Content area
The spatial form of historic blocks comprises multi-scale elements, and its evolutionary process is the result of residents" long-term adaptation to and coexistence with the external environment. This micro-level, dynamic, and complex system requires an interpretation grounded in theoretical frameworks that align more closely with real-world logic. This paper integrates Conzenian urban morphology, the adaptive cycle theory of social-ecological systems, and the complex adaptive system theory to develop a research framework for analyzing the morphological evolutionary characteristics, mechanisms, and pathways of historic blocks from a complex adaptive perspective. Taking the Hui-Fang area in Xi'an, a historic block, as an example, the paper employs quantitative methods, such as fractal dimension analysis and space syntax, to uncover both the complex external manifestations of the morphological system and the underlying adaptive cycle mechanisms of the agent system. Furthermore, it identifies three distinct adaptive evolutionary pathways of spatial form: unidirectional control, bidirectional guidance, and multi-level symbiosis. This research expands epistemological understandings of morphological evolution and provides theoretical support for the development of urban spaces in historic blocks that reflect regional cultural significance.
Abstract The spatial form of historic blocks comprises multi-scale elements, and its evolutionary process is the result of residents' long-term adaptation to and coexistence with the external environment. This micro-level, dynamic, and complex system requires an interpretation grounded in theoretical frameworks that align more closely with real-world logic. This paper integrates Conzenian urban morphology, the adaptive cycle theory of social-ecological systems, and the complex adaptive system theory to develop a research framework for analyzing the morphological evolutionary characteristics, mechanisms, and pathways of historic blocks from a complex adaptive perspective. Taking the Hui-Fang area in Xi'an, a historic block, as an example, the paper employs quantitative methods, such as fractal dimension analysis and space syntax, to uncover both the complex external manifestations of the morphological system and the underlying adaptive cycle mechanisms of the agent system. Furthermore, it identifies three distinct adaptive evolutionary pathways of spatial form: unidirectional control, bidirectional guidance, and multi-level symbiosis. This research expands epistemological understandings of morphological evolution and provides theoretical support for the development of urban spaces in historic blocks that reflect regional cultural significance.
Keywords adaptive cycle; spatial form; complexity; historic blocks
A historic block is a specific area within a city that holds historical significance and contains living functions. Unlike other areas, it typically follows a development pattern shaped by a combination of government guidance, resident self-organization, and gradual renewal. Its morphological evolution primarily arises from collective adaptive behavioral rules that develop over time as residents adapt to and coexist with the external environment. These behavioral rules frequently influence micro-level architectural transformations, becoming the central force driving morphological changes. Therefore, a theoretical framework that better aligns with the dynamic morphological value of historic blocks is needed to fully interpret their evolutionary processes.
Current urban morphological inquiries have progressed along distinct trajectories, molded by varied disciplinary traditions and motivated by heterogeneous research agendas. Nonetheless, different schools of thought share conceptual and analytical similarities. At the epistemological level, both the German-British historical geography school, represented by Conzen, and the Italian historical architecture school, represented by Muratori, regard urban morphology as a discipline concerned with the study of urban structure, open space layout, and spatial composition, or the tangible outcome of ongoing interactions between human activities and their environments - encompassing physical, cultural, political, and economic dimensions. This discipline requires consideration from three aspects: morphological elements, mechanisms, and time (Zhang, Tian, and Gu, 2012). At the methodological level, morphological studies follow a process of knowledge acquisition, analysis, and validation. Common research methods include historical-geographical analysis, building typology analysis, and complex systems-based approaches that evaluate the influence and dynamic changes of factors affecting morphological elements. These conceptual foundations, theoretical frameworks, and analytical models offer multifaceted insights for this study. Nevertheless, existing research has paid insufficient attention to the complexity, dynamism, and hierarchy of interactions between human behaviors and physical spatial form. Accordingly, integrating the adaptive cycle theory into the traditional analytical framework of urban morphological evolution requires a clear analytical structure - encompassing theoretical construction, research pathways, and model validation - in order to achieve a deeper understanding of morphological evolution.
1. Theoretical construction of an adaptive cycle model for spatial form in historic blocks
1.1 Key concepts
1.1.1 The morphological system
In line with the conceptual implications and core attributes of urban morphology and the complex adaptive system (CAS) theory, this paper conceptualizes spatial form as a multi-level nested complex adaptive system characterized by a multi-layered hierarchy, nonlinearity, and adaptation-driven complexity.
