanjing, Hangzhou and Hefei. These cities gradually form a spatial combination of the four major metropolitan areas. In the spatiotemporal evolution of the spatial structure of the urban agglomeration in the Yangtze River Delta, the scale space and development factors have grown synchronously and synergistically, the population and total economic output have continued to increase, and industries have unceasingly gathered and undergone structural upgrading and adjustment [20,31]. The urban agglomeration in the Yangtze River Delta presents a highly urbanised area with economic vitality, an urbanisation process, technological innovation ability and a high level of human habitation. It also has competitive development advantages, such as a developed transportation network, numerous scientific research institutions, a concentration of high-end talents, a complete industrial system and huge economic volume.
The urban agglomeration in the Yangtze River Delta is a typical multiscale spatial structure with certain spatial levels and amplitudes (Table 2). Figure 2 shows the multiscale spatial structure of the urban agglomeration in the Yangtze River Delta and the metropolitan area of Nanjing, with four and three levels of multiscale spatial structure, respectively. Spatial synergy and factor fluctuations motivate the spatial structure. The urban agglomeration in the Yangtze River Delta is the first level of regional space, and its spatial range covers the entire geographical range and development factors of the area. In terms of functional positioning, development factors and technological innovation, the metropolitan area of Nanjing is different from the metropolitan areas in Shanghai, Hangzhou and Hefei, which belong to the second-level regional space, to promote a synergistic relationship amongst circles [32]. The third level of regional spatial range includes four central cities: Shanghai, Nanjing, Hangzhou and Hefei, with Nanjing being the second level of regional space in the metropolitan area of Nanjing. Correspondingly, small and medium-sized cities, such as Zhenjiang, Yangzhou, Huai’an, Ma’anshan, Chuzhou, Wuhu and Xuancheng, are the fourth-level regional space of the urban agglomeration in the Yangtze River Delta and the third-level regional space of the metropolitan area of Nanjing. The spatial structure of the urban agglomeration in the Yangtze River Delta has a certain level of elasticity and growth potential. When the fluctuation amplitude of development factors and control parameters changes, the regional space can generate new structural levels.
The same scale space of a multiscale spatial structure is homogeneous, whereas different scale spaces are heterogeneous. The metropolitan areas of Nanjing and Shanghai share the same spatial structure and scale characteristics. They are not dependent on changes in spatial geographic scope and spatial function, demonstrating homogeneity. The spatial structure and scale spatial characteristics of Nanjing and small and medium-sized cities are different, especially in terms of spatial functions, indicating heterogeneity. From the perspective of network structure characteristics, nodes and relationships in the same scale space are the same but different from those in different scale spaces [33]. The basic development factors are evenly distributed throughout the entire scale space, reflecting the homogeneity of the same scale space. The characteristic development factors lie in specific scale spaces, suggesting the heterogeneity of different scale spaces. For example, small and medium-sized cities must develop characteristic industries [34,35] while also having certain basic industries to form a synergistic relationship with other cities.

