1. Introduction
With the rapid economic development and urbanization since China's Reform and Opening-up, urban space has shifted from rough expansion to intensive connotative development. Urban renewal is an important means of achieving sustainable urban development [
1]. The shaping and enhancement of spatial quality is the main goal of the urban renewal movement [
2], and urban spatial quality is closely related to vitality [
3]. Vibrant cities have higher well-being indices for their residents and tend to be more attractive to investment and talent inflows, enhancing urban competitiveness [
4]. Maintaining and enhancing the urban vitality of urban centers is particularly important for the realization of urban development [
5]. Urban vitality is a spatial characteristic that results from the interactions between human activities and space; it is also an important manifestation of the potential for urban development and urban quality [
6,
7]. Subdis-trict-scale urban vitality provides a finer-grained reflection of the activity of people in-ter-acting with the urban space [
8].
Urban vitality is a classic topic in urban planning and development, and the creation of vibrant cities and vibrant spaces has long been a concern in the fields of urban planning, environmental science, and geography [
9,
10]. The characterization and measurement of urban vitality is the focus of scholars' attention, and the characterization of urban vitality is generally based on the related theories of Jacobs and Jan Gehl, according to whom urban vitality is qualitatively embodied in the composite qualities of the explicit and objective existence of the city, and is quantitatively characterized by the people and their activities in the city [
7,
11,
12]. In terms of measurement indicators, novel big data is often used as a measure of urban vitality. Kim et al. [
12] used cell phone traffic signals and Wi-Fi access points to measure urban vitality intensity, based on the perspective of combining virtual and real. Levin et al. [
13] used remote sensing data to find that nighttime light images can effectively reflect the intensity of urban residents' activities and that it changes over time. Wu et al. [
14] explored the difference between daytime and nighttime vitality by using Easygo crowd-sourced travel data. Concerning the object of study, administrative divisions are often used as a basic measure of urban vitality. [
15,
16] There are also more extensive measures of vitality for specific spatial types, such as urban vitality measures for historic districts [
17], parks [
18], and waterfronts [
19]. Urban vitality exhibits a high degree of concentration where there is a high and overlapping density of population as well as commercial and public service facilities [
5]. Vitality measurement methods include interview questionnaires [
20], the entropy method vitality evaluation model [
21], Jane Index [
5], Projection Pursuit Model (PPM) [
22], kernel density estimation [
23], and other methods. In addition, Li et al. [
7] and Qi et al. [
24] have over the course of time been applied in related research.
Established studies have shown that the factors affecting the urban vitality of cities mainly include social development, economic restructuring, and characteristics of the built environment and other aspects. In the area of social development, social policies and institutions are important factors influencing the urban vitality of cities. With the loss of vitality in inner cities, due to the aging of the urban physical environment and to functional imbalance, countries around the world have been promoting urban renewal campaigns since the middle of the last century to restore the vitality of cities [
25]. Effective intervention by governmental agencies leads to an orderly urban renewal movement, which is a mandatory and effective strategy for revitalizing urban center spaces [
26]. Negative events, such as major social emergencies, such as the outbreak of New Crown Pneumonia (COVID-19) [
27,
28] and wars and conflicts [
29], can reduce the vitality of urban spaces and have lasting impacts. Economic restructuring often brings about corresponding changes in the spatial dynamics of cities. Traditional economic development indicators such as Gross Domestic Product (GDP) and disposable income per capita are partly indicative of economic vitality, and elevated levels of these indicators mean that social activities such as consumption and innovative behaviors of the population can be promoted to influence the urban vitality [
30]. Emerging online economies such as urban takeaways in recent years also contribute to the clustering of urban vitality as a consumer activity for the population [
31]. In terms of the built environment, scholars have chosen to measure different types of urban built environment indicators to analyze their impact on urban vitality, such as reasonable urban texture [
8] and spatial structure [
32] that can lead to the gathering of vitality, to intense urban development and construction [
33], and to supporting facilities [
34] that are also considered to be closely related to urban vitality, including high-density buildings that create intensely fertile ground for crowd activities [
16]. In terms of analytical methods, Geographically Weighted Regression (GWR) [
34], Structural Equation Modeling (SEM) [
32], and Spatial Lag Modeling (SLM) [
7] are often used in related studies. For example, Wang et al. [
35] used multi-scale GWR to explore the spatial and temporal influence mechanism of 24-hour urban vitality in Beijing. In addition, Geodetector is increasingly being used in related research. For example, Li et al. [
36] used Geodetector to analyze the interaction effects among the drivers of nighttime light expansion dynamics. Overall, it seems that studies on the level of the city have been more frequent, while fewer studies have looked at indicator systems of factors influencing urban vitality at the neighborhood level and could therefore be further supplemented.