(1) Multi-layered hierarchy
Complexity in a complex adaptive system takes the form of hierarchy, which serves as an essential condition. The evolution of system hierarchies can be understood as a process of continuous complexity generation (Simon, 1987). In terms of historic blocks, first of all, they often exhibit more diverse social and cultural organizational structures. Second, their morphological elements inherently possess complex hierarchical relations. In accordance with Conzenian urban morphology, urban form consists of fundamental morphological elements - including blocks, streets, plots, and buildings - which interact in a way that moves from parts to wholes, ultimately forming a sophisticated hierarchical structure (Kropf, 2014). Such hierarchy reflects the complexity and diversity of external spaces. The interconnected layers of hierarchy collectively constitute the overall morphological characteristics of urban areas.
2) Nonlinearity
In line with the CAS theory, a CAS is composed of discrete hierarchical levels or subsystems, wherein higher levels tend to maintain relatively stable structures, whereas lower levels undergo frequent partial changes (Walker and Salt, 2006). In urban morphology, different morphological elements follow distinct temporal cycles of change, with morphological elements at various hierarchical levels evolving simultaneously but at different rates. For instance, when slowchanging elements, such as cultural organizational patterns, social order, and social relations, are affected by external disturbances, they influence fast-changing elements, including management systems and morphological components. Streets function as slow variables, constraining the potential reconfiguration of other morphological elements (Scheer and Ferdelman, 2001). In contrast, buildings act as fast variables, bringing about frequent innovations, yet they have limited capacity to trigger overall changes in spatial form (Vale and Campanella, 2005). Therefore, spatial form, like a complex adaptive system, has the capacity to maintain relatively stable structures at higher levels while generating frequent changes at lower levels across temporal dimensions. This cross-level nonlinear dynamic drives a system to exhibit complex evolutionary characteristics.
(3) Adaptation-driven complexity
According to the CAS theory, the adaptive behaviors of agents are the primary force driving the evolution of a complex adaptive system (Garcia, 2013). In the morphological evolution of historic blocks, two dynamics are at play. On the one hand, the cultural identity and behavioral logic held by original residents - reflected in their values, norms, and lifestyles - are intrinsic elements that sustain social order and constitute the most stable framework of the spatial structure. On the other hand, residents actively respond to external stimuli - such as changing socio-economic conditions and market demands - by adapting their behaviors and generating new rules, which in turn drive the morphological system toward greater complexity. Therefore, the core driver of morphological evolution lies in residents' adaptive behaviors, which reflect the continuous and multi-layered interactions between social dynamics and physical space.
In summary, the spatial form of historic blocks embodies multiple social relations, a hierarchical structure of morphological elements, and a dynamic social environment. The interplay among these three aspects contributes to the increasing complexity of spatial form. This complexity arises from a continuous process in which adaptive residents, as active agents, adjust their internal behavioral rules in response to external environmental stimuli, resulting in spatial expressions that are highly localized. Simultaneously, this evolutionary process enhances the morphological capacity to adapt to changing environmental conditions. Accordingly, the CAS theory offers a robust explanatory framework for understanding the intrinsic logic of morphological evolution in historic blocks. On this basis, this paper defines spatial form as a CAS composed of two interrelated subsystems: a physical morphological system and an adaptive agent system. The adaptive behaviors of spatial agents drive changes in the complexity of the external physical space, resulting in adaptive spatial characteristics that correspond to different historical stages. The adaptive agent system includes individual residents, as well as the broader social and cultural orders of higher-level ethnic groups formed through the aggregation of micro-level individuals. Meanwhile, the physical spatial form comprises three morphological elements derived from Conzenian urban morphology: town plan, land utilization, and building fabric (Conzen, 1960).
1.1.2 The adaptive cycle of spatial form in historic blocks
In urban morphological studies, the city is conceptualized as a complex organic system that grows and evolves through metabolic processes (Yu, 2009). According to Conzen's burgage cycle, this evolution occurs through four successive phases: repletive, climax, recessive, and fallow (Conzen, 1960). In 1973, Holling introduced the adaptive cycle theory for complex systems, which delineates four phases of self-organized development: exploitation (r), conservation (K), release (Ω), and reorganization (α) (Gunderson and Holling, 2002). This theory effectively elucidates the internal system linkages, the responsiveness of the key system dynamics, and systems' capacity to adapt to disturbances. Furthermore, it facilitates the identification of distinct phase-specific characteristics under varying pressures.
