The cities and villages of developed countries, like Britain, France, the USA, and Japan, have broadly evolved into a state of balance [1
]. However, influenced by the extreme differences between urban and rural areas [3
], the contradiction of urban-rural development in China remains prominent. Urban and rural areas are equally essential and should be combined organically [5
]. Therefore, how to intensively make use of urban-rural resources and spaces, rationally optimize the overall layout of urban and rural areas, protect and rectify the urban-rural ecological environment, and coordinate the relationship between urban and rural development to promote the sustainable and healthy development of urban-rural society and economy, is one of main tasks confronting China at the moment. The urban-rural spatial structure, a regional reflection of humans’ social, economic, and cultural activities under specific circumstances, illustrates the spatial dichotomy between urban and rural areas. In addition, with the rapid development of urbanization, urban sprawl has brought great challenges to urban and rural eco-environments [6
], especially in China [9
]. How to protect and maintain ecosystem security during the process of urban growth, to resolve the contradiction between ecological protection and urban—rural development, is another major problem that is necessary to solve. Against this background, studies on urban-rural spatial structure face new challenges, that is, using the bottom-line thinking of taking “three areas (urban, agricultural, and ecological spaces) and three lines (the red line of the ecological areas, the red line of permanent basic farmlands, and the urban development boundary)” as impassable boundaries of spatial development while ensuring the national layout of urban-rural spaces. This new challenge would first require the land use in urban and rural areas to be arranged from the perspective of ecology, so that the urban-rural spatial structure can evolve toward sustainability and ecological security.
It is an historical idea that the urban spatial structure can be arranged from the perspective of ecology. The ancient Chinese “Feng Shui [11
]” model and Western “garden construction [13
]” model, Howard’s “Garden Cities”, Saarinen’s “Theory of Organic Decentralization”, and Soleri’s “Bionic City” in the late nineteenth and early twentieth centuries are representations of this idea. After the 1950s, the outbreak of the global environmental crisis made human beings rethink their relationships with nature. McHarg suggested that urban design should be connected with nature and described how to conduct land development via natural processes in his book [15
] where his ideas initiated the exploration of urban ecological space. Thereafter, words like “Ecological Priority”, “Ecological Infrastructure”, and “Green Infrastructure” emerged and are now used in ecological urban studies [16
]. Green infrastructure is the network of natural and semi-natural green and blue areas [19
], which play an important role in the process of urban and rural sustainable development. Green infrastructure provides support for ecological processes and can contribute to better human health and wellbeing [18
]. The application and protection of green infrastructure is beneficial to mitigate urban sprawl and promote the rational development of the urban spatial structure [24
]. However, previous studies have mainly concentrated on discussing the optimization or reconstruction of urban spatial structures from an ecological perspective. It is a complex and long process to change or shape the spatial structure of a city, which is why some researchers have developed simulation studies on urban development in the future using Cellular Automata (CA) models [25
]. The research objects in these studies are actually urban land expansion or urban interior spatial structures. The Lowry model can simulate the urban spatial structure in terms of land use and transport [28
]. However, this model is limited to explaining the formation mechanism of current urban spatial structures. With reference to existing research [29
], the urban-rural spatial structure of this study refers to the distribution and combination of the point-line-surface in space, where points are settlements, lines are traffic lines, and surfaces are the land use division corresponding to the hierarchical size structure, the traffic network structure, and the land use zoning structure, respectively.
In this study, in order to promote the sustainable development of urban and rural space, an integrated simulation model from an ecological perspective was proposed. This model primarily includes four parts: (1) a green infrastructure assessment; (2) hierarchical size structure extraction; (3) traffic network structure simulation; and (4) land use zoning structure determination. Using a quantitative evaluation method, the CA model, the Least Cost Path (LCP) model, and a classification of ecological importance, coupled with the spatial analysis technology of Geographic Information System (GIS), this study simulated the spatial structure of the Harbin city territory in 2035. The quantitative evaluation method was used to assess green infrastructure. The results of this green infrastructure will be an ecological constraint on the model, which is the fundamental basis of the integrated model. The CA model is used to simulate urban-rural growth, from which the hierarchical size structure can be extracted. The LCP model can simulate potential traffic paths, from which the traffic network structure can be obtained, combined with the present and planned traffic situations. The classification of ecological importance is the basis of the land use zoning, aiming to give priority to determining the scope of ecological spaces. It can be concluded that this integrated model has limitations, but can reflect the pattern of urban-rural spatial structures in the future to some extent. Furthermore, in 2035, the urban-rural spatial structure of Harbin will show an increasing development tendency toward a single-center, and traffic systems above the county level will also be improved.