Changsha is the representative city of the middle reaches of the Yangtze River in China [
37]. Strategically, it is an important nodal city of the urban agglomeration in the middle reaches of the Yangtze River and has rapidly developed into a megacity with a resident population of more than 10 million under the guidance of the "The Belt and Road " and "The Yangtze River Economic Belt" national strategies [
38]. As a popular city for travel on the internet, it has been vigorously developing its nighttime economy in recent years, with the city's attractiveness and economic competitiveness rising year by year, and playing an exemplary role in economic transformation. Therefore, Changsha City is representative of its geographical location and economic development characteristics and is a typical city in China's fast-growing central region [
39]. As a representative of China's new first-tier cities, Changsha is facing urban problems, such as deteriorating road traffic and loss of vitality in the old urban areas after experiencing rapid industrial and economic development [
40]. This makes Changsha a typical city for measuring the factors influencing the spatial and temporal evolution of urban vitality.
Summarizing existing research, it was found that a large number of studies have been conducted on urban vitality and its driving factors based on different study areas, scales, and perspectives. However, current research on urban vitality and its driving factors is mainly based on cross-sectional data, and the research scale focuses on the prefecture or district scale. In terms of study areas, nationwide studies are mainly explored at the provincial scale, while studies at the prefecture, city, county, and district scales are mostly confined to developed regions. The research methodology is dominated by the research paradigm of environmental geography, but there is a lack of exploration of the interactive effects of neighborhood form-function and urban vitality relationships. At the same time, urban vitality agglomeration does not evolve in the short term but is formed over a long period by way of various spatial factors [
41], with a strong temporal sequence [
42]. Most current studies lack consideration of the spatial and temporal characteristics of the evolution of urban vitality and the factors influencing it over a multi-year span. Based on this, this paper uses exploratory spatial data analysis to investigate the evolution of urban vitality and spatial and temporal influences at the Subdistrict scale in Changsha, a city in central China. We use Geodetector and spatial regression analyses to explain the interactive effects and spatio-temporal heterogeneity of the three dimensions of subdistrict form, subdistrict function, and subdistrict economy on urban vitality. Our aim is to provide targeted planning recommendations as a reference for the creation and enhancement of vibrancy in the construction of sustainable human settlement and urban renewal.
7. Conclusions
This study aims to investigate the spatial driving factors of subdistrict space in the spatio-temporal evolution of urban vitality in Changsha City. To achieve the research objective, the entropy weight method and spatial autocorrelation are used to conceptualize and measure the urban vitality and characterize its spatio-temporal evolution using two indicators, namely, social media check-ins and nighttime lighting index. In addition, Geodetector and GTWR models are used to explore how three dimensions of subdistrict form, subdistrict function, and subdistrict economy influence the aggregated characteristics of urban vitality, and how these influences evolve. Data from 81 subdistricts in the central city of Changsha, China in 2013, 2017, and 2021 were collected for empirical analysis. The results of the study show that:
(1) The spatial and temporal distribution of urban vitality in the central district of Changsha City shows spatial differentiation characteristics and the urban vitality was gathered in the south-central and southwestern districts in 2013, with a sporadic distribution. The south-central and southwestern regions show a gradual agglomeration and distribution trend after 2017 and continue to spread to the west and south in 2021. Low urban vitality subdistricts have long been concentrated in Wangcheng District in the north and Changsha County in the east, and the urban vitality value of the mountainous area in the southwest of Yuelu District is also low.
(2) There is a significant spatial correlation in the distribution of urban vitality. The high–high type of urban vitality subdistricts are clustered and distributed at the junction of the Furong, Yuhua, and Tianxin Districts in the central part of the city, and then gradually spreads out from the center to the surrounding area. The low–low type of urban vitality subdistricts are mainly located in the northern Wangcheng District, Kaifu District, and the northern part of Changsha County, before spreading to the south in 2021.
(3) Subdistrict form and subdistrict function have a significant effect on urban vitality, while the effect of the economic dimension of the subdistrict is not significant. The aggregation of urban vitality is the result of a variety of factors. The interaction of entertainment, recreation, and business office at the functional level of the subdistrict is the dominant factor affecting the intensity of urban vitality, while factors related to the level of income of the city's inhabitants have the lowest interaction at the economic level.
(4) The contribution of SHDI to the urban vitality intensity was most prominent, followed by DP. From the perspective of temporal evolution, the street pattern shows a positive contribution to the urban vitality. As for the function of the subdistrict, except for the density of DS facilities, which has both a positive and a negative inhibitory effect on the urban vitality at different times, the density of DP, DRL, and DBO all show a positive contribution to the urban vitality. In terms of the spatial distribution of the intensity of the role, the functional aspects of the subdistrict, SHDI, and RD, fill an important role in promoting the urban vitality of the central subdistricts, as well as inhibiting the surrounding remote subdistricts, with SHDI playing a stronger role in promoting the urban vitality. In terms of subdistrict form, the effects of DP and DS on urban vitality are high in the west and low in the east, while the effects of DRL and DBO show a spatial distribution pattern of gradual increase from the west to the east.