A comparative analysis of the CAS theory and the adaptive cycle theory, as proposed by urban morphologists, reveals that the exploitation phase corresponds to the repletive phase, the conservation phase to the climax phase, the release phase to the recessive phase, and the reorganization phase to the fallow phase (see Figure 1). The specific evolutionary trajectory is as follows.
(1) Forward loop (from r to K phase)
During the exploitation and conservation phases, changes in spatial form are gradual and predictable, achieving the system's maximal production and accumulation. The spatial form is influenced by factors such as location, historical culture, and market economy. These factors interact with and intertwine among the main spatial adaptive agents. A large number of adaptive agents follow simple operational rules. As the system grows, agents' connectivity within economic and social networks strengthens, promoting individual independence and adaptive capacity. During this phase, new orders emerge from the previously chaotic system. Adaptive agents leverage new knowledge and resources for incremental innovations and establish connections with other agents through interactions. With the intensification of external disturbances, the number of agents increases and new rules continuously emerge, eventually reaching a peak characterized by high interconnectivity among agents. This fosters enhanced collaboration and interaction among agents within the spatial form, driven by nonlinear dynamics, which ultimately cause the emergent properties of the entire morphological system to exhibit remarkable complexity.
(2) Back loop (from Ω to α phase)
During the release and reorganization phases, morphological changes occur rapidly and become unpredictable, enabling the maximal reorganization of the morphological system as agents' adaptive capacity begins to strengthen. The high degree of self-reliance that the system has developed during the conservation phase has also become increasingly susceptible to large-scale environmental disturbances. Intense external shocks render existing rules inadequate, destabilizing the system and triggering a bottleneck period with minimal adaptability. The level of interconnection between system elements declines, and the entire system begins to descend into disorder. Meanwhile, shifts in external conditions tend to generate new systemic attractors, offering the morphological system new developmental opportunities, gradually guiding it into a new non-equilibrium cycle. Driven by agent behaviors, morphological elements start to innovate continuously - starting with building arrangements - developing new structures and functions to adapt to internal and external environments. This process involves substantial accumulation at the foundational level, generating cumulative effects that foster functional and structural changes. During this period, the increasing adaptability of agents accelerates the system's rapid restoration of equilibrium and facilitates its transition into the next cycle.
In summary, this paper defines the adaptive cycle of spatial form as an evolutionary process in which the adaptive agent system, through continuous and incremental interactions with the external environment, drives spatial form through the phases of exploitation, conservation, release, and reorganization across different scales. This cyclical process promotes the evolution of the entire system, demonstrating characteristics of a complex adaptive system.
1.2 Research methodology
Using different historical periods as a chronological framework and building upon Conzenian urban morphology, this paper integrates the adaptive cycle theory to identify the complex adaptive characteristics of morphological systems in historic blocks. It focuses on the adaptive mechanisms and cyclical pathways that drive the evolution of these systems. Following the research logic of identifying characteristics, analyzing mechanisms, and uncovering evolutionary pathways, the paper applies both qualitative and quantitative methods to provide an in-depth interpretation of the morphological evolution of historic blocks.
1.2.1 Identification of morphological characteristics
Morphological characteristics include the adaptive behaviors of the agent systems and the complex attributes of morphological elements. First, a qualitative analysis is conducted to examine the adaptive behaviors of agents in response to environmental changes in different periods. Second, drawing on core concepts and measurement methods from the cutting-edge science of complexity, this paper focuses on three plan elements defined in Conzenian urban morphology - plots, streets, and block plans - and employs methods such as scaling law, fractal dimension, space syntax, and kernel density analysis to identify the complex attributes of morphological elements in different periods (Batty, 2013; Batty and Longley, 1994). Specific measuement indicators include the Shannon-Wiener Index, cumulative distribution function, box-counting dimension, area-perimeter dimension, and hierarchical dimension (see Table 1).