4.2. Characteristics of a Multiscale Spatial Structure Stage

The growth stage of an urban agglomeration spatial structure has two dimensions: spatial and temporal. In terms of the spatial dimension, the growth of an urban agglomeration spatial structure can be described as the extension and expansion of the central city spatial structure and functional differentiation; the formation of spatial synergistic network structure between small and medium-sized and central cities; the flow and exchange of development factors across space, administrative regions and organisations; and the formation of a ‘central, gradient and circular’ spatial structure. In terms of the temporal dimension, the development of urban agglomerations is influenced by external forces of human social development, namely, control parameters mentioned in systems science. Policies can be used as proactive control parameters to intervene in the growth path of urban agglomerations.
The urban agglomeration system is a dissipative structure that continuously exchanges development factors with the external environment [36]. The fluctuations in development factors are amplified into huge fluctuations by nonlinear mechanisms, which promote the central city to radiate to surrounding cities through development factors and form the initial spatial structure [36,37]. The order parameter of an urban agglomeration system dominates the system behaviour. The synergistic relationship between subsystems within the urban agglomeration is upgraded through the development of factor resource allocation. The order parameter system reaches the synergistic relationship again, which triggers a phase transition of the system state. A multiscale spatial structure is generally characterised by the formation of spatial synergy between multiple central cities and metropolitan areas. The trend of urbanisation based on the division of labour is accelerating, and the gradient division of labour from the core to the periphery is further strengthened. Distance diffusion gradually replaces level diffusion as the dominant factor in the exchange of development factors, and a multiscale structure extends in regional space with a certain spatial level and spatial amplitude range (Figure 3).
The spatial structure evolution path of the urban agglomeration can be divided into four stages: central city, urban agglomeration, metropolitan area and urban agglomeration. The central city is a large city with influence, cohesion and radiation capabilities within a certain geographical spatial range [38]. Urban clusters are multiple urban agglomerations that radiate from small and medium-sized cities, with the central city as the core [2]. They are an unstable form of spatial combination and may develop into urban agglomerations. The metropolitan area is the central city and surrounding cities that achieve geographical and transportation connectivity, exchange of development factors and functional complementarity to achieve highly integrated development. Urban agglomeration is a multiscale spatial structure that involves the exchange of development factors and spatial synergistic relationships amongst multiple metropolitan areas, and it has a diverse and complex functional factor structure.

5. Growth Motivation for the Multiscale Spatial Structure of Urban Agglomerations

5.1. Changes in Development Factor Fluctuations

The spatial structure of urban agglomerations is developing towards complexity and functionalisation, which can be roughly divided into three stages: urban isolation, urban integration and urban agglomeration integration. Portnov and Schwartz (2009) [14] found that population cluster and flow are related to the formation of urban agglomerations. Fang and Yu (2017) [2] proposed the criteria of urban agglomerations, which include having more than three large cities with populations exceeding 20 million and a per capita GDP of more than USD 10,000. Population and GDP can be used as order parameters to reveal the effects of development factors in formation [19]. Figure 4 and Figure 5 show urban agglomerations, metropolitan areas and central cities to reflect the trend of population and GDP during the formation of the urban agglomeration in the Yangtze River Delta.
The spatiotemporal growth stages of the urban agglomeration in the Yangtze River Delta include control parameters, fluctuations in development factors and the formation process of the spatial structure (Table 3). The time series shows that the changes in central cities of the urban agglomeration in the Yangtze River Delta (229–1911 AD) during the first stage of urban isolation have the following transfer order: Nanjing, Yangzhou, Suzhou–Hangzhou and Nanjing–Hangzhou–Suzhou. Under the influences of external environmental forces, such as political reform, transportation, system reform, economic model, the Westernisation Movement and industrialisation, the central cities rose and fell alternately in time and space. The input of development factors leads to the concentration of population and economy, which results in urbanisation that exhibits centrality [10]. This situation shapes the spatial structure of the urban agglomeration, which gradually moves towards a multicentre and multilevel configuration.
The second stage is the period of urban integration (1912–1990 AD). From 1927 to 1937, the Nationalist government strengthened the economic construction of Nanjing, Shanghai and Hangzhou, and the ethnic industry and commerce developed rapidly in these cities. The three cities exhibit centrality in regional urbanisation and development factor aggregation, which contributed to the initial formation of the Nanjing–Shanghai–Hangzhou urban agglomeration model. Urban integration occurs between cities, with railways connecting the three central cities of Shanghai, Nanjing and Hangzhou. Transportation accelerated diffusion and the flow of development factors between small and medium-sized cities and central cities [39,40], and urban communities emerged before metropolitan areas. Cities in southern Jiangsu and eastern Zhejiang attracted foreign investment and accelerated the process of industrialisation and urbanisation. The clustering of development factors, especially for clustered industry, enabled the growth of urban agglomerations and metropolitan areas.
The third stage is the period of urban agglomeration integration (1990 AD–present). Urban agglomeration and its internal metropolitan area simultaneously formed a multiscale spatial structure. During the period of the Shanghai Pudong Development, industrial transfer and division of labour collaboration and the adjustment of urban functional structure led to the diffusion of development factors. Nantong, Suzhou, Zhoushan and Jiaxing undertook industrial transfer and development factors that accelerated urbanisation integration, and the spatial structure of the Shanghai metropolitan area was forming. Then, the metropolitan areas of Nanjing, Hangzhou and Hefei emerged. Subsequently, the extension of spatial structure and the flow of development factors facilitated the progress of the spatial structure of the urban agglomeration in the Yangtze River Delta towards multiscale, multicentre and multi-metropolitan areas. The fluctuation state of development factor is upgrading, especially for central cities based on urban function, to enhance the structure of development factors for reaching a spatial synergistic relationship.
The changes in development factor fluctuation state influence the formation of multiscale spatial structures for urban agglomerations. Fluctuations in development factors occur in four states: factor input, factor aggregation, factor diffusion and factor upgrading. Cao (2015) found that the aggregation of population and economy motivates the process of urban expansion [7]. Wu et al. (2023) also revealed that population density and GDP growth are the key driving factors of urban expansion [4]. Therefore, mutational changes in development factor fluctuation state motivate the growth of urban expansion towards generating a new spatial structure. Based on the change laws of development factor fluctuation, supportive policies should be implemented by governments to accelerate the change in states [19].

