Spatiotemporal Analysis of Urban Blue Space in Beijing and the Identi ﬁ cation of Multifactor Driving Mechanisms Using Remote Sensing

: With rapid urban development in Beijing, there is a critical need to explore urban natural resources and understand their underlying mechanisms. Urban blue space (UBS) has gained increasing a tt ention due to its potential to drive microcirculation, mitigate heat islands, and enhance residents’ well-being. In this study, we used remote sensing data to extract UBS in Beijing and employed exploratory spatial data analysis (ESDA) methods to examine its spatial and temporal development over the past two decades. We adopted a mesoscopic perspective to uncover the full spectrum of landscape pa tt erns and quantitatively simulate the mechanisms in ﬂ uencing the area of UBS and landscape pa tt erns. Our ﬁ ndings are as follows: (1) The UBS area in Beijing exhibited ﬂ uctuating growth from 2000 to 2020. (2) Spatial clustering of UBS was stable with subtle changes. (3) The ecological conditions in Beijing improved over the last 21 years, indicated by increased habitat diversity and richness, while notable landscape fragmentation posed signi ﬁ cant challenges. (4) Science and technology management-related factors, such as UEM, EDUI, and STI, emerged as the most in ﬂ u-ential mechanisms for the UBS area. The coe ﬃ cients for these factors were 0.798, 0.759, and 0.758, respectively. Following closely were vegetation conditions (NDVI) with a coe ﬃ cient of 0.697 and an annual average temperature (T) with a coe ﬃ cient of 0.692. (5) Precipitation was identi ﬁ ed as the most vital in ﬂ uencing factor for the UBS landscape, with a signi ﬁ cant correlation coe ﬃ cient of 0.732. It was followed by residential population (POP), with a coe ﬃ cient of 0.692, and economic conditions represented by gross domestic product (GDP), with a coe ﬃ cient of 0.691.


Introduction
Urban blue space (UBS) refers to spaces of surface water within urban areas, including lakes, channels, and pools [1].UBS plays a crucial role in various aspects, such as biodiversity conservation [2], climate change mitigation [3], provision of ecosystem services [4], and public health benefits [5].It also contributes to reducing the heat island effect and regulating the local climate [6][7][8].Consequently, the management of UBS holds great importance in urban planning and development [9,10].In addition to its functional benefits, UBS significantly enhances the aesthetics [11] and cultural value [12] of urban environments.It provides a sense of comfort and tranquility, offering respite from the pressures of modern city life [13,14].Furthermore, UBS helps mitigate flood risks associated with the expansion of artificial surfaces, optimizes land use patterns, improves public satisfac-tion, and promotes residents' well-being [15][16][17][18][19]. Beijing, as a globally recognized metropolis renowned for its fast-paced lifestyle, serves as an exemplary case study for examining the role of UBS in urban environments.
In terms of the content of existing studies in this field, the focus has primarily been on changes in urban blue spaces (UBS) [20] and patch connectivities [21][22][23].However, UBS in metropolises serve not only as functional outdoor water bodies, but also as unique urban landscapes and spaces with aesthetic and emotional significance.Therefore, it is crucial to pay more attention to the area and landscapes of UBS.Regarding the time scale, most previous research has concentrated on specific years or short-term timelines [24][25][26][27][28].However, UBS changes occur over long-term and gradual processes, which cannot be adequately captured within a short-term study spanning only three to five years [29].Additionally, UBS is subject to irregular impacts from extreme weather events such as droughts and floods, which are often overlooked when using equal time interval methods [30].Hence, conducting long-term studies with more detailed information is essential in this field.In terms of primary data, previous research on UBS landscapes has primarily relied on traditional data sources, such as historical maps and aerial images with large spatial resolutions and limited information [31,32].These sources only support studies at a patch scale [33,34].However, with advancements in remote sensing technology, high-quality primary data have become more accessible.Therefore, there is a demand for studies that utilize more detailed information and focus on a smaller scale [35,36].Considering the potential interference of fragmented and temporal water patches resulting from high-resolution data, as well as the inability of low-resolution data to capture detailed information, this study adopts remote sensing data with a resolution of 30 m × 30 m to accurately extract UBS.
The analysis of component mechanisms, including population, economics, climate, and land use [37], deserves more attention compared to single-factor studies [38] since the spatiotemporal characteristics of UBS are influenced by multiple resource factors.Understanding the interactions and contributions of these factors is crucial in comprehending UBS dynamics.Moreover, qualitative mechanisms hold greater value for policymakers and stakeholders [39][40][41][42][43] involved in urban management and hydrological projects compared to quantitative mechanisms [44,45].Qualitative insights provide a deeper understanding of the underlying processes and offer more meaningful guidance for decisionmaking.In many existing studies, the differentiation in vegetation density has been overlooked [46].However, vegetation density directly affects the water holding capacity and the ability of ecosystems to regulate runoff.Considering that the area of low-density vegetation in Beijing is significantly larger than that of high-density vegetation, and that the availability of vegetation density data is limited, we have chosen NDVI, which is more sensitive to low-density vegetation surfaces, and EVI, which is more sensitive to highdensity vegetation surfaces [47], as proxies to distinguish vegetation density in this study.
Compared to previous studies, this research makes several significant contributions.Firstly, all sections are quantitative, including the spatial distribution, the main aspects of the landscape indexes, and the impact mechanisms.Secondly, we studied UBS from double perspectives: the area and the landscapes temporally and spatially.Finally, this research explored the heterogeneity in vegetation density within the mechanism simulation, and put forward insights into the impact of vegetation densities on UBS dynamics.