1.2.2 Analysis of morphological interaction mechanisms
Within the adaptive cycle theory, the Panarchy model is employed to elucidate the interaction mechanisms among subsystems in a system's adaptive cycle (Zhang, 2020). This model offers a valuable perspective for interpreting the complexity, dynamics, and adaptability inherent in and between different systems. Drawing on the Panarchy model, this paper conceptualizes the adaptive cycle of spatial form as a dual process involving the self-adaptive behaviors of agents and the "remember" and "revolt" mechanisms of the macro system (see Figure 2). "Remember" refers to the retention and review of the system's past states, which is crucial for understanding the stability and variability of system agent behaviors. "Revolt" drives changes in agent behaviors by urging the system to introduce new conditions. Therefore, the CAS evolution is influenced both by the overall innovative behaviors exhibited by lower hierarchical systems through "revolt" and by the constraints, guidance, and control on elements and subsystems exerted by higher hierarchical systems through "remember." Building on these insights, the mechanisms driving the morphological evolution of historic blocks are analyzed through two primary aspects: the self-adaptive behaviors of agents and the "remember" and "revolt" mechanisms of the macro system.
(1) The self-adaptive behaviors of agents
In the morphological system of historic blocks, the customary practices of local residents shape the rules governing the construction and operation of the built environment. These basic operational rules are related to the cross-influence among various internal operational rules, the complexity of the external environment, and the intervention of individual complex behaviors. Therefore, the spatial form reflects adaptive responses that emerge organically, without centralized control, characterized by adaptive transformations. The driving force behind the evolution of the morphological system arises from the layered emergence and progression of these small and simple actions.
(2) The "remember" and "revolt" mechanisms of the macro system "Remember" and "revolt" exhibit varying degrees of influence at different levels and phases. At the level of internal adaptive agents, "remember" and "revolt" are reflected in the process of agents adjusting their knowledge structure in response to changes in the external environment, as well as taking certain actions to adapt to the objective social environment (Bai, 2011). At the level of physical spatial form, on the one hand, building functions begin to revolt in response to changes, leading to innovations in building functions and structures, which generate a large number of new building types. When enough new buildings emerge, higher-level plots and blocks begin to exhibit complex, diversified evolutionary characteristics. On the other hand, the spatial structure formed in the previous period retains a "remember" function, which causes the morphological elements to demonstrate certain characteristics of historical continuity.
Therefore, the morphological evolution is shaped both by bottomup "revolt" from the micro system and by top-down "remember" from the macro system. This macro-level overall structure and micro-level local behaviors condition each other, and their mutual coordination and symbiotic cooperation promote the emergence of morphological complexity.
1.3 Study area
1.3.1 Hui-Fang area in Xi'an
This study takes Xi'an Hui-Fang area as a case study to investigate the evolutionary characteristics of its spatial form. The Hui-Fang area, also known as the Muslim Quarter, is located in Beiyuanmen Historic Area in Xi'an - one of the three major historic areas in Shaanxi Province. Situated near the Bell and Drum towers in the city center, it covers an area of 1.3 square kilometers and is home to approximately 34,000 Hui people (see Figure 3). This area is bounded by Dongzaoci Alley in the west, Shehui Road in the east, Hongbu Street in the north, and West Street in the south. Within these boundaries are 12 mosques. Focusing on urban morphological transformation and incorporating key historical events, this paper divides the spatial evolution of the Hui-Fang area after 1949 into three distinct periods: the adaptation period following national institutional changes (1949 - 1981), the adjustment period during the economic reform era (1982 - 1997), and the period characterized by responses to complex social relations (1998 - present).
1.3.2 Applicability of the CAS theory to the Hui-Fang area
The applicability of the CAS theory to the Hui-Fang area is reflected in three key aspects. First, the spatial form of the Hui-Fang area consists of a wide variety of elements. It has multiple cultural attributes and complex social relations, including geographical, religious, economic, and kinship-based social structures, as well as rich spatial hierarchical relations among species. Second, there is a complex feedback mechanism between spatial form and social structure. Third, the core force driving the complexity of morphological changes is Hui residents. Their adaptability, along with the social, economic, and cultural order they embody, forms the root of morphological complexity, which in turn further enhances the overall spatial form's capacity to adapt to the environment.
