As a global leader in sustainable development, the World Commission on Environment and Development (WCED) proposed that the environmental policies and urban development of every country should actively respond to the goals of sustainable development [
1]. Over the past few decades, China has undergone rapid urbanization, leading to significant changes in the spatial pattern of urban areas [
2]. However, this development has also brought about ecological degradation, environmental pollution, resource scarcity and other issues. In order to achieve the Sustainable Urban Development Goals by 2030 [
3], the Chinese government has put forward new requirements for territorial spatial planning, namely the delineation of “Three Zones and Three Lines”
1 [
4] to construct a comprehensive and coordinated spatial control system. Urban space, as the carrier of the regional economy and environment, is crucial for sustainable development. Guiding the development of urban areas in a rational manner and harmonizing the relationship between urban development and ecological protection are key to achieving sustainable economic and social development. For cities located in ecological functional areas, there is a higher degree of overlap between nature reserves and poverty-stricken areas, leading to obvious conflicts between resource protection and development. The conflicts between urban spatial development and ecological/agricultural spaces are more pronounced, and focusing solely on urban development planning may have negative impacts on natural resource conservation and ecological sustainability [
5]. The new planning system emphasizes the integrity and systematic nature of territorial spatial planning and highlights the classification and governance of land. The changes in ecological space, agricultural space, and urban space (“Three Zone Space”) derived from land use abstractions reflect the extent and manner of the influence of human activities. This represents an important link between the macroscale (main functional areas) and microscale (land use planning). Therefore, it is necessary to study the evolution and driving mechanisms of urban spatial development from the perspective of the systemic integrity of “Three Zones” to effectively control the pace and scale of urbanization.
The Three-River-Source Region, located in the hinterland of the Qinghai–Tibet Plateau, is the origin of the Yangtze River, Yellow River, and Lancang River [
6]. It is often referred to as the “Asian Water Tower” [
7] and is recognized as an area sensitive to global climate change and an important ecological functional zone in China [
8]. The region boasts numerous natural protected areas, accounting for 42% of the total area, while human settlement space is extremely limited. However, with the implementation of strategies such as the Western Development and the continuous improvement of socio-economic conditions, urban space in the Three-River-Source Region has rapidly expanded, encroaching upon ecological and agricultural areas and leading to conflicts between development and conservation, resulting in strained human–land relationships. To address this issue, it is necessary to establish a structurally rational and spatially ordered land resource element pattern from the perspective of the “Three Zone Space”, Specifically, it is important to describe the direction and quantity relationship between urban space and the other two types of spaces. By analyzing the distribution range, centroid, clustering degree, and directional changes of urban space at each time slice, the overall direction of urban-space change can be determined. This will help establish an optimized pathway for improving the urban spatial pattern. The use of a spatial cross-conversion matrix [
8] and standard deviation ellipse [
9] methods is suitable for achieving this goal. Currently, research on the Three-River-Source Region primarily focuses on areas such as ecosystem services, climate change, vegetation, and biodiversity [
10,
11,
12], with limited attention given to land use, particularly in relation to urban space [
13,
14,
15,
16]. In contrast, research methods concerning urban space are relatively mature, with refined measurement approaches represented by spatial expansion-related indices and compactness [
17,
18,
19,
20]. Additionally, spatial autocorrelation methods are widely used to identify, explain, and analyze patterns and trends in land use changes [
21]. The most commonly used methods are Moran’s I index (local Moran’s I index) and G coefficient (Getis-Ord local G). Among them, the former is often used to reveal the spatial patterns of the entire study area and is suitable for describing the long-term spatial pattern evolution characteristics in the Three-River-Source Region. Therefore, in this study, Moran’s I index was selected for global spatial autocorrelation assessment and significance testing. Regarding driving mechanisms, spatial econometric models and system dynamics models are widely employed, primarily focusing on the driving forces of socio-economic, population, and natural factors, while neglecting the quantification of cultural, policy, and other human factors. This poses certain limitations for the Three-River-Source Region, which is heavily influenced by ecological policies and Tibetan culture [
22,
23]. In comparison, Geodetector exhibits high detection sensitivity and its principle ensures the model’s immunity to multicollinearity among multiple independent variables. This model has been widely applied in various fields, including the analysis of urban spatial expansion, prediction of ecological risk factors, spatiotemporal evolution of land use/land cover types, spatial differentiation characteristics, and spatial layout. Importantly, this method can detect both numerical and qualitative data, which gives it an advantage in measuring policy and cultural factors. Therefore, this study utilizes an optimized parameter Geodetector tool to explore the impact of ecological conservation policies and Tibetan cultural factors on the evolution of urban space, aiming to provide a more comprehensive understanding of the spatiotemporal evolution characteristics and mechanisms of urban space in the Three-River-Source Region. In summary, there is currently a lack of comprehensive, long-term, and functionally integrated research on the spatial patterns, evolutionary processes, and mechanisms of urban space. Therefore, this study, within the framework of the “Three Zone Space,” examines the land resource element pattern of the Three-River-Source Region from the establishment of the market economy system to the present (1992–2020), quantitatively analyzes the scale changes, spatial transformations, and inherent driving factors of urban space, and fills the research gap in the long-term analysis of urban space in the context of territorial spatial planning in the Three-River-Source Region.
Urban areas are complex and open systems, and the Three-River-Source Region has its own unique characteristics in terms of urban spatial development due to its special natural geographical environment and diverse ethnic cultural background. These characteristics include prominent ecological features and significant regional transportation restrictions. The development of urban areas can provide employment opportunities and income sources for herders. With increasing population and input from herders, small towns play an increasingly important role in promoting pastoral area development and improving herders’ livelihoods. Since 2000, the Chinese government has implemented a series of major ecological projects in the Three-River-Source Region, constructing and completing a natural protected area system primarily based on national parks. These measures have had a positive impact on the development of urban tourism and public facilities, promoting the development and construction of urban spatial areas [
24]. In the context of ecological civilization construction and the establishment of national parks in the Qinghai–Tibet Plateau, studying the urban spatial evolution and driving mechanisms in the Three-River-Source Region is not only significant for ecological environment protection, economic development, urban planning and construction, and cultural heritage conservation but also helps to better understand the process of urbanization and the implementation of relevant policies. It provides support for urban planning and management and promotes the achievement of sustainable urban development goals. The objectives of this study are as follows: first of all, to construct an analysis framework for the spatiotemporal evolution of urban spatial development in the Three-River-Source Region from the perspective of the ecological-agricultural-urban spatial system. Secondly, to describe in detail the spatiotemporal evolution characteristics of urban spatial development in the Three-River-Source Region. Lastly, to quantify the driving forces of natural changes and human activities on the evolution of urban spatial development.