1. Introduction
Rural settlement is the key research field of rural geography. It is a complex system composed of social economy, natural ecology and other subsystems. It has a certain scale, structure, form and function, and has the main characteristics of complexity and dynamics [
1,
2]. Since the reform and opening up, with the rapid industrialization and urbanization process in China, the interactive flow of urban and rural resource factors has accelerated, the structure of production factors in rural areas is changing, and the development and evolution of rural settlements are undergoing rapid transformation. Rural settlements are also gradually shifting from “homogeneous” to “heterogeneous”, and the transformation and development of rural settlements present a variety of scenarios [
3,
4]. To promote the implementation of the rural revitalization strategy and build a beautiful countryside suitable for living, working and visiting, the development of rural settlements is an important basis for rural revitalization [
5]. Therefore, exploring the spatial characteristics, influence mechanism and reconstruction path of rural settlements in typical regions is a realistic proposition under the background of rural revitalization.
Rural settlement geography is the main branch of rural geography. With the development and transformation of rural geography, the focus of rural settlement geography research is also constantly changing [
6]. In the 18th century, geographers began to explore the relationship between people and land in rural areas. In this process, they began to study rural settlements, mainly involving the origin, distribution, type, evolution of rural settlements and their relationship with the environment [
7,
8,
9]. After the 1950s, research in developed countries began to focus on spatial classification and measurement, research on rural sustainable development and the impact of human decision-making behavior on rural settlements [
10,
11,
12,
13]; the research content was constantly enriched. By the 1990s, under the influence of many philosophical trends, especially postmodernism, existentialism, idealism, humanistic geography, structuralist geography and critical realism geography, the research paradigm of rural geography in western countries began to transform to the social human direction at this stage [
14,
15,
16]. With the rapid development of rural industrialization and urbanization, the rural settlement system and spatial structure have been constantly changed. The research on rural transformation and reconstruction has attracted scholars’ attention, involving rural economy, society, space and other aspects [
17,
18,
19,
20]. In terms of distribution characteristics, landscape index, spatial syntax, fractal theory and other research methods are used to study the distribution characteristics of rural settlements from different aspects. For example, Hudson [
21] analyzed the distribution system of rural settlements in Iowa on the basis of central place theory and diffusion theory; Weisler [
22] studied the settlement space structure of Polynesian bacteria states from the perspective of historical evolution; Conrad [
23] used remote sensing technology to measure the expansion of the spatial scale of rural settlements in Uzbekistan; Gallarati [
24] combed the context of landscape and environment at various scales and studied the space and type of rural settlements. The research focused on the spatial layout of settlements [
25], rural land use [
26], settlement scale and form [
27], etc. In terms of influencing factors, scholars in western countries started to study the influencing factors of rural settlement layout early. As early as the 1940s, they carried out a discussion on the relationship between rural settlement distribution and geographical environment [
28]. With the development of social economy, the focus of research has gradually shifted to the human, economic and social directions, paying more attention to the relationship between population density and rural settlement distribution [
29], the relationship between economic transformation and rural settlement distribution [
30], the relationship between policy system and rural settlement distribution [
31], the relationship between farmers’ behavior and rural settlement distribution [
30]. The research on influencing factors of rural settlement has experienced a change from focusing on natural factors to comprehensively considering various factors, such as natural, social, economic and cultural factors, and the perspective of research tends to be comprehensive.
The study of rural settlements in China started relatively late. Influenced by the academic research trend of geography in western countries, Lin [
32] and other geographers of the older generation began to study rural settlements in the 1930s. Summarizing the research results of rural settlements in China, the study can be roughly divided into four stages: embryonic start (before 1949), preliminary development (1949–1978), rapid development (1978–2000), transformation and reconstruction (2000-present) [
33,
34,
35]. Taking a general view of the research achievements related to rural settlements in China, a summary was made from the three dimensions of research contents, methods and scales. (1) In terms of research contents, the research mainly focused on the spatial pattern [
36,
37], evolution characteristics [
38], influencing factors [
39] and optimal regulation [
40] of rural settlements. (2) In terms of research methods, multiple methods such as GIS spatial analysis [
41], econometric analysis model [
42] and field investigation method [
43] were applied to the spatial analysis of rural settlements, showing a trend of cross-integration of multidisciplinary research methods. (3) In terms of research area, the current research mainly focused on the Pearl River Delta [
44], Yangtze River Delta [
45], Beijing-Tianjin-Hebei [
46], etc. The research area mainly focused on the developed coastal areas in the east, and the typical areas (hilly area [
47], loess area [
48]) were also involved. Some scholars have also paid attention to the spatial pattern of rural settlements in metropolitan areas [
49,
50]. Through the review, it can be found that the current research focused more on the description of the spatial pattern characteristics of rural settlements. The discussion on the spatial characteristics and formation mechanism of rural settlements in different typical regions was relatively weak.
