Assessment of Ecological Benefits of Urban Green Spaces in Nanjing City, China, Based on the Entropy Method and the Coupling Harmonious Degree Model

Abstract

Urban green spaces (UGSs) are an important spatial carrier for carbon sequestration in the national land space. The urban ecosystem is a highly harmonious composite ecosystem of nature, society, and the economy. Therefore, this study constructed an evaluation system of the UGS ecological benefit based on two subsystems–the natural environment and the social economy–in order to quantitatively evaluate the construction level of the UGS ecological benefit in Nanjing City and to reveal its temporal evolution characteristics. The entropy method was applied to assess the ecological benefits of UGSs in Nanjing City, China, from 2011 to 2020. The coupling harmonious degree model was utilized to analyze the dynamic coordination relations among subsystems. The Robust regression analysis was used to verify the evaluation results. The results showed that: (i) Between 2011 and 2020, UGS ecological benefits in Nanjing City exhibited a substantial and consistent upward trend. (ii) Between 2011 and 2020, the coupling harmonious degree among the subsystems of UGS ecological benefits in Nanjing City showed an overall rising trend of fluctuation. With the enhancing of the coupling harmonious degree among the subsystems of UGSs, the ecological benefit of UGSs will be significantly improved. (iii) The comprehensive evaluation score of the social and economic subsystem of UGS in Nanjing City was higher than that of the natural environment subsystem; this highlights the significant constraints posed by the natural environment on the construction of UGSs in Nanjing City. The research conclusion provides a decision basis for realizing the collaborative optimization of the natural environment and the social economy in Nanjing City and further promotes the sustainable development of the UGS ecological environment.

1. Introduction

The rapid development of China’s economy and society has led to a steady increase in urbanization and industrialization. However, this growth has also exacerbated several ecological and environmental issues, including air pollution, rain and flood disasters, heat island effects, and landscape fragmentation [1,2]. Moreover, environmental mass incidents caused by urban environmental pollution occur periodically [3], posing a huge threat to regional ecological security. UGSs serve as a crucial spatial component for carbon sequestration in the national land space. Studying the evaluation index system of ecological benefits of UGSs has significant theoretical implications for enhancing the ecological environment of these spaces, formulating accurate economic and environmental development strategies, and optimizing the UGS pattern. Moreover, it has a critical role in promoting the efficient carbon sequestration by UGSs. Therefore, the restoration and protection of urban green spaces (UGSs) have been increasingly recognized by all sectors of society.

UGS is a concept that only appeared in modern times. In the early studies of Britain, the United States and Japan, UGS was generally called “open space”. After the emergence of the modern urban planning system, scholars took green space as a urban space typology and defined its connotation from the perspective of urban land [4,5]. The Standard for Classification of Urban Green Space (CJJT85-2017) released and implemented by China in 2018 divides UGSs into five categories: park green spaces, protective green spaces, green squares, affiliated green spaces, and regional green spaces [6]. Both domestic and foreign researchers have conducted extensive studies and applications on the ecological evaluation of UGSs, which can be broadly categorized into different types. These include the ecological benefit evaluation [7], ecological structure evaluation [8], ecological function evaluation [9], ecosystem service evaluation [10], ecological health evaluation [11], and sustainability evaluation of UGSs [12]. Such diverse approaches reflect the multidimensional nature of UGSs and their contribution to the overall well-being of urban ecosystems. UGS ecological benefits refer to the degree of improvement in human survival and socioeconomic development conditions caused by UGS ecosystems and their changes [13]. Ecological benefits are the foundation of economic and social benefits. The high-quality development of UGS ecosystems can be truly achieved only by coordinating the relationship between the three. A review of the literature reveals that since the beginning of the 21st century, the research direction of UGS ecological benefit evaluation has evolved towards greater detail, primarily driven by the increasing use of 3S technologies such as remote sensing (RS), geographic information systems (GIS), and global positioning systems (GPS). These technologies have enabled researchers to conduct more accurate and comprehensive assessments of the ecological benefits of UGSs, considering various spatial and temporal factors. It has shifted from large-scale single-time pure forests such as forests, forests returned from farmland, and ecological public welfare forests (referring to forests and shrubbery whose main management objectives are to protect and improve the human living environment; maintain ecological balance; preserve species resources, scientific experiments, forest tourism and national land protection; and whose construction, protection and management are mainly invested by the government) to focusing on UGSs of various scales and specific garden plant applications [14,15]. Current research in this field has focused primarily on investigating the impact of various factors on the ecological benefits of UGSs. These factors include green-space shape characteristics [16], vegetation structure [17], external factors such as urban built environment, weather conditions [18], and biodiversity [19]. At present, the most widely used methods for evaluating the ecological benefits of UGSs include the value assessment method [20], the model evaluation method [21], and the indicator system method [22]. The commonly used methods for value assessment include the CTLA (Council of Tree and Landscape Appraisers) method [23], the AVTW (Amenity Valuation of Trees and Woodlands) method [24], and the Burnley method [25]. The commonly used models for model evaluation include the CITYgreen model [26], the i-Tree model [27], and the UFORE (Urban Forest Effects) model [28]. In the 1980s, the Canadian statistician Rapport first proposed the Pressure State Response (PSR) framework for evaluating ecosystem quality, which was widely used in ecological benefit evaluation research [29]. These methods offer different approaches for assessing the various ecological functions and services provided by UGSs, each with its strengths and limitations. Among the value assessment method, the model evaluation method, and the indicator system method, the indicator system approach has gained significant popularity due to its ability to provide fundamental research and application data for various types of UGS ecological measurements. It enables an intuitive reflection of the excellence of UGS ecological indicators and facilitates quick decision-making in UGS evaluation. The indicator system approach provides a wider scope of application for evaluating the ecological benefits of UGSs. Determining the weight assigned to each index is a crucial aspect of ecological benefit evaluation research using this method. The rationality of the weight determination will directly affect the evaluation results. Subjective methods for determining weights include the expert consultation method [30] and analytic hierarchy process [31], while objective methods include principal component analysis (PCA) [32] and the entropy method [33].