1.3.3 Data sources
Research data includes both quantitative and qualitative sources. Basic geographic information is derived from 1:1000 CAD topographic maps from 1997 and 2020, as well as the 1967 keyhole spatial imagery map. Additional data includes 2022 point-of-interest (POI) data from Amap (Gaode Map), 2023 statistical bulletins and yearbooks, government work reports, historical records, and local chronicles. This study employed purposive sampling to select 232 households across 12 designated residential areas, ensuring that the sample reflected the typical range of contextual settings, individual profiles, and critical practices. Data collection was carried out through one-on-one in-depth interviews. Residents of the Hui-Fang area, particularly longstanding indigenous inhabitants, participated in unstructured interviews that facilitated the indirect disclosure of household demographics, land inheritance norms, living patterns, and plot quantities. For cultural elites and government officials, free-form interviews were conducted to document local Hui cultural conventions and organizational governance frameworks. The skewed distribution of respondents by age, occupation, and length of residence may compromise the findings' capacity to fully capture the living characteristics of Hui-Fang residents, thereby constraining the generalizability of subsequent conclusions regarding morphological evolution mechanisms.
2. Complex adaptive characteristics of urban form in the Hui-Fang area of Xi'an
2.1 Adaptive evolutionary characteristics of the internal agent system
Influenced by external environments and interactions among agents, agent behaviors in different periods exhibit different adaptive characteristics. Overall, the following features are observed.
First, social order transitions from ethnically-based social networks to more diverse social structures. The traditional Hui ethnic neighborhood is a production and living community grounded in religious and kinship ties. In spaces structured around the mosque-Fang model, the shared value system and cultural identity form the foundation of group identity. However, entering the 21st century, the traditional residential model based on kinship and religious bonds is no longer sufficient to meet development needs. The Hui-Fang area has gradually become more open, with the increasing complexity of social networks leading to greater knowledge accumulation and prompting the area's economic network to become more complex and diversified. The social structure, originally based on kinship, has gradually shifted towards more economic-driven social relations as a result of the transformation brought about by modern consumer society. Social-spatial relations have evolved from small-scale relations primarily within the indigenous Hui ethnic group to larger-scale social-spatial relations among the Hui people from different regions.
Second, cultural order transitions from a narrow regional identity to a broader cultural identity. In the past, residents in the Hui-Fang area shared a strong sense of unity in terms of values and social customs. Following the implementation of the reform and opening-up policy in 1978, Xi'an's steady socioeconomic development, coupled with the influence of globalization, facilitated cultural exchanges through various means. This process has deepened interactions between Hui and Han people, enhancing inter-ethnic interdependence and fostering greater cultural inclusivity. While preserving their own values, the Hui ethnic group has actively integrated elements from other cultures, forming a multicultural system that aligns with social development.
Third, organizational order transitions from a singular ethnic management system to a multi-tiered ethnic community management system. Since the reform and opening-up, the mosque-Fang management system? has gradually improved. At the same time, the establishment of urban community systems has prompted the Hui-Fang area to embrace the administrative management and leadership of subdistrict office, local government, and the religious affairs department of the people's government. This has facilitated the establishment of more standardized norms for social interactions. After 2000, there gradually formed a multi-tiered management network structure characterized by broad resident participation through residents' committees, governmental/non-governmental religious management organizations, and household units.
2.2 Complex evolutionary characteristics of the physical morphological system
The complexity of the physical spatial form across different periods is measured (see Table 2 & Figure 4), revealing the following characteristics.
First, spatial patterns present historical inheritance. The Hui-Fang area's spatial form exhibits unique, stable patterns shaped by natural environment and historical interactions, demonstrating path dependence. This is specifically reflected in the historical continuity of the street network and the mosque-Fang structure.
Second, spatial functions shiftfrom uniformity to diversity. The Hui- Fang area has undergone a significant transformation in the three periods, evolving from an exclusively ethnic residential quarter to an ethnic tourism neighborhood, and later to a historic and cultural tourism block. Its functions have become increasingly complex, incorporating more diversified social, economic, and cultural elements. During these three periods, the number of plots increased by more than 12 times, and the number of buildings grew by more than three times. As a result, the number of building owners also increased, facilitating the construction of differentiated buildings to meet individual needs. This has enriched the internal functions within the Hui-Fang area, leading to more diverse spatial forms.