As a transitional zone between urban and rural areas, the metropolitan fringe is faced with an overall transformation of economic, social and spatial structures. However, the spatial evolution of rural settlements located in the metropolitan fringe was influenced by rural urbanization. Compared with the traditional rural settlements, the rural settlements located in the metropolitan fringe were affected by the radiation and driving effect of the urban core area, and their spatial characteristics were characterized by complexity and diversity [
51]. Nanjing is located in the Yangtze River Delta urban agglomeration, which is a metropolis in the coastal development area of eastern China. In the process of rapid urbanization, the rural settlements in the urban fringe are facing transformation and reconstruction, which can better reflect the characteristics of the rural settlements in the metropolitan fringe. In view of this, this paper chose Nanjing as a typical case, based on the interpretation of remote sensing image, using R statistics, kernel density analysis, hot spot detection analysis and semi variogram function, the paper analyzed the distribution characteristics and influencing factors of rural settlements in the metropolitan fringe area. The objective of these analyses is to address the following research goals: (1) What are the distribution characteristics of rural settlements in the metropolitan fringe? (2) What are the factors affecting the distribution of rural settlements in the metropolitan fringe? The research structure of this paper is as follows (
Figure 1).
What are the distribution characteristics of rural settlements in the metropolitan fringe area? Using R statistic, kernel density analysis, hotspot detection analysis and semi-variogram, from three different dimensions: scale distribution, space distribution and morphological distribution, this paper analyzed the spatial distribution characteristics of rural settlements in the metropolitan fringe area.
What are the factors affecting the distribution of rural settlements in the metropolitan fringe area? On the basis of theoretical analysis of influencing factors, terrain, river system, traffic, economic, social development, cultural and policy factors were adopted to analyze the internal relationship between them and the distribution of rural settlements, and revealed the influencing mechanism of the distribution pattern of rural settlements in the metropolitan fringe area.
4. Discussion
(1) Rural settlement is the evolution of the long-term integration of human and nature. Rural settlement is a complex system involving social, economic, ecological, resource and other factors. The distribution of rural settlements is characterized by regional differences. At present, the academic community has carried out research on the distribution law of rural settlements in different regions. Tang [
67], Guo [
68], Ma [
69] and others have focused on the distribution law of rural settlements in northwest China. They have selected Yulin City in Shaanxi Province, Qin’an County in Gansu Province, Tongwei County in Gansu Province, such as case to carry out empirical research. They found that rural settlements in northwest China presented a small-scale decentralized distribution pattern; the natural geographical environment (elevation, slope, river, farmland, etc.) had a decisive impact on the distribution of rural settlements in northwest China, and the change of natural environment directly affected the distribution of rural settlements.
By comparison, this paper chose the rural settlements in the metropolitan fringe area as the research object. Through the empirical analysis of Nanjing, it was found that the distribution of rural settlements in the metropolitan fringe area presented a large-scale agglomeration distribution pattern, which was mainly affected by economic and social development factors, while the natural environment factors had less impact on the distribution of rural settlements. This conclusion was quite different from the distribution law of rural settlements in northwest China. This difference was mainly attributed to the fact that the metropolitan fringe area was a transitional zone between cities and villages. The rural settlements in the metropolitan fringe area were a concentrated reflection of the human-land relationship, with significant characteristics of rapid economic development and urbanization. Under this influence, the rural settlements in the metropolitan fringe area were facing or experiencing dramatic spatial evolution and modern transformation, and their spatial distribution and evolution patterns were different from those of the rural settlements in other regions.