Recent years have witnessed an increasing interest in studying the construction of ecological civilization for the entire country. Ecological civilization construction means that in the face of the severe situation of tightening resource constraints, serious environmental pollution, and ecosystem degradation, we must establish the concept of ecological civilization that respects, conforms to, and protects nature, and takes the road of sustainable development [34]. Considerable achievements have been made in environmental theory and indicator system methods. However, from existing research, there are still some limitations in the research on the indicator system for the ecological benefits of UGSs: first, current research focuses more on ecological civilization [35], forest resources [36], land improvement [37], and other aspects, and there is less research on UGSs; second, a significant amount of the literature now focuses on the relationship between economic growth and environmental pollution [38], but in terms of indicator selection, more environmental pollution indicators are used, and there are few social and economic indicators, which lacks certain rationality. For example, Shen et al. evaluated the ecological benefits of green spaces in Fuzhou City from four aspects: cooling benefits, humidifying benefits, shading benefits, and wind protection benefits [39]. Zhou et al. evaluated the ecological benefits of the urban greenway in Ningbo City from the aspects of energy saving benefits, carbon sequestration benefits, air-quality improvement benefits, rainwater interception benefits, and aesthetic benefits [40]. Urban ecosystems are complex ecological systems highly coordinated between nature, society, and the economy. Their construction has shifted from large-scale planning to micro-ecological design and small-scale fine control [41]. The development of an evaluation indicator system for the ecological benefits of UGSs needs to consider the natural environmental characteristics of UGSs and important factors that affect the quality of the natural environment. This study aims to evaluate the ecological benefits of UGSs in Nanjing City over the past decade by introducing a novel evaluation system tailored to the current development context. This system is informed by existing urban ecological assessment frameworks, with focuses on the ecological protection of UGSs, and coordinates diversified economy and social development, designing two subsystems of the natural environment and social economy. The goal is to provide a theoretical basis for researchers and urban managers in the field of urban ecology to make decisions, to promote the sustainable development of UGSs, and to provide a valuable reference for the establishment of UGSs in similar cities worldwide.

2. Study Area and Method

2.1. General Description of the Study Area

Nanjing City, situated at the crossroads of the eastern coastal economic belt and the Yangtze River economic belt in Eastern China (31°14′ N to 32°37′ N, 118°22′ E to 119°14′ E), serves as the central economic hub of the metropolitan areas of Nanjing City (Figure 1). It holds the distinction of being the first officially approved cross-provincial urban zone in China. Nanjing City also serves as a significant national center for science education and comprehensive transportation, and has long been the political, economic, and cultural center of southern China. Nanjing City has a north subtropical monsoon climate, with low mountains and gentle hills as the main landforms. It has four distinct seasons and abundant rainfall. The average annual rainfall is 117 days, and the water area reaches over 11%. As the first sub-provincial city in China to receive the title of “National Ecological City”, Nanjing City encompasses 11 districts covering a total area of 6587.02 square kilometers. As of 2020, the city boasts a green coverage area of 1022.97 square kilometers, a permanent population of 9.3197 million, and an urbanization rate of 86.80%. In recent years, Nanjing City has followed the guidelines of the Xi Jinping’s Thought on Ecological Civilization, emphasizing the importance of balancing economic development and environmental protection. The city has been actively pursuing decision-making arrangements for ecological civilization construction at the central, provincial, and municipal levels. Its city construction goal is to promote “strong, prosperous, beautiful, and high-quality” development, with improving ecological environment quality as a core component [42].