5.2. Upgrade of Spatial Synergistic Relationships

The growth of spatial structures or changes in external forces can lead to variations in the fluctuation amplitude of development factors. Scholars have realised the importance of the effect of superlinear growth in cities [10,16]. This study proposed its counterpart in systems science, that is, the nonlinear mechanism, which amplifies the fluctuation amplitude of factors [38]. This mechanism causes the system fluctuation to deviate from the original trajectory to a nonequilibrium state. From the perspective of efficient allocation of development factors, the urban agglomeration in the Yangtze River Delta has encountered problems of uneven development factors, such as land tension, traffic congestion, cost increase, ecological imbalance and environmental degradation [23]. From the perspective of subsystem functions, the homogeneous development of industrial ecology, population structure and spatial structure between cities and metropolitan areas results in a vicious competition, which deters the formation of spatial synergy. To solve the abovementioned issues, a new spatial synergistic relationship is generated between the subsystems of the urban agglomeration [41], whereas the fluctuation trajectory of development factors drives the system to transition to a new equilibrium state.
The industrial division of labour and collaboration are important carriers of spatial synergy and serve as the basis for spatial functional differentiation, which also optimises the allocation of development factors and maximises spatial synergy effects. The spatial synergy within urban agglomerations occurs sequentially within cities, between cities, between metropolitan areas and between multiple metropolitan areas; the core goal is to optimise the allocation of development factors and improve the efficiency of spatial resource utilisation [42]. Figure 6 shows the four types of spatial synergistic relationships in urban agglomerations: internal, point-to-point, circle-to-circle and multicircle synergies. An urban agglomeration not only utilises spatial synergies to develop into national-level technological innovation and industrial spatial structure but also synergises with other world-class urban agglomerations to drive global economic and social development.
The process of the spatial structure of urban agglomerations growing into a higher-level, orderly and balanced state has generated new scale spaces and spatial synergy has been upgraded into new relationships. The spatial synergistic relationship of the urban agglomeration in the Yangtze River Delta is mainly point and point-to-point synergies, amongst which circle-to-circle and multicircle synergies are still growing. The level and amplitude of the spatial structure of the agglomeration in the Yangtze River Delta urban agglomeration are in a continuous growth stage [39], and a clear boundary of development factors and spatial synergistic relationships has not yet been formed. The reasons are threefold. Firstly, a certain mismatch exists in the fluctuation trend between population and industrial development factors within the scale space because the speed of population aggregation is considerably faster than industrial aggregation, whereas the technological content of industries and emerging industries is insufficient. Secondly, the homogenisation of functions between central cities and metropolitan areas, unclear division of labour and co-operation in industries, and the lack of spatial synergy amongst the four metropolitan areas have caused certain issues. Thirdly, the change in the fluctuation trajectory of development factors advances to the upgrading of the spatial synergistic relationship, which affects the formation process of multiscale spatial structures.
Each scale existing in the spatial synergistic relationship forms the multiscale structure of urban agglomerations. Policy is a control parameter that accelerates the upgrading of the spatial synergistic relationship [19]. National and local governments implement policy regulations to increase the growth momentum of urban agglomeration as the external forces (Table 4). Since 2010, the State Council has issued relevant plans to clarify the geographical spatial scope, functional structure and strategic positioning of the urban agglomeration in the Yangtze River Delta. This situation has created policy conditions for the formation of an orderly and advanced spatial structure by changing the external environment to intervene in the fluctuation trajectory of factors and the upgrading path of spatial synergistic relationships [32].
Spatial synergistic relationships impel urban expansion towards an orderly spatial structure. Fang and Yu (2017) [2] deemed that policies, plans and infrastructure construction can be co-ordinated from macrolevel to secure sustainable development and long-term integration of urban agglomeration. Zou and Xu (2024) [19] indicated that synergising the management of spatial structure and promoting industrial division of labour are essential to the growth of urban expansion [4]. The spatial synergistic relationship must be classified into four types: internal, point-to-point, circle-to-circle and multicircle synergies. These interactions form a multiscale spatial structure during the growth of urban agglomeration.