Third, the spatial structure shifts from hierarchy to networked connection. In the first two periods, building footprints in the Hui-Fang area exhibited distinct fractal characteristics within the range of 0.5 to 256 meters. The fractal dimension is 1.68 and 1.79. This indicates that simple elements at different scales, through nested layers and overlays, formed a spatial hierarchy with similar complexity, order, and continuity. However, in the third period, building footprints showed fractal characteristics only within the range of 1 to 256 meters, with the fractal dimension of 1.788, indicating a reduction in the continuity of spatial hierarchy. Furthermore, with the rapid development of tourism after 2000, the economic space of the Hui-Fang area began to integrate into the broader economic spatial structure of Xi'an, especially the southeastern area, which has increasingly intermingled with the city's commercial core.
Fourth, the plan elements shiftfrom organic order to multidimensional complexity. This transformation is reflected in multiple dimensions, including changes in street networks, plot patterns, and building arrangements.
1 Weakening the organic order of street networks. Axial maps were generated using DepthmapX 0.8.0, with axial line lengths subsequently extracted via the Calculate Geometry Attributes tool in ArcGIS Pro 3.1. Lengths (in meters) were stored in new fields for cumulative distribution function analysis. In 2020, the number of short street axes decreased, and the distribution of axial lengths shifted towards longer axes, with the power-law exponent down by 0.04 (see Figure 5). At the same time, there was a progressive deviation of road branching ratio (from 3.74 in 1967 to 2.71 in 2020) from the natural optimum. This suggests reduced hierarchical differentiation between road levels, leading to weakened hierarchical relationships.
2 Enhanced heterogeneity of plot pattern. The fractal dimension of plot boundaries has gradually increased from 0.461 in 1967 to 0.6 in 2020, indicating enhanced heterogeneity. In terms of the plot area distribution, it shows a transition from a logarithmic distribution (simple distribution) to a power-law distribution (complex distribution) (see Figure 6). The distributions indicate substantial transformations in the cumulative distribution frequency (CDF) over the three time frames (see Figure 7). The nearly overlapping curves of the first two periods (1967 and 1997) indicate that plot sizes lacked a central tendency and were dispersed over a wider range, evidencing a highly skewed or heavy-tailed distribution. In stark contrast, the distribution curve for the third period diverges significantly, characterized by a linearly decaying tail. Furthermore, plot sizes in this later period assume a considerably wider range of values compared to the earlier years, marking a transition from a relatively simple, uniform distribution to a more complex one. This suggests that as the scale of plots in the Hui- Fang area expanded, the increasing number of plot elements and the denser development gave rise to more diverse and hierarchical spatial forms.
3 Emergence of building arrangement complexity. In the Hui- Fang area, the behavioral rules of renovations and expansions under different requirements from main agents vary, leading to a greater diversity in the combination patterns from basic architectural components to entire buildings. Analysis of the CDF of the building size reveals that the curves of the first two periods show substantial overlap (see Figure 8). However, the curve of the third period is significantly different, with the tail showing a linear decline, transitioning from a relatively uniform simple distribution to a complex power-law distribution (see Figure 9).
3. The Adaptive cycle mechanism of spatial form in the Hui-Fang area of Xi'an
3.1 The adaptive behavior of the agent system
The adaptive behavioral rules of agents enable the morphological evolution of the Hui-Fang area to maintain overall coherence and unity - manifested as self-similarity across the scales of building footprints, street systems, and courtyard spaces. At the same time, these rules bring about greater diversity and complexity in morphological changes, with variables such as plot size and building scale gradually exhibiting power-law characteristics in terms of their rank-size distribution.
(1) The evolution of spatial patterns brought about by family inheritance behavioral rules
The cultural inheritance model, organizational system, and economic production modes are all family-centered, creating an isomorphic relationship between culture, economy, and organization at this hierarchical level. The stable family-centered cultural structure fosters industrial development, giving rise to a livelihood model bound by household-based economies, thus catalyzing the formation of economic communities. These family-based units, bound by kinship ties, generate bottom-up, small-scale multifaceted cohesion, which enhances the stability of the social structure and the closeness of neighborhood relations in the Hui-Fang area. These units constitute the most robust spatial structure within the Hui-Fang area.
