Objective With the continuous socioeconomic development after entering the 20th century, intensive development of land and rapid change of land use mode, human disturbance to the ecosystem becomes increasingly severe, and the ecosystem is facing the dilemma of imbalance and function degradation, which directly affects the regional landscape pattern and sustainable development. It is urgent to optimize the function of ecosystem to maintain the stability of ecosystem structure. Ecological security pattern (ESP) is considered as an appropriate approach to solve these intractable problems. ESP composed of ecological source, ecological corridor and ecological node, is an effective way to maintain natural ecological process, improve and realize regional ecological security. The construction process of ESP has important ecological significance for maintaining and controlling a certain regional ecological process. Urban fringe is the outer part of urban built-up area, characterized by dynamic and unstable structure. The surrounding ecosystem is particularly sensitive to the changes in the area of urban fringe. The unstable structure of ecosystem in this area has aroused wide attention, for which the rational green space planning is particularly critical. In this research, Shijiazhuang metropolitan area is taken as the research object. Under the background of Beijing-Tianjin-Hebei integrated development, Shijiazhuang will usher in a new round of urban construction. The contradiction between urban expansion and ecological conservation is increasingly prominent, and the competition for land use is becoming fierce in the metropolitan area. In view of this, it is urgent to establish regional ESP to optimize regional space.

Methods The research framework is developed from the two aspects of ESP construction and optimization. The framework involves three steps. 1) Integrate ecological sensitivity indexes of key ecosystem services to extract regional ecological sources; 2) identify regional ecological corridors and build a regional ESP by the minimum cumulative resistance (MCR) model; 3) calculate the relative importance and connectivity of regional ecological corridors in combination with interaction force and landscape connectivity to determine their priorities, and based on the radiant capacity of ecosystem services and landscape connectivity, calculate the radiant force and connectivity of regional ecological sources to determine their priorities, based on which optimize the regional ESP.

Results The ESP in Shijiazhuang metropolitan area is composed of 20 ecological sources and 24 ecological corridors. These ecological sources are dominated by those distributed in the western shallow mountain area and along the banks of the Hutuo River, and are interconnected in the northwestern area, with a widespread ecological resource connecting with other ecological resources in the eastern farmland area. The research finally identifies 10 ecological corridors and 7 ecological sources with the highest priority, and finally concludes as follows. 1) The ecological sources with the highest priority cover 90% of local water body, indicating the importance of water body in the metropolitan ecosystem. 2) The research delineates a buffer zone of 300 m for each ecological source, wherein the ecosystem services have the strongest radiation force, and the buffer zone can be used as the potential development land of ecological source. 3) In the priority assessment of ecological sources and ecological corridors, the core ecological corridors and sources performing well in connectivity, relative importance and radiation force are mainly distributed in the western shallow mountain area and along the Hutuo River, which are important areas deserving special attention. It is supposed to establish protection zones for relevant sources, with a regional green space network based on connectivity assessment to strengthen the stability of regional ecosystem. 4) Performing poor in connectivity, relative importance and radiation force, ecological sources and ecological corridors with low priority are concentrated in the southern part of the metropolitan area, while in the northern part, there is a lack of ecological sources and ecological corridors. Due to urban expansion and the change of surrounding land use, ecological sources in the southern and northern parts of the metropolitan area are mostly fragmented small-scale plots. In the future development, attention should be paid to the protection and promotion of green space, so as to maintain ecosystem functions and provide more ecosystem services for high-density cities. 5) It is noteworthy that some scattered ecological sources and ecological corridors near urban areas also show excellent radiation power, connectivity and relative importance, which can not be ignored in future urban expansion.

Conclusion The research can help deepen the understanding of regional ecological resources in future territorial spatial planning and ecological civilization construction, and provide guidance for corresponding green space planning. In addition, although the Shijiazhuang government has defined the ecological conservation redline at the municipal scale, there is still a lack of refinement of regional characteristics at the metropolitan area scale. This research provide reference for the optimization and adjustment of the ecological red line in the metropolitan area.