Study Area
The study area of this research is Beijing, which is a prominent political, economic, and cultural center in China.Beijing is situated between 115.7°E-117.4°Elongitude and 39.4°N-41.6°Nlatitude [48].It shares borders with Tianjin in the east and Hebei in the remaining directions (see Figure 1).The city covers an area of 16,410 square kilometers, and had a permanent resident population of 21.89 million as of 2021 [49].Beijing experiences a monsoon-influenced humid continental climate.Summers in Beijing are hot, humid, and prone to rainfall, while winters are cold, dry, and characterized by clear skies.The average annual rainfall in Beijing is approximately 698.4 mm, and the average annual temperature ranges from 9 °C to 19 °C [50].Beijing serves as a typical case for studying urban blue space (UBS) in a metropolis.The city's UBS plays a crucial role in several aspects.First, due to the frequent intense rainfall and extreme precipitation events that occur during the summer, UBS serves as a vital component of natural reservoirs, helping to absorb and regulate excess water, thereby reducing the risk of flooding.Additionally, UBS in Beijing provides valuable mental and recreational benefits to the public.As a special urban landscape, UBS is a source of mental relaxation and entertainment for the city's residents amidst their fast-paced lives.These blue spaces create a serene and tranquil environment, offering an escape from the hustle and bustle of urban life.The presence of UBS in Beijing contributes to the overall well-being and quality of life for its inhabitants.Considering the dual functions of flood mitigation and mental well-being, studying UBS in Beijing provides valuable insights into the multifaceted role of blue spaces in metropolises.