2.2. Research Methods

2.2.1. Standardized Processing of Indicators

It is essential to standardize the original data to eliminate the dimension differences across different indicators. This involves transforming the attribute values of each indicator to a common scale within the range of [0, 1], ensuring comparability across indicators [43]. Evaluation indicators can be broadly classified into two categories: positive indicators, where larger values are considered better, and negative indicators, where smaller values are considered better. The standardization formula is as follows:

The formula for processing positive indicators:

$$Y_{ij}=\frac{X_{ij}-\mathrm{m}\mathrm{i}\mathrm{n}\left(X_j\right)}{\max \left(X_j\right)-\mathrm{m}\mathrm{i}\mathrm{n}\left(X_j\right)}(i=\mathrm{1,2},\dots ,n)$$

(1)

The formula for handling negative indicators:

$$Y_{ij}=\frac{\mathrm{m}\mathrm{a}\mathrm{x}\left(X_j\right)-X_{ij}}{\max \left(X_j\right)-\mathrm{m}\mathrm{i}\mathrm{n}\left(X_j\right)}(i=\mathrm{1,2},\dots ,n)$$

(2)

where $Y_{ij}$ denotes the standardized value of the jth indicator in the ith year; $X_{ij}$ denotes the current value of the jth original indicator in the ith year; $\max \left(X_j\right)$ denotes the maximum value of the jth original indicator for all years; and $\min \left(X_j\right)$ denotes the minimum value of the jth original indicator for all years. To ensure meaningful data processing, zero and negative values must be eliminated.

Following the research method of Zhang Yanfei et al. [44], this study performs an overall shift on the dimensionless data, that is, $X_{ij}=X_{ij}+d$. In order to preserve the intrinsic regularity of the original data to the greatest extent, d must be as small as possible, and this study takes d = 0.001.

2.2.2. Entropy Method

The concept of entropy originates in thermodynamics, primarily reflecting the degree of disorder within a system [45]. Shannon, drawing inspiration from thermodynamics, proposed “information entropy.” In information theory, information represents a measure of a system’s order, while entropy quantifies the degree of disorder. Although the absolute values of the two are equal, their signs are opposite. The entropy value method is employed to determine the weights of each index for standardized data, with the specific steps outlined below:

• Calculation of the proportion of the ith year’s index value under the jth index:

  $$P_{ij}=\frac{Y_{ij}}{{\sum }_{i=1}^m{Y}_{ij}}$$

  (3)
• Determination of the entropy value ($e_j$) for the index denoted by j:

  $$e_j=-\frac{1}{Inm}{\sum }_{i=1}^m{P}_{ij}In{(P}_{ij}),e_j\in [0,1]$$

  (4)
• Computation of the coefficient of variation ($d_j$) for the index represented by j:

  $$d_j=1-e_j$$

  (5)
• Computation of the weight ($w_j$) assigned to the index denoted by j:

  $$w_j=\frac{d_j}{{\sum }_{j=1}^n{d}_j}$$

  (6)
• Computation of the comprehensive evaluation score ($U_i$) through weighted summation:

  $$U_i={\sum }_{j=1}^n{Y}_{ij}{w}_j$$

  (7)

  where $U$ represents the comprehensive score, n is the number of indices, and $w_j$ represents the weight value of the jth index. The higher the value of $U$, the higher the comprehensive score, and the more favorable the evaluation result. Finally, the evaluation results were compared according to all $U$ values.
• Classification criteria:

The quantification of evaluation indicators is a top-down process, and the scores of each subsystem and the comprehensive evaluation score can be calculated based on the weights. The urban ecological benefit evaluation reported in the existing literature both in China and abroad has been reviewed. The grading design is divided into five grades, and the corresponding grading evaluations are given: excellent, good, fair, poor, and very poor. The classification is presented in

Table 1. Comprehensive grading standards for urban green spaces’ ecological benefits.

Grade	Comprehensive Score of Ecological Benefits	Graded Evaluation
I	0.80–1.00	Excellent
II	0.60–0.80	Good
III	0.40–0.60	Average
IV	0.20–0.40	Poor
V	<0.20	Very poor

.