(2) The fragmentation of plots brought about by property rights iteration behavioral rules
The inheritance laws of the Hui ethnic group have profoundly influenced plot subdivision principles, with properties being subdivided in accordance with a sophisticated shareholding system. After decades or even centuries of repeated subdivisions, this process continues until it reaches the smallest functional components of buildings. As the population increased, originally completed buildings were subdivided along perpendicularly intersecting exterior walls or partition walls. These subdivision mechanisms caused continuous shifts in the property boundaries and positions of partitions. The longer the history, the shorter and more irregular these partition walls became, leading to an increase in the number of dead-end alleys, the number of plots, and greater complexity in building footprints and a gradual weakening of the organic order within street spaces.
(3) The evolution of buildings brought about by decision-making bifurcation behavioral rules
The construction rules of the Hui-Fang area follow a private decisionmaking system, characterized by a nonlinear system with a high degree of bifurcation that leads to increased uncertainty. Although the number of rules and steps is limited, the combination of different rules produces unpredictable results over time. This decision-making system has laid a stable foundation for the non-geometric patterns of spatial form in the Hui-Fang area, enriching its spatial hierarchy and enhancing the internal organic order and organization.
3.2 The "remember" and "revolt" mechanisms of the macro system
In the first period, under continuous disturbance from macro-level national policies, a family inheritance "revolt" mechanism began to emerge. The Hui-Fang area's living and production spaces retreated into domestic spheres, where specific loci, such as family courtyards and dead-end alleys, retained personal emotional significance and social relations. This spatial retreat stabilized the kinship-based social order and living patterns to some extent. Correspondingly, its physical spatial form preserved the traditional pattern characterized by dwellings clustered around mosques and commercial activities thrived along Fang (neighborhood) streets.
In the second period, with the establishment of the socialist market economy system in 1992, the "remember" mechanism inherited from the previous period sustained the stability of the Hui-Fang area's social structure and tight-knit neighborhood relations, and further strengthened the mosque's role as the core of both production and living spaces. The spatial organization - featuring deep courtyard plots, traditional quadrangle dwellings, and the economic spatial layout emphasizing proximity to mosques - embodied the ethnic identity. At the same time, the "revolt" mechanism of "autonomous economic development" began to adapt to changes in the economic system. In terms of physical spatial form, while maintaining the previous "front store, back residence" building typology, a new building arrangement of "below store, above residence" emerged, reflecting further integration of production and living spaces.
In the third period, the development of tourism and the integration of multicultural influences contributed to the emergence of a new "revolt" mechanism: "economic complementary symbiosis." While maintaining its characteristic small-scale flexibility, the economic models progressively incorporated new economic operation modes, evolving into an economic system characterized by broader participation of community residents, interactions with external economies, and integration with ethnic enterprises. The physical spatial form was marked by higher degrees of spatial overlap between production and living spaces, diverse spatial types, and a decentralized, networked spatial structure.
In summary, under the framework of alternating "remember" and "revolt" mechanisms (see Figure 10), the morphological evolution of the Hui-Fang area exhibits a typical spatial pattern characterized by the "mosque-Fang structure and layered courtyards," and the street system marked by "a three-tiered hierarchy with the integration of commercial and residential spaces." At the same time, it maintains moderate flexibility in response to environmental changes, showing a complex evolutionary trend of diversity, nonlinearity, and selforganization.
3.3 The adaptive cycle pathways of spatial form
Building on these insights, the evolutionary process of spatial form in the Hui-Fang area is placed within the adaptive cycle framework, to gain a deeper understanding of its transformation (see Figure 11).
(1) First period: adaptive evolutionary pathway under unidirectional control
During the conservation-to-release (K-Ω) phase, under continuous top-down interventions from macro-level national policies, the sociocultural system temporarily lost its equilibrium, and its primitive accumulation potential was gradually depleted. Main agents retreated into domestic spaces, leading to weakened external connections and reduced interconnections among system elements. Consequently, the proactivity and adaptability of agents diminished, resulting in insufficient sustainable momentum for the overall system and a gradual loss of vitality. In terms of physical spatial form, large- and mediumscale elements (blocks, streets, and plots) remained aligned with policy directives, while small-scale elements (buildings) underwent slow, passive adaptation, maintaining historical continuity.