Data and Resources
We chose Beijing as the study area to explore the spatial and temporal characteristics and the mechanisms of urban blue space.Firstly, we used the remote sensing dataset JRC Monthly Water History to extract urban blue space, described the area development from 2000 to 2020, scheduled Moran's I to analyze the spatial autocorrelation, and used Getis-Ord General G* to reveal the spatial clustering pattern.Secondly, using Fragstats 4.2, we calculated ten landscape indexes and extracted the main aspects of those indexes by PCA.Thirdly, we used grey relation analysis to simulate the mechanisms of the UBS area and landscapes, then classified the main influencing factors as strong, medium, and weak using the natural break method (Figure 2).Remote sensing images from the Google Earth engine data catalog were used to extract influencing factors.The statistical data were retrieved from the statistical yearbook and official websites (Table 1).To address the spatial and temporal resolution differences between remote sensing images and statistical data, the influencing factors derived from remote sensing images were aggregated to the county level from the pixel scale.This aggregation process ensures that the data align with the resolution of the statistical data available.Furthermore, to account for the temporal variations within the remote sensing images, the data were further derived to annual averages.This averaging process provides a representative value for each influencing factor, smoothing out short-term fluctuations and capturing the overall trends over time.By aggregating and deriving the data, this study ensures compatibility and consistency between the remote sensing images and the available statistical data, enabling a comprehensive and integrated analysis of the influencing factors at the district level on an annual basis (Figure 3).We used multi-resource data, including remote sensing and statistics data with different spatio-temporal resolutions.We extracted the annual average at the county scale to unify the temporal and spatial scales of all data.
Referring to the existing research, considering the actual situation and data availability in Beijing, we have selected the following indicators as influencing factors for studying UBS.
Population (POP): Population is a critical factor influencing UBS scope and intensity [51,52].Domestic water consumption and modifications to surface runoff by human activities significantly impact UBS.Therefore, population is a relevant indicator in this study.
Precipitation (PREP): Urban precipitation plays a vital role in the groundwater recharge and overall water circulation in cities. Extreme weather events associated with global climate change can generate temporary urban blue spaces, such as groundwater puddles.Hence, precipitation is commonly considered an influential indicator in UBS research [53].
Temperature (T): UBS and temperature represent a complex system interaction.UBS helps regulate the local microclimate, mitigating high temperatures and providing substantial cooling effects to the surrounding areas [54].Higher temperature accelerates waterbody shrinking through increased evaporation.Thus, temperature is a significant factor to consider in UBS studies [55].
Fractional vegetation cover (FVC): Vegetation plays a crucial role in slowing surface runoff and enhancing water conservation capacity [56,57].Considering the positive influence of vegetation on UBS, FVC is an essential indicator in this study.
Artificial surface proportion (ASP): The proportion of artificial surface in a city significantly affects its surface temperature, leading to either warming or cooling effects.Analyzing ASP helps in understanding urban ecological health and the impact of ASP on UBS [58].
Normalized difference vegetation index: NDVI is closely associated with the cooling effect of urban ecological spaces and precipitation [59].It is more sensitive than the enhanced vegetation index (EVI) in regions with sparse vegetation, which are often found in metropolises such as Beijing.Thus, NDVI is a suitable indicator for UBS research in Beijing.
Enhanced vegetation index (EVI): EVI is a robust remote sensing index that reflects vegetation density and is especially effective for dense vegetation surfaces.It is closely related to urban microcirculation and blue spaces [60].
Gross domestic product (GDP): GDP measures the gross product of a country and indirectly reflects water consumption, wastewater discharge, and water-use efficiency [61,62].Considering the implications for water management and efficiency, GDP is a relevant indicator in this study.
Urban environmental management (UEM): UEM encompasses water conservancy, public facilities, and land use planning to ensure that population growth is in alignment with access to natural resources, basic infrastructure, and shelter.UEM encompasses the water management sector, flood control facilities management, water resources management, natural water collection and distribution, as well as hydrological services.It also includes the ecological protection and environmental governance industry, covering aspects such as ecological protection and environmental governance.Additionally, UEM involves public facilities management, which comprises municipal facilities management, environmental health management, urban and rural appearance management, and greening management.
Educational investment (EDUI): Education investment significantly promotes science and technology, which in turn affects production methods and water consumption efficiency.It is closely related to UBS and its sustainability.
Scientific and technical investment (STI): STI drives the application of technologies such as the Internet of Things (IoT), YunOS IoT, and big data.These technologies optimize water consumption patterns and UBS planning, making STI a relevant indicator in understanding UBS dynamics.