2.2.3. Coupling Harmonious Model

The harmonious degree is a concept originating from physics. However, due to the similarity of coupling relationships between different systems, it has also been widely applied in other research fields. The coupling harmonious model is utilized to measure the interaction between two or more systems and the coordination level of each system in the development process [46]. This study employs the coupling and harmonious degrees to analyze the ecological benefits of various subsystems of UGSs.

• As there are two subsystems, the calculation formula is:

  $$C=2\times \sqrt{\frac{U_1\times U_2}{{(U_1+U_2)}^2}}$$

  (8)

  $$T=\alpha U_1+\beta U_2$$

  (9)

  $$D=\sqrt{C\times T}$$

  (10)

  where C denotes the coupling degree, reflecting the strength of the interaction between subsystems. U₁ and U₂ represent the comprehensive scores of the natural environment and social and economic subsystems, respectively. T represents the comprehensive harmonious index, and α and β are coefficients whose values are yet to be determined. In this study, both coefficients are set to 0.5, as the natural environment and social and economic subsystems are equally important [47]. D represents the harmonious degree, which can provide a more comprehensive evaluation of the development status of the two systems.
• Grade classification criteria:

This study categorizes the coupling and harmony levels of each subsystem of UGSs into four levels based on the strength of the relationship between the natural environment and social–economic subsystems and draws from the value ranges for coupling and harmony degrees found in the existing literature. The classification is presented in

Table 2. Classification standards for coupling and harmony levels.

Coupling Degree	Coupling Stage	Reference	Harmony Degree	Harmony Level	Reference
0 ≤ C ≤ 0.30	Low-level coupling stage	[48]	0 < D ≤ 0.30	Decline and maladjustment type	[49]
0.30 < C ≤ 0.50	Antagonistic stage	[50]	0.30 < D ≤ 0.50	Imminent maladjustment type	[51]
0.50 < C ≤ 0.80	Running-in stage	[52]	0.50 < D ≤ 0.80	Basic harmony type	[53]
0.80 < C ≤ 10	High-level coupling stage	[54]	0.80 < D ≤10	High-quality harmony type	[55]

Note: Among the low coupling level, if C = 0, it would correspond to no coupling.

.

2.2.4. Robust Regression Analysis

Robust regression can explain the dynamic relationships of multiple correlated variables and can be used to analyze the dynamic effects of time-series variable systems. In order to further clarify the stability of the research conclusions, the Robust regression analysis was performed by using the SPSSAU (Version 23.0) software, and the evaluation results of the urban green space ecological benefit and the results of the coupling coordination degree among subsystems were tested.

2.2.5. Correlation Analysis

Correlation analysis helps to identify the presence of a linear relationship between two sets of variables. The correlation values range from −1 to 1. A value approaching 1 indicates a strong positive correlation between the variables, while a value nearing −1 signifies a strong negative correlation. A value closer to 0 implies a weaker correlation [56]. In this study, 20 indicators from the UGS ecological benefit evaluation index system were selected, and the Origin 2022 software was used to process the data and to generate a correlation heatmap.

2.3. Data Sources

The data used for the evaluation indicators mainly came from the Nanjing Statistical Yearbook and the Jiangsu Statistical Yearbook from 2011 to 2020. On the basis of the collected data, the entropy method was applied to evaluate the ecological benefits of UGSs in Nanjing City from 2011 to 2020. Additionally, the coupled harmony degree model was employed to examine the interrelationships and evolutionary patterns between the subsystems of ecological benefits of UGSs.

3. Development of an Evaluation Index System for Assessing the Ecological Benefits of UGSs

3.1. Principles for Selecting Indicators

Reasonable indicator selection is key to constructing a good evaluation indicator system. When constructing the UGS ecological benefit evaluation indicator system, selecting too few or arbitrary indicators is not enough to reflect the overall characteristics of an UGS, and too many indicators are not practical or necessary. According to the structure, function, and regional characteristics of the object, the indicator system should follow the following principles:

(i) Hierarchy: the ecological benefits of an UGS involve a wide range of complex mechanisms, and hierarchical processing should be implemented when constructing the indicator system to meet different evaluation needs. (ii) Operability: the selected indicators should be easy to monitor, with simple content and easy access. (iii) Systematization: the ecological benefit evaluation indicator system is a multi-attribute, multi-level, and multi-change system, which should not only reflect the mechanism of the UGS ecosystem but also the correlation and harmony with social and economic benefits. Therefore, it is necessary to start from the system theory when constructing the system and to build a complete indicator system. (iv) Independence: to avoid reusing certain indicators and data overlap in calculating indicator benefits, the selected indicators should have a certain degree of independence from each other. (v) Representativeness: the specific situation of different cities should be considered when selecting indicators, and the indicators that fully highlight local characteristics should be chosen.