5.3. Growth Motivation of Spatial Synergy and Development Factors

The growth of multiscale spatial structures in urban agglomerations requires spatial synergy and the development of factor fluctuations. The fluctuation in development factors in urban agglomeration systems reaches a critical point and the nonlinear mechanism amplifies into huge fluctuations [13,32]. Population, industry and natural resources are slow variables that determine the growth behaviour and trend of subsystems as order parameters. The formation of new trajectories of development factor fluctuations and the upgrading of the spatial synergistic relationship may reach a new equilibrium state. During this process, development factor fluctuations and spatial synergy always exist in every growth stage of urban agglomerations. The development factor fluctuations and spatial synergy alternately dominate the growth process in urban agglomeration.
The total number of development factors between the urban agglomeration in the Yangtze River Delta and the San Francisco Bay Area is compared (Table 5); the former is better in total quantity, whereas latter is advantageous in average value. This finding indicates that determining the multiscale spatial structure of urban agglomerations is a qualitative mutation of order parameters rather than a simple accumulation of quantity, which is critical for the growth of spatial structure. This nuance is different from the seven criteria of urban agglomeration proposed by Fang and Yu (2017) [2]. Urban agglomeration entails the fluctuations in development factors to sustain the multiscale structure, and the changes in the fluctuations create the growth motivation.
The growth path of multiscale spatial structures in urban agglomerations is a systematic growth in which the changes in development factor fluctuation state and the upgrading of spatial synergistic relationships are intervened by control parameters and nonlinear mechanisms (Figure 7). The changes in control parameters cause instability to the system, and nonlinear mechanisms temporarily replace the linear fluctuation trajectory. The development factor fluctuation state changes in synchrony with the upgrading of spatial synergistic relationships. The growth stages of multiscale spatial structures can be presented as ‘central city → urban communities → metropolitan area → urban agglomeration’, and one of the key factors in the growth of spatial structures is the continuous upgrading of spatial synergistic relationships. Spatial synergy motivates the growth of the spatial structure of urban agglomerations and braces the multiscale features.
Urban agglomerations are complex open systems that are driven by spatial synergy and factor fluctuations. Firstly, city systems far from equilibrium input development factors from the outside, and the differentiation of internal spatial functions and spatial synergy promote the formation of central cities in multiscale spatial structures. Secondly, the nonlinear mechanism amplifies the fluctuation in development factors, and the development factors of central cities orderly spread to small and medium-sized cities. A point-to-point spatial synergistic relationship exists between cities, and central cities grow into urban community forms in spatial structures [2]. Through the increased aggregation of development factors and the amplification effect of nonlinear mechanisms, the upgrading of spatial synergistic relationships promotes the evolution of spatial structures from urban communities into metropolitan areas and may lead to circle-to-circle spatial synergistic relationships. Thirdly, the upgrading of development factors promotes the multiscale spatial structure of urban agglomerations through multicircle synergy, which achieves differentiated and complementary development strategies for the functional positioning of metropolitan areas. Fourthly, the control parameters for the growth of urban agglomeration systems include controllable and uncontrollable parts, and the growth path of the controllable part can be regulated by policy tools [35]. The uncontrollable parts include technological changes and natural resources, and policy tools can be adjusted and modified in accordance with their change laws.
The synergistic effect between the upgrading of spatial synergistic relationships and the changes in development factor fluctuations forms the growth motivation of multiscale spatial structures. Wu et al. (2023) realised that spatial synergy would be beneficial to the growth of spatial structures [2,4]. The development of urban agglomeration needs to promote new types of urbanisation and spatial synergy [19]. Wang and Zhang (2018) studied land market, economic development and local governments’ behaviour as the driving forces of land urbanisation quality in urban agglomerations [43,44]. At different growth stages of urban agglomerations, development factors and spatial synergy generate corresponding driving forces, which promote the emergence of new spatial structures.