Spatial Autocorrelation Analysis and Spatial Clustering Analysis
Spatial autocorrelation detects the convergence or dispersion of observations [63,64].Moran s  is a widely used classical spatial autocorrelation index.For a series of n variable samples,  is the observation at location i, and wij is the spatial weight matrix (SWM).Then, Moran s  is calculated as follows: Moran s  ranges from −1 to 1. Moran s  >0 indicates a positive spatial correlation.The closer it is to 1, the more significant the positive spatial autocorrelation.Moran s  < 0 indicates a negative spatial correlation.The closer it is to −1, the more significant the negative spatial autocorrelation.Moran s  = 0 indicates a random distribution [65].The high/low clustering (Getis-Ord General G*) tool is an effective method for spatial aggregation simulation.The calculation formula is as follows: ̅ is the average of observations,  ,  , ⋯ ,   is the spatial weight of  , and  , ,  1,2, ⋯,n.The higher G* is, the higher the observation clustering, and vice versa.The null hypothesis of General G* assumes that the observations do not cluster spatially [66].The p value determines whether the null hypothesis should be accepted or not.The z score reflects the dispersion of observations [67].

Principal Components Regression Analysis
Principal component regression analysis (PCR) is used to solve multivariate collinearity problems [68].Principal component analysis, or PCA, is a dimensionality reduction method that is often used to reduce the dimensionality of large data sets by transforming a large set of variables into a smaller one that still contains most of the information in the large set.Principal component analysis (PCA) converts multiple indexes into several comprehensive indexes by orthogonal rotation transformation, following the premise of minimizing information loss.Generally, the results of PCA are independent variables called principal components [69].The geometric interpretation and model of PCA are as follows (Figure 4):

Grey Relation Analysis
A complex system always involves various elements; the mechanism of each element is hard to simulate quantitively in practice because of the associated interactions.Grey system theory attempts to look for quantitative relationships based on the curve geometry [70][71][72].Sequences are closely related when they have tight geometry curves and similar trends, and vice versa.Thus, grey correlation analysis is an effective classical quantitative measure for dynamic series.The formula is as follows: where    ,   is the grey correlation coefficient at location , and  and  are the observations of data sequences  and  , respectively. refers to the resolution coefficient.The value of  is inversely proportional to the difference between sequence phases.In classical statistical theory, for a set of observations with a large sample size, the probability of the sample is approximately equal to the frequency.Thus,  is equal to 0.5.

Development Characteristics of the UBS Area in Beijing
The UBS area (S) in Beijing showed a slight increasing trend from 2000 to 2020, with a stable trend from 2004 to 2016.It has been clearly increasing since 2016.The area of UBS in Beijing was 124.4 km 2 in 2000, reducing to 99.08 km 2 in 2004, rising slightly to 121 km 2 in 2016, and increasing to 183.4 km 2 in the last four years (Figure 5).

Spatial Autocorrelation Analysis of the UBS in Beijing
Considering a confidence level of α=0.05,Moran's I is always lower than 0.2, which indicates the weak spatial autocorrelation of the UBS in Beijing.

Spatial Clustering Pattern of the UBS in Beijing
From 2000 to 2020, the cluster analysis of "high/low" revealed that the agglomeration characteristics of UBS were relatively stable at the county level.However, the significance of clustering in Tai Shitun decreased prominently (Figure 6).

Analysis of Landscape Indicators
The landscape pattern comprehensively reflects landscape spatial heterogeneity.Patterns reveal the spatial distribution and combination of different patches.These patches are always of various sizes, shapes, and attributes.
In this study, multiple landscape indexes were computed.Specific indicators included LPI, SPLIT, CONTAG, AI, PD, NP, LSI, SHDI, SHEI, and PAFRAC.The detailed descriptions and formulas of these indicators are presented in Table 2 [73].