This study offers two contributions to knowledge: (1) This study attempts to construct an integrated simulation model to simulate urban-rural spatial structures; and (2) this study offers an analysis of Harbin’s urban-rural spatial structure on a regional scale from an ecological perspective. Previous research on urban spatial simulations have focused on central Harbin city [31
] or Harbin city [32
]. The scope of this study will be the administrative area of Harbin, that is the Harbin city territory. This territory contains Harbin city and nine counties (or county-level cities). Research on the spatial structure of Harbin aims to reveal the spatial expansion and evolution of the construction land in central Harbin city [33
] without considering rural areas and ecological problems. Additionally, some studies have placed Harbin city into a bigger scale and broader context, analyzing the urban agglomeration’s spatial structure evolution [36
] and its patterns [37
The main purpose of the study was to understand the patterns of the urban-rural spatial structure under ecological security via the constructed simulation model to propose optimization and implementation strategies in the next step. This study also aims to accelerate harmonious urban and rural development through overall consideration of urban-rural settlements, traffic systems, and land use zoning, thus further promoting the achievement of the final target of urban-rural integration. This is a significant and key aspect of Chinese national spatial planning.
This study simulated and predicted the overall arrangement of the urban-rural spatial structure under the basis of ecological constraints in order to maintain the lowest effect on ecosystem security when urban-rural development is in progress. Controlling urban growth is a common approach for promoting urban-rural sustainable development [73
]. Green infrastructures in this study were assessed in terms of resource protection and ecological services, instead of only from the perspective of ecological services [51
]. The results showed that key areas, like natural reserves, gained a higher value of importance. This would be especially helpful for the establishment of subsequent development strategies.
This study considered settlements, traffic systems, and land use zoning as the three determinants of urban-rural spatial structures. In the process of hierarchical size structure extraction, although the Moran I index, which can reflect the extent of spatial concentration or dispersion, is lower for 2035 than for 2015, medium and small cities of Harbin will be further developed by 2035. These results are in accord with the national strategy in China. The rapid development of medium and small cities could have positive effects on the development of surrounding rural areas [76
], which is of great significance for realizing balanced regional development and urban-rural integration, especially for developing countries. Meanwhile, the urbanization progress of the central city (Harbin city) is also continuing, whose speed is faster than that of medium and small cities. Consequently, the urban primacy index increased, which indicates that the centrality of the central city will still increase. The traffic network supports urban-rural material flow and is the necessary basis for urban-rural integration. Many studies have predicted traffic flow and explained spatial-temporal correlations [77
], or forecasted traffic volume through a significant number of surveys [79
], but their aims were not to explore the spatial layout of the traffic network. According to the simulation result of this study, the traffic network structure of Harbin in 2035 tends to be more complex. The grade and quality of the roads will also be enhanced. Drawing on the experience of urban-rural integration in developed countries [80
], the current service capacity including the grade and quality of highways and railways will be improved to connect all settlements through the traffic network. It should be noted that because the Harbin railway development will mainly be fulfilled by upgrading existing railway facilities, the layout of the railway in this study was maintained the same as the current situation. This could be further studied when more data are available. The land in this study was zoned in terms of living, productive, and ecological functions responding to national spatial planning in China. Finally, the overlaid result showed that the urban-rural spatial structure still featured a significant single center. This is currently a common phenomenon for the development of regions in China. A review of urbanization in developed countries has found that the tendency to develop a single center could be an inevitable process preceding urban-rural integration [82
In accordance with the equivalents of the ecosystem service values supplied by the per unit area of the ecosystem [50
], water bodies can offer a superior water resources supply service, hydrological adjustment service, biodiversity support service, and cultural service; forests can provide a better gas regulation service, climate regulation service, and biodiversity support service; while agricultural land can offer a better food supply service. The application of the integrated model in this study can lead to policy recommendations that may support more sustainable development in Harbin. For example, through the policy of prohibiting development activities in water bodies and forest areas and the restricting misuse of agricultural land, multiple ecological services including water resource supply services, hydrological adjustment services, biodiversity support services, and so on, can be preserved to support sustainable development in Harbin.
Previous research on spatial structure only simulated land use structures from the perspective of the surface [83
], while the integrated model developed in this study comprehensively predicts the urban-rural spatial structure in the future from a point-line-surface perspective. In addition, the model of this study can determine the sequence of land development in advance on the basis of the results of a green infrastructure assessment. Then, the urban-rural spatial structure can be simulated. Therefore, the integrated model in this study can be considered as a model of sustainable development. The simulation results based on this model could effectively protect important ecological resources. To further explain the strength of the integrated model in this study, a comparison between the ecological constraint scenario that we developed and the scenario of urbanization as usual should be undertaken. This work is being done at present. Furthermore, to realize the results of the simulation in this study, some optimization measurements must be undertaken, for example, how to promote the fast development of medium and small settlements in Harbin, how to improve the service capacity of the traffic network in Harbin, and how to control and manage developing activities in ecological, agricultural, and construction spaces for Harbin. All of these questions may be addressed in future research in this area.
This study focused on problems at the macro scale and considers urban areas and rural areas in a comprehensive way. To some extent, by giving priority to the protection of areas with higher ecological importance to restrict the location of construction land development, the problem of mixed urban-rural land uses (which entails more detailed and complicated issues such as fragmentation, leap-frog development, and low-density urbanization) can be modified. However, this study is not concerned about problems of large-scale development within the city. For these problems, some restoration and compulsory measures will be necessary. This will also be studied more deeply in the future.