5.4. Limitations

This study discusses the growth motivation of urban agglomerations from the perspective of synergy theory. However, this study still has some shortcomings. Firstly, in addition to population growth and GDP discussed in this paper, other development factors may probably influence the formation process or urbanisation level. Zou and Xu (2024) [19] argued that artificial intelligence technology and fiscal decentralisation also matter in urban expansion. Secondly, in addition to urban synergy, Shi et al. (2020) considered that the level of spatial synergy is conducive to urban–rural integration and healthy development in urban agglomerations [45]. The rural–urban synergistic relationship is a promising topic in spatial synergy. Thirdly, this study lacks a synergistic model to evaluate the co-ordination level of GDP and population growth at the multiscale space. Such a model can generate more accurate results for the growth motivation of urban agglomerations.

6. Policy Implications for Promoting the Growth of Urban Agglomerations

Firstly, incentive policies are required to facilitate the fluctuation in development factors. The allocation of development factors must be optimised through integrated urban policies to reduce the institutional costs of the cross-spatial and cross-organisational flow of development factors, such as talents, technology and industrial resources. The spatial utilisation efficiency of the urban agglomeration in the Yangtze River Delta must be emphasised. Top-level planning for spatial hierarchy and amplitude is needed to optimise the spatial layout and flow direction of development factors. The high-quality development of urban agglomerations particularly relies on incentive policies to facilitate the fluctuations in talent, technology and advanced industries, amongst which talent is a key development factor. For example, according to official statistical data in 2020, only 35% of the population in Nanjing have a college degree or above, whereas 50% have a four-year college degree or above in San Francisco. Changes in development factor fluctuations should be towards quality structure rather than quantity accumulation. Local governments must enact incentive policies to upgrade the composition of development factors, particularly in the aspects of educated population, high-technological industry and advantageous economy [19,46].
Secondly, integrated development planning is needed to differentiate the positioning of urban agglomeration functions that promote spatial synergistic relationships. Implementing regional and functional planning at the urban scale can achieve cross-spatial synergistic relationships. Metropolitan areas should be developed to form spatial synergistic relationships with differentiated and complementary functions [18] and differentiated positioning of the metropolitan areas of Nanjing, Hangzhou, Shanghai and Hefei to form spatial functional synergistic relationships. The multiscale spatial structure of the urban agglomeration in the Yangtze River Delta should maintain appropriate spatial levels and amplitudes. This approach promotes the development of point and point-to-point to circle-to-circle and multicircle synergies’ spatial synergistic relationships to enhance the spatial synergy effect of the urban agglomeration. Spatial synergistic relationships amongst urban agglomerations shift the development model of cities from mainly competition to competition and co-operation [6]. Therefore, local governments can differentiate local industry and spatial function.
Thirdly, integrated industrial policies are necessary to cultivate spatial synergy. Industrial clustering and industrial division of labour facilitate urban agglomerations and spatial synergistic relationships, respectively [47]. From an urban agglomeration perspective, integrated industrial policies reallocate industrial resources to avoid homogeneous and vicious competition and strengthen spatial synergistic connections to build a world-class competition industrial ecosystem. Industrial collaboration intensifies spatial synergy and should thus be increased to enable the development of advantageous industrial and key technology clusters. The development should focus on attracting and cultivating core talents and embedding core links or key technologies into the global industrial production and manufacturing supply chain.
Fourthly, top-level planning of multiscale spatial structures of urban agglomeration is a must. Central and local governments need to enact top-level planning that specifically manifests the scopes of spatial levels and amplitudes for multiscale spatial structures of urban agglomeration. Key planning focuses on the aggregation of emerging industries and the extension of industrial value chains, with industries supporting the differentiation of urban functions. In the planning process, large-scale spatial synergy needs to be utilised to reduce distance attenuation effects, the spatial structure of urban agglomerations with multi-metropolitan areas and multicentral cities should be appropriately developed, and the process of small-scale spatial urbanisation must be accelerated. In addition, by using systematic scientific methods, the fluctuation trajectory and critical points of development factors can be predicted and a dynamic adjustment of the integrated development plan of urban agglomeration can be achieved in a timely manner [48].