SPLIT (Splitting index)
Dispersion among different patches at a landscape scale.The higher the value of SPLIT, the more separation between studied patch types.

CONTAG (Contagion index)
Spatial collection and decentralization.The smaller the value of CONTAG, the sparser each patch type.

AI (Aggregation index)
Connectivity between patches of all patch types.The lower the value is, the more discrete the landscape.

PD (Patch density)
Patch density in the landscape reflects the degree and type of landscape fragmentation.Patch density represents the spatial heterogeneity of the landscape per unit area.

NP (Number of patches)
Number of all patches distributed in the landscape.

LSI (Landscape shape index)
Indicates the change in landscape form.The higher the value, the more complex the shape.

SHDI (Shannon's diversity index)
Reflects how many different quantitative measures are in a dataset.

SHEI (Shannon's evenness index)
Describes the extent of the landscape controlled by minority patch types.

PAFRAC (Perimeter area fractal dimension)
The intensity index reflects the disturbance in landscape patterns due to human activities.The higher the value, the greater the landscape's external disturbance.
The elements with upward trends are LPI, SPLIT, PD, NP, LSI, SHDI, SHEI, and PAFRAC (Figure 7).Their changes show that the UBS landscape pattern in Beijing developed stably in the first two decades.The maximum landscape patch area is increasing.Patches are more complex and have a significant change in intensity.Diversity and richness are improved.The patches are distributed more evenly.Landscape patch types have become more diverse because of human effects.As a result, the extent of separation, fragmentation, and spatial heterogeneity indexes was more remarkable and higher.
The indicators with downward trends are AI and CONTAG (Figure 7).In the last 21 years, the landscape connectivity of UBS in Beijing has been shallow, and the downward trend was kept up with the sprawl of urban construction.

Principal Component Analysis of the UBS Spatial Landscape Indices
The cumulative contribution rate of the first two principal components (Z1 and Z2) is 93.9% (Table 3), indicating that the first two principal components contain 93.9% of the information of the 10 original components.Thus, the landscape indexes of the UBS could be significantly extracted to the two component indicators (Formula 1, 2).Using principal component loadings, we can calculate UBS landscape index (Z).
Z1 is highly positively related to NP, PD, and SHEI, and is negatively related to CON-TAG, indicating a spatial distribution structure at a landscape scale.According to formula (1), NP, PD, CON, and SHEI contribute to Z1 significantly, with coefficients greater than 0.8, referring to the fact that Z1 is closely related to the integrality of the landscape.The higher the Z1, the greater the NP, PD, and SHEI indexes, and the smaller the CONTAG value, indicating more patches; the higher the patch density, the lower the agglomeration degree of various patches.Z2 is positively related to PAFRAC, LPI, and SPLIT, reflecting the spatial distribution structure at the patch scale.According to formula (2), PAFRAC, LPI, and SPLIT contribute to Z2 significantly, with coefficients greater than 0.8, referring to the fact that Z2 is closely related to the malconformation of the landscape.The higher the Z2, the greater the PAFRAC, LPI, and SPLIT indexes, meaning a more complex patch shape, a more extensive patch area, and a greater distance between patches.Z is the UBS landscape index.The greater Z is, the larger Z1 and Z2 are, representing a greater NP, PD, SHEI, SHDI, PAFRAC, LPI, SPLIT, and LSI, and a smaller CONTAG and AI, reflecting more severe UBS fragmentation and weaker spatial aggregation.
Z1 increased steadily from 2000 to 2014 and decreased until 2020, with a downward trend overall.The results revealed that the UBS patch number, density, and diversity had increased at fourteen years, and then declined in the last six years; the agglomeration weakened and then decreased.Overall, UBS in Beijing has faced severe fragmentation, which is expected to slow in recent years.
In contrast, Z2 decreased in the first decade and increased in the second decade, trending upward.The results showed that the shape complexity, area, and distance decreased first and then increased.From the perspective of the whole period, UBS in Beijing was disturbed from 2000 to 2020, and has been declining in the last decade.