(2) Second period: adaptive evolutionary pathway under bidirectional guidance
During the release-to-reorganization (Ω-α) phase, with the establishment of the socialist market economic system and the revival of public cultural life, the agent system began to regain vitality and became more open. The increasing connectivity of social networks created new attractors, prompting the transformation of the agent system. As the number of agents grew, the system expanded, and social and economic interactions intensified. Agents broke free from previous developmental constraints, gaining greater adaptability but also encountering higher uncertainty. Owing to the still-limited carriers for resource flow, top-down policy guidance at the macro level remained necessary to facilitate the system's transition to a more resource-rich phase. As a result, the distribution of plot sizes and building scales, though still relatively simple, exhibited higher uncertainty and reduced stability. Changes in medium- and smallscale morphological elements were not yet rapid enough to trigger emergent behaviors at higher spatial levels, and block functions remained primarily residential. Specifically, at larger scales (blocks and streets), a relatively stable structure persisted; at the plot scale, there was slow, self-organizing renewal; and at the building scale, rapid self-organizing renewal occurred.
(3) Third period: adaptive evolutionary pathway in multi-level symbiosis
During the reorganization-to-exploitation (α-r) phase, the sustained development of tourism since the 1990s triggered economic expansion and increased population mobility. As the system grew, the connectivity of economic and social networks among agents strengthened, enhancing individual independence and gradually strengthening adaptive capacities. Agents began to exhibit a surge in tourism- and consumption-oriented social behaviors. In terms of physical spatial form, building arrangements underwent continuous innovation, with accumulation effects emerging through mass aggregation. This drove block-wide morphological changes, characterized by: scale expansion, functional differentiation, and networked connectivity. Specifically, at the macro scale, influenced by the "remember" mechanism, blocks and streets maintained slow self-organizing renewal, with fractal dimensions and power-law exponent of the street network remaining stable compared to the first period. At the medium and small scales, buildings continued rapid, self-organizing renewal, which in turn drove changes in higher-level plot systems and block functions.
4. Conclusion
At the epistemological level, this paper conceptualizes spatial form as a complex adaptive system, interpreting its evolutionary process as one driven by the adaptive behaviors of micro-level individuals that push the macro-level morphological system to undergo complex changes. At the methodological level, the core attributes of complex systems, such as self-organization, nonlinearity, and diversity, are extended to spatial quantitative analysis, revealing its external complexity. Drawing on the Panarchy model, the paper further summarizes the mechanisms of morphological evolution as the selfadaptive behaviors of micro-level systems and the "remember" and "revolt" succession mechanisms of the macro system. This approach enables a re-recognition of the internal logic of morphological evolution. Key findings show that since the founding of the People's Republic of China in 1949, the Hui-Fang area has gone through fundamental transitions from regional identity to ethnic identity, from religious ties to economic network ties, and from cultural belonging to symbolic production. The previously homogeneous population structure has shifted towards differentiated interpersonal relations, and social relation networks have become more complex. The spatial pattern of the mosque-centered residential pattern has transformed into an economically embedded multi-ethnic community. The morphological elements present complex evolutionary characteristics, such as path-dependent alley networks, heterogeneous plot patterns, and diverse building arrangements. Employing the adaptive cycle theory, it identifies that the evolution of spatial form in the Hui-Fang area has gone through three phases: the slow, passive adaptation in the first period - conservation-to-release phase; the top-down and bottom-up interactive adaptation in the second period - release-toreorganization phase; and the multi-level symbiotic adaptation in the third period - development phase.
This paper attempts to bridge the adaptive cycle theory from complex systems with the burgage cycle theory in urban morphology. However, existing research has yet to extensively explore how urban morphological phases correlate with different societal development phases. Further research is needed to uncover: the specific mechanisms linking social-ecological system states to agent behaviors that drive the transformations of physical spatial form, as well as the underlying logic connecting the stages of the social-ecological system to morphological changes.
(This research was supported by the General Program of the National Natural Science Foundation of China (No. 52478070) and the Humanities and Social Science General Program of the Ministry of Education (No. 22YJC760134).)
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Notes:
1 The term "Fang" in this paper specifically refers to the mosque-centered structure typical of the Hui ethnic group - a living and production space formed by a group of Hui people who share a common culture and gather around mosques.
2 The unique social organization form of the Hui-Fang area is characterized by the "mosque-Fang" structure formed by a mosque and its surrounding residential blocks. Its social, cultural, and economic structures essentially arise and develop around this mosque-centered neighborhood framework. The unique structure has consequently led to the formation of a special localized management system within the Hui-Fang area - the Mosque Democratic Management Committee.
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