7. Conclusions

This study explores the growth motivation for the multiscale spatial structure of the urban agglomeration in the Yangtze River Delta from the perspective of spatiotemporal evolution. The research findings are described as follows. (1) The urban agglomeration in the Yangtze River Delta has gradually formed a multiscale spatial structure of ‘small and medium-sized cities, central cities, metropolitan areas and urban agglomerations’. (2) The nonlinear mechanism at the critical point amplifies the development factors as huge fluctuations, which deviate from the original fluctuation trajectory. These changes in state cause the urban agglomeration system to reach a state far from equilibrium. (3) The upgrading of spatial synergistic relationships enables urban agglomeration systems to reach orderly spatial structures. (4) The synergy effect of the upgrading of spatial synergistic relationships and the changes in development factor fluctuation state is the growth motivation for multiscale spatial structures.
Sustainability is essential to the development of global urban agglomeration. The theoretical contribution of this study is as follows. The upgrading of the spatial synergistic relationship and the changes in development factor fluctuations act as the growth motivation for the multiscale spatial structures of urban agglomerations. These findings help explain the scientific problems of spatial structure formation of urban agglomerations. Additional case studies in different contexts are needed to develop the theory further. In addition, a quantitative method is required to validate the findings in the future. This topic is a promising research field that will be beneficial to urbanisation worldwide.

Author Contributions

Conceptualization, L.W.; methodology, L.W.; formal analysis, L.W.; writing—original draft preparation, L.W. and Y.H.; writing—review and editing, Y.H.; funding acquisition, Q.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was jointly funded by the Decision Consultation Project in Jiangsu Province, grant number 24SSL021; General projects of Qinghai Provincial Social Science Fund, grant number 23YQA-004; Hunan Provincial Natural Science Foundation of China, grant number 2024JJ5046; and Decision-making consulting research base for Jiangsu service-oriented government construction, grant number AB15051.

