Objective The most significant problem caused by urban shrinkage is the phenomenon of large-scale urban spatial vacancy, which has generated a large quantity of vacant lands, buildings, and infrastructure in cities. As “gray area” in urban space, vacant land is prone to arouse waterlogging and rainwater siltation, which will not only decrease the quality of urban space, affect cityscape and result in landscape fragmentation, but also generate economic and social problems. Indeed, the renewal of vacant land into green infrastructure (GI) has been proven to be an effective way to alleviate negative effects caused by urban shrinkage. At present, there exists a research gap on the evaluation of the potential for renewal of vacant land from the perspective of both function and structure. It is of great scientific significance to explore the potential for renewing urban vacant land into GI from this perspective to realize the high-quality development of shrinking city.
Methods Based on the characteristics of GI, urban vacant land is evaluated from the perspective of both function and structure. At the functional level, ecosystem services are selected as the evaluation indicator. While at the structural level, landscape connectivity is selected as the evaluation indicator. The potential for renewing urban vacant land into GI is calculated by equally weighted overlap analysis. This research selects 2 regulation services (air purification and water purification), 3 support services (soil conservation, biodiversity, and vegetation coverage), and 3 cultural services (landscape aesthetics, recreational potential, and recreational accessibility) as main ecosystem services evaluated targeting the renewal potential of urban vacant land. Also, this research selects patch importance as an indicator of landscape connectivity. Finally, ArcGIS 10.8 is used for calculations to reflect the spatial distribution and potential evaluation results.
Results In terms of functionality, the overall results show a distribution trend of being high in the north and low in the south. Almost all areas with high ecosystem service supply are lands with minimal human activity damage, such as forests and grasslands; meanwhile, areas with low ecosystem service supply are mostly the lands left behind under the most severe impact of human activities. The vacant lands with high ecosystem service supply account for 26.47% of the total. In terms of structure, the overall landscape connectivity of vacant lands in the central urban area of Hegang is not high, with only a few vacant lands being of medium or higher patch importance. Vacant lands with high connectivity are distributed scatteredly in each district, which account for 16.26% of the total. There exists a correlation in spatial distribution between the evaluation results of the renewal potential of the vacant lands and the evaluation results of function and structure in the central urban area of Hegang. The renewal potential of the vacant lands is not high overall. Vacant lands with high potential only account for 22.19% which almost owned large area in each district. The spatial distribution pattern of the potential evaluation with respect to the renewal of urban vacant land into GI is similar to that of the functional evaluation, both showing a pattern of being high in the old urban area and gradually decreasing from urban center to the suburb. Most of the vacant lands in the central urban area of Hegang are of low potential for renewal into GI, with the vacant lands with high potential being distributed scatteredly in each district. There is only one vacant land with the highest renewal potential, located at the junction of Xiangyang District and Dongshan District in the northern part of the central urban area with an area of 105.39 hectares, accounting for 7.87% of the total. Vacant lands with high renewal potential are also quite few, with a total area of 191.88 hectares, accounting for 14.32% of the total.
Conclusion For each potential level, this research proposes four renewal modes of increasing, merging, inserting and reserving for vacant lands to be renewed into GI. The research believes that it is possible to strengthen the function and optimize the pattern of GI network in the central urban area of Hegang by adding network centers, generating new connecting corridors, widening the original connecting corridors, or building small sites. For urban vacant lands with each potential level, the research proposes to base corresponding potential evaluation on resource endowment and maximize their green advantages; assist ecological restoration and achieve moderate and comprehensive development; meet the needs of residents and adapt to local conditions; prioritize ecological restoration and ensure a flexible and resilient planning strategy. The research quantifies the functional and structural levels of each vacant land in the central urban area of Hegang, identifies the styles of such vacant lands, and proposes suitable improvement mode to improve the GI network. Based on the location and prominent functional and structural characteristics of each vacant land, corresponding strategies are proposed, aiming to provide a basis for urban vacant land renewal.