Mechanisms Driving the Area of UBS
According to the correlation coefficients, the influencing factors rated from most to least importance are as follows: UEM > EDUI > STI > NDVI > T > GDP > POP > FVC > EVI > PREP > ASP.
The influencing factors were identified as strong factors (UEM, EDUI, STI), medium factors (NDVI, T, GDP, POP), and weak factors (FVC, EVI, PREP, ASP) according to the Jenks Natural Breaks Classification.
The results showed that scientific technology factors greatly influenced the UBS area, with correlation coefficients greater than 0.7.The strongest factor is UEM, with the highest coefficient of 0.798, followed by EDUI and STI, with coefficients of 0.759 and 0.758, respectively.The coefficients of NDVI and EVI indicated that the sparse vegetation surface magnified the UBS area more than the dense vegetation surface.POP and GDP have influenced UBS area, with correlation coefficients of 0.68 and 0.689, respectively.With correlation coefficients of 0.5, the influence of ASP on UBS area is smaller than that of natural and technology factors FVC and PREP.

Mechanisms Influencing the UBS Landscape
From the perspective of UBS landscapes, the influencing factors were rated as follows: PREP > POP > GDP > STI > T > EDUI > UEM > ASP > NDVI > EVI > FVC (Table 4).According to the results of the Jenks Natural Breaks Classification, the strong factors influencing the UBS landscape are PREP, POP, GDP, STI, and T, the medium factors are EDUI, UEM, and ASP, and the weak factors are NDVI, EVI, and FVC.Thus, it is reasonable to conclude that precipitation and human activities influence the UBS landscape more than vegetation factors.
POP and GDP have a great impact on UBS landscape, with correlation coefficients of 0.692 and 0.691, respectively, following the greatest indicator of 0.732, PREP.Technological factors have influenced the UBS landscape significantly, with correlation coefficients greater than 0.664.ASP is the next, with a correlation coefficient of 0.656.The smallest coefficient is 0.493, indicating that FVC has the weakest impact on UBS landscape.

Discussion
Urban blue space is essential to human life, especially for mental health and quality of life, which has caught eyes worldwide [74].The UBS area initially decreased from 2000 to 2004 due to significant water consumption resulting from population growth and industrial activities, leading to water shortages [75].However, a series of measures aimed at improving water resources and protecting urban water bodies subsequently led to a steady expansion of the UBS area.The implementation of the South-North Water Diversion Project, which began in late 2014, significantly contributed to the increase in UBS area [76].The year 2016 stands out in particular, with a sharp increase in UBS area, which can be explained by the significantly greater annual precipitation since 2015 [77] and the changes in water availability and management practices after the South-North Water Diversion Project (Figure 8).These findings align with previous research [78].The landscape pattern of UBS in Beijing showed stability with increasing diversity, richness, and evenness indexes from 2000 to 2020, indicating the generation of more water bodies with diverse properties and purposes.However, the dispersion and spatial heterogeneity indexes were poor, indicating severe fragmentation potentially caused by urban expansion and the erosion of ecological spaces [79].Water pollution policies implemented since 2015 have helped improve water microcirculation, leading to a reversal of negative trends in 2017, with decreasing density and diversity indexes and increasing connectivity and aggregation indexes.
The UBS in Beijing appeared to have a weak spatial autocorrelation, potentially influenced by artificial water bodies created in heat-island-reducing projects over the past two decades [80].Stable UBS clustering patterns were observed, with unique spots identified, such a shrinking clustering in Taishitun County, extinct clustering in Huairou District, and expanded clustering in Miyun District.Regulation policies such as vegetable cultivation, reservoir water network development, and reclaimed water usage are likely related to stable development and clustered expansion.Conversely, decreasing and extinct clusterings could be attributed to disturbances from industrial and agricultural consumption, river channel changes, and artificial water bodies [81].
According to the grey relation analysis results, science and technology management factors such as UEM, EDUI, and STI strongly impact the UBS area, in which UEM is the most vital factor, in accordance with the result of past research.Vaeztavakoli et al. emphasized that urban management is vital to urban blue space [82], while precipitation and human activities strongly influence UBS landscapes.Scientific technologies enhance production efficiency and water utilization efficiency, reducing water consumption and expanding the UBS area.
Precipitation replenishes water storage in urban water bodies and promotes vegetation growth, directly affecting the landscape pattern of the UBS.Human activities, including land use changes and surface modifications, have a direct and significant impact on UBS landscapes.Medium influencing factors, such as sparse vegetation, temperature, economy, and population, have a moderate influence on UBS area and landscapes.Weak factors are associated with land use patterns, particularly in dense vegetation areas, reflecting the limited correlation in these regions [83].The weak influencing factors in UBS landscapes are closely related to vegetation.