Data Availability Statement

The original contributions presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Research roadmap.
Figure 1. Research roadmap.
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Figure 2. Multiscale spatial structure of the urban agglomeration in the Yangtze River Delta: Nanjing–Metropolitan Area–Nanjing–small and medium-sized cities (the urban agglomeration in the Yangtze River Delta includes Shanghai, Nanjing, Hangzhou and the metropolitan area of Hefei. This figure takes the metropolitan area of Nanjing as an example).
Figure 2. Multiscale spatial structure of the urban agglomeration in the Yangtze River Delta: Nanjing–Metropolitan Area–Nanjing–small and medium-sized cities (the urban agglomeration in the Yangtze River Delta includes Shanghai, Nanjing, Hangzhou and the metropolitan area of Hefei. This figure takes the metropolitan area of Nanjing as an example).
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Figure 3. Spatial structure characteristics of urban agglomerations in different evolution stages.
Figure 3. Spatial structure characteristics of urban agglomerations in different evolution stages.
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Figure 4. Trend of population fluctuation in the multiscale spaces of the urban agglomeration in the Yangtze River Delta (in 2014, cities such as Hefei were included in the urban agglomeration in the Yangtze River Delta, which resulted in a remarkable increase in population).
Figure 4. Trend of population fluctuation in the multiscale spaces of the urban agglomeration in the Yangtze River Delta (in 2014, cities such as Hefei were included in the urban agglomeration in the Yangtze River Delta, which resulted in a remarkable increase in population).
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Figure 5. Trend of GDP fluctuation in the multiscale spaces of the urban agglomeration in the Yangtze River Delta.
Figure 5. Trend of GDP fluctuation in the multiscale spaces of the urban agglomeration in the Yangtze River Delta.
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Figure 6. Upgrading path of spatial synergy in the urban agglomeration in the Yangtze River Delta.
Figure 6. Upgrading path of spatial synergy in the urban agglomeration in the Yangtze River Delta.
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Figure 7. Schematic of the growth motivation of the spatial structure of urban agglomerations.
Figure 7. Schematic of the growth motivation of the spatial structure of urban agglomerations.
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Table 1. Data sources of the case.
Table 1. Data sources of the case.
ContributionsSources
Plannings, urban positioning of cities and spatial structure of the urban agglomeration in the Yangtze River DeltaPortnov and Schwartz (2009); Fang and Yu (2017); China Development and Reform Commission websites; Chinese State Council websites
GDP and polpulations of the urban agglomeration in the Yangtze River DeltaChina Bureau of Statistics
Spatial structure of the metropolitan area of NanjingChina Development and Reform Commission websites
GDP and population of the San Francisco Bay Area Census Bureau, Bureau of Economic Analysis in the US
Table 2. Multiscale structure of the urban agglomeration in the Yangtze River Delta.
Table 2. Multiscale structure of the urban agglomeration in the Yangtze River Delta.
Spatial LevelSpatial Amplitude
Urban agglomeration (First level)A total of 26 prefecture-level cities, 104 counties (cities) and thousands of towns.
Metropolitan area (Second level)Metropolitan areas of Shanghai, Nanjing, Hangzhou and Hefei
Central city (Third level)Cities of Shanghai, Nanjing, Hangzhou and Hefei
Small and medium-sized cities (Fourth level)Hundreds of counties (cities) and thousands of towns (except for the four central cities)
Table 3. Urbanisation of the urban agglomeration in the Yangtze River Delta in different periods.
Table 3. Urbanisation of the urban agglomeration in the Yangtze River Delta in different periods.
Years *Growth StageControl ParametersFluctuations in FactorsResults of the Urbanisation Process
229Urban isolation periodDongwu established its capital in Nanjing.Commercial centreNanjing has become a political, cultural and economic centre.
610The Grand Canal was built.Water transport hubYangzhou has become a north–south transportation hub and an economic and commercial centre in Jiangnan.
1127Hangzhou became the capital of the Southern Song Dynasty.Political and commercial centresHangzhou has become the political, cultural and economic centre of southern China, whereas Suzhou is the economic centre.
1368Ming Dynasty established its capital in Nanjing.Political centre, salt transportation, grain production and silk industryNanjing is the political, cultural and economic centre of the country; whereas Suzhou, Hangzhou and Yangzhou are the centres of silk, salt and grain production, respectively.
1861Westernisation MovementOrdinance manufacturing, machine textile and shipbuildingThe Jiangnan Manufacturing Bureau, Anqing Internal Ordnance Bureau, Machine Weaving Bureau and Ship Investment Promotion Bureau have promoted the rise of modern industry in Shanghai and Nanjing.