(2) The spatial distribution of settlements can be used as a basis to show the comprehensive relationship between human activities and the natural environment in a region and has reference value for the optimization of the spatial pattern of settlements. Based on the analysis of the distribution characteristics and influencing factors of rural settlements, combined with the field research in this typical region, the optimization of the layout of rural settlements in the metropolitan fringe area was attempted to be divided into four types: urban transformation type, key development type, limited development type, and relocation type [
70,
71,
72].
Urban transformation type. This type refers to the rural settlements distributed at the edge of the county seat and the central town and near the main traffic arteries. Suggestions for optimizing layout: bring rural residential areas close to built-up areas or central towns into the urban planning system, actively guide the transformation of rural residential areas into urban residential areas, increase the construction of transportation and other infrastructure, emphasize the functional zoning of internal land, and form an all-round and multi-level land use pattern.
Key development type. This model mainly refers to the rural settlements which are far away from the urban center, large scale, transportation location and good level of economic development. Suggestions for optimizing layout: the rural residential areas with small scale and poor conditions in towns and townships should be relocated to the central village nearby, focusing on the construction of the central village within the city scope; and improve the basic and public service facilities of central village, based on the resource advantages of central village, develop and expand the characteristic industries, attract the surrounding small natural villages to gather in the central village.
Limited development type. On the premise of the stability of the original spatial pattern of rural residential areas, this type of rural residential areas should be rebuilt and reasonably developed. Through promoting the renovation and construction of rural residential areas, the potential of the village’s internal land use should be fully exploited, and the village, especially the hollow village, should effectively “lose weight” to improve the intensive use of rural residential land in hilly areas.
Relocation type. This type of rural residential area is mostly located in areas with poor suitability level of urban residential areas, with shortage of cultivated land resources, inconvenient transportation and more villagers going out to work. Suggestions for optimizing layout: gradually move to another place by taking multiple approaches such as urban resettlement, central village resettlement, small villages merging into large villages and building independent new villages.
5. Conclusions
This paper took the rural settlements in the metropolitan fringe area as the research object, taking Nanjing as a typical case. From three aspects of spatial distribution, scale distribution and form distribution, this paper analyzed the distribution characteristics of rural settlements in the metropolitan fringe area. On this basis, this paper tried to reveal the influencing factors in the distribution of rural settlements in the metropolitan fringe area. The following conclusions were drawn:
(1) The spatial distribution of rural settlements was significant in the metropolitan fringe area, it showed the characteristics of “agglomeration” spatial pattern. The spatial distribution of rural settlements generally presented a “multi-core” center, and a spatial distribution trend of stepwise declined from the core to the periphery, showing a typical “core-edge” structure. The core of rural settlements in the metropolitan fringe was mainly distributed in the agricultural counties in the outer suburbs, while the surrounding villages in the main urban area were radiated by the city, and the population was urbanized locally. Rural settlements gradually evolved into urban settlements, resulting in less spatial distribution of rural settlements.
(2) There were significant differences in the scale distribution of rural settlements in the metropolitan fringe area, showing that the scale of rural settlements gradually decreased with the increase in the distance from the central city. The overall distribution pattern was that the scale of rural settlements in the near suburbs was large, the scale of rural settlements in the outer suburbs was moderate, and the scale of rural settlements in remote areas was small. The closer the rural settlement was to the built-up area, the more affected by human activities and policy factors, the easier it was to form a large-scale rural settlement distribution.
(3) The morphological distribution of rural settlements had good stability in the metropolitan fringe area, and the spatial self-organization of the distribution of rural settlements was strong, which showed that structural factors (topography, geomorphology and other geographical environmental factors) and random factors (economic development, policy system, etc.) jointly played a role in the differentiation of rural settlements in the metropolitan fringe area. The morphology of rural settlements mainly included strip, arcbelt, cluster, scatter types, the formation of different settlement types was closely related to the natural geographical environment, historical and cultural factors.
(4) The distribution of rural settlements in the metropolitan fringe area was mainly affected by topography, river system, traffic, economic development, cultural and policy. Among them, the distribution of rural settlements had the location orientation of “low altitude, close to river and close to road”, and the natural geographical environment has laid the foundation for the distribution pattern of rural settlements in the metropolitan fringe area. The increase in agricultural population and the development of rural economy played a leading role in the distribution of rural settlements in the metropolitan fringe area; the cultural and policy factors played an important guiding role in the distribution and reconstruction of rural settlements in the metropolitan fringe area.