Conclusions
We employed remote sensing techniques to extract urban blue space (UBS) in Beijing and conducted a comprehensive analysis of its spatial and temporal development over the past two decades using ESDA methods.From an ecological perspective, we examined a wide range of landscape patterns, including area and composition, and quantitatively simulated the underlying mechanisms.The main findings are as follows: (1) The UBS area in Beijing experienced a decline from 2000 to 2004, followed by a steady increase over the next decade and a significant surge since 2016.(2) The spatial clustering of UBS exhibited overall stability with subtle variations.(3) With the exception of artificial surface proportion (ASP), UBS results in a more than 0.6% change in UBS area for every 1% change in those factors.(4) In terms of UBS area, urban environmental management (UEM), educational investment (EDUI), and scientific and technical investment (STI) were identified as strong influencing factors; normalized difference vegetation index (NDVI), temperature (T), gross domestic product (GDP), and population (POP) were classified as medium influencing factors; fractional vegetation cover (FVC), enhanced vegetation index (EVI), precipitation (PREP), and ASP were considered weak influencing factors.In terms of UBS composition, PREP, POP, GDP, STI, and T were factors; EDUI, urban environmental management (UEM), and ASP were classified as medium influencing factors; NDVI, EVI, and FVC were considered weak influencing factors.(5) Dense vegetation cover and sparse vegetation cover had distinct impacts on UBS area while exhibiting similar effects on UBS landscape patterns.
This study successfully uncovered the spatiotemporal characteristics of the UBS area and landscapes in Beijing from 2000 to 2020 and elucidated the multifactorial mechanisms driving these changes.However, the analysis resolution was limited to counties due to the availability of policy number and statistics data, rather than at the pixel level.Further research is needed to explore UBS at a finer scale.Additionally, while the study analyzed 11 influencing factors, it is acknowledged that there may be other factors that should have been explored, such as the number and effectiveness of policies.Furthermore, due to data limitations, the study focused on a 21-year period, and longer-term research is warranted in the future.

Figure 1 .
Figure 1.Study area of Beijing.

Figure 4 .
Figure 4. Illustration of principal component analysis theory.The distribution of a series of n binary observations ( ,  , ⋯ ,  ,  in the coordinate space composed of  and  is shown in Figure 4. Along the  or  axis, observation points have large discretization indicated by the variance of  or  , respectively.Axes  and  are rotated counterclockwise to axes  and  following formula X.The dispersion of n observation points on the  axis is the largest, indicating that variable  retains most of the information of the original data.  =           =

Figure 5 .
Figure 5.The area of UBS in Beijing from 2000 to 2020.

Figure 6 .
Figure 6.Spatial clustering pattern of the UBS in Beijing.

Table 3 .
Results of principal component analysis.

Table 4 .
Correlation analysis of UBS area and landscapes.