1912Urban integration periodThe Shanghai–Nanjing–Hangzhou Railway was completed.Railway transportation, finance andmodern industryShanghai has become a central city, and the flow of development factors along the cities of Shanghai, Nanjing and Hangzhou has accelerated, which expedited the process of industry modernisation.
1927The Nationalist government established its capital in Nanjing.Political centre, modern industry, ethnic industry and commerceThe urban cluster of Nanjing–Shanghai–Hangzhou has emerged with the rapid development of modern industry, ethnic commerce and service industry, which made Shanghai the financial centre of China.
1978Reform and Opening upUrban functions and transportationDifferentiation of spatial functions, industrial division of labour and flow of development factors in the urban agglomeration in the Yangtze River Delta under market economy.
1990Period of urban agglomeration integrationPudong DevelopmentIndustrial upgrading and population mobilityThe integrated urban communities of Shanghai, Nantong, Suzhou and Jiaxing are gradually forming and the external population input is accelerating.
2013Shanghai–Nanjing–Hangzhou–Hefei High-speed Railway UnicomHigh-speed rail technology and mobile InternetThe geographical and spatial connectivity of the urban agglomeration in the Yangtze River Delta has become integrated and the construction of integrated public services has begun.
2021Approval of Nanjing Metropolitan Area PlanningCross provincial planning and
technological innovation		The integration of the metropolitan area of Nanjing is accelerating and the spatial structure of the urban agglomeration in the Yangtze River Delta is developing towards multiple metropolitan area models.
* Year refers to the start date, not the duration of the event.
Table 4. Integration progress of the urban agglomeration in the Yangtze River Delta.
Table 4. Integration progress of the urban agglomeration in the Yangtze River Delta.
YearPolicy NameSynergistic Effect
2008Guiding Opinions of the State Council on Further Promoting the Reform, Opening up and Economic and Social Development of the Yangtze River Delta RegionA major decision and deployment to further enhance the overall strength and international competitiveness of the Yangtze River Delta region.
2010Regional Plan for the Yangtze River Delta RegionThe State Council has clarified that Shanghai, Jiangsu and Zhejiang compose the urban agglomeration in the Yangtze River Delta and are positioned as world-class competitive urban agglomerations.
2014Guiding Opinions of the State Council on Promoting the Development of the Yangtze River Economic Belt by Relying on the Golden WaterwayHefei has been officially incorporated into the urban agglomeration in the Yangtze River Delta, which further expanded the division of labour between industries and cities.
2016Development Plan for the Yangtze River Delta Urban AgglomerationShanghai plays the role of a central city, which realises the upgrading and optimisation of industries driven by scientific and technological innovation and promotes the collaboration development of metropolitan areas.
2021Implementation Plan for the 14th Five-year Plan for the Integrated Development of the Yangtze River DeltaRegional balance, ecological environment, public services, infrastructure and strategic technology are considered.
2021Request for Approval on the Development Plan of Nanjing Metropolitan AreaThe aim is to achieve high-quality, co-ordinated development of cities within the metropolitan area and the circle-to-circle synergy between the metropolitan area of Nanjing and other metropolitan areas.
Table 5. Comparison of GDP and population between the urban agglomerations in the Yangtze River Delta and the San Francisco Bay Area (2021).
Table 5. Comparison of GDP and population between the urban agglomerations in the Yangtze River Delta and the San Francisco Bay Area (2021).
IndexNanjingMetropolitan Area of NanjingUrban Agglomeration in the Yangtze River DeltaSan FranciscoMetropolitan Area of San FranciscoSan Francisco Bay Area
Population (K)9310.035,490.0235,000.0870.04820.07760.0
Area (km2)6587.0064,600.00380,000.00121.009880.0018,000.00
GDP (T$)0.20 0.57 3.26 0.19 0.56 0.92
Per capital GDP (K)22.8216.1813.90221.65117.16118.93
Population density (km2)1413.00549.00618.007190.00488.00431.00
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Wu, L.; Huang, Y.; Cheng, Q. Growth Motivation of Urban Agglomerations in Multiscale Spatial Structures from the Perspective of Synergy Theory. Sustainability 2024, 16, 6190. https://doi.org/10.3390/su16146190

AMA Style

Wu L, Huang Y, Cheng Q. Growth Motivation of Urban Agglomerations in Multiscale Spatial Structures from the Perspective of Synergy Theory. Sustainability. 2024; 16(14):6190. https://doi.org/10.3390/su16146190

Chicago/Turabian Style

Wu, Lufeng, Yao Huang, and Qian Cheng. 2024. "Growth Motivation of Urban Agglomerations in Multiscale Spatial Structures from the Perspective of Synergy Theory" Sustainability 16, no. 14: 6190. https://doi.org/10.3390/su16146190

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