3.2. Indicator Selection

This study reviewed the authoritative indicator systems related to ecological civilization construction that were released by the government, selected high-frequency indicators, and constructed preliminary indicators based on the natural environment and social and economic subsystems. The government documents referenced by the index selection include the “Beautiful China Construction Evaluation Index System and Implementation Plan”, “National Ecological Civilization Construction Demonstration City and County Construction Indicators”, “Assessment Target System for Ecological Civilization Construction”, “Index Evaluation Indicator System for The Two-Mountain Theory (lucid waters and lush mountains are invaluable assets)”, “Evaluation Index System for Circular Economy Development” (2017 Edition), the “Green Development Indicator System”, “Green City Evaluation Indicators”, and the United Nations’ “Sustainable Development Indicator System”. In addition, the study also referred to a series of references [57,58,59,60,61,62].

3.3. Index System Construction and Weight Calculation

Based on the literature collection and policy document sorting, the frequency statistical method was used for specific indicator selection, and the indicators were further screened through expert consultation and data availability. Finally, 20 indicators were selected to construct the UGS ecological benefit evaluation indicator system. The weight coefficients of each secondary indicator were calculated using the entropy method, as shown in

Table 3. Evaluation index system for ecological benefits of urban green space.

System	Primary Index	Secondary Index	Unit	Attribute	Weight
Natural environment subsystem A1	Air quality B1	Number of days with good or better air quality C1	Days	+	0.0480
		Annual mean sulfur dioxide concentration C2	mg per m3	−	0.0532
		Annual mean nitrogen dioxide concentration C3	mg per m3	−	0.0580
		Annual mean inhalable particulate matter concentration C4	mg per m3	−	0.0367
	Water environment quality B2	Total water resources C5	Billion cubic meters	+	0.0378
		Annual precipitation C6	Millimeters	+	0.0525
	Noise environment quality B3	Average value noise in the region C7	Decibels	−	0.0229
	Biological environment quality B4	Green coverage area C8	Hectares	+	0.0408
		Public green space area per capita C9	Square meters	+	0.0470
		Green coverage rate of built-up areas C10	%	+	0.0642
Social and economic subsystem A2	Population factors B5	Urban population density C11	People per square kilometer	+	0.0721
		Population growth rate C12	%	−	0.0415
	Pollution control B6	Comprehensive utilization rate of industrial solid waste C13	%	+	0.0224
		Sewage treatment rate C14	%	+	0.0506
		Harmless treatment rate of household garbage and excrement C15	%	+	0.0397
	Resource allocation B7	Number of personal cars owned per 10,000 people C16	10,000 vehicles	−	0.1202
		Per capita road area C17	Square meters	+	0.0440
	Economic development B8	Per capita GDP C18	CNY	+	0.0447
		Proportion of secondary industry in GDP C19	%	−	0.0521
		Proportion of tertiary industry in GDP C20	%	+	0.0515

.

4. Results

4.1. Comprehensive Scoring Results of Ecological Benefits of UGSs in Nanjing City

Figure 2 reveals that the overall ecological benefits of UGSs in Nanjing City displayed a fluctuating upward trend from 2011 to 2020. Between 2011 and 2015, these benefits followed a “U”-shaped pattern, while a linear upward trend was observed from 2016 to 2020. Prior to 2015, the comprehensive evaluation score for the ecological benefits of UGSs fell below 0.4, suggesting suboptimal overall ecological benefits of UGSs; however, in 2015, it rose to 0.5157, indicating a shift from poor to average ecological benefits of UGSs. From 2016 to 2020, during the important development phase of the 13th Five-Year Plan, key development directions and main tasks focused on energy conservation, environmental protection industries, advanced environmental protection industries, and resource recycling industries. During this time, the comprehensive evaluation score of the ecological benefits of UGSs in Nanjing City ranged between 0.5 and 0.8, with a steady annual increase. In 2020, the rating reached “excellent,” indicating a significant improvement in the ecological benefits of UGSs in Nanjing City over the five-year period, as shown in

Table 4. Comprehensive scores and evaluation grades of the ecological benefits of urban green spaces in Nanjing City from 2011 to 2020.

Year	Classification Index		Comprehensive Score	Ranking	Status Description
	Natural Environment	Social Economy
2011	0.1673	0.2289	0.3963	7	Poor
2012	0.1093	0.1828	0.2921	9	Poor
2013	0.0735	0.1954	0.2689	10	Poor
2014	0.1347	0.2129	0.3476	8	Poor
2015	0.2550	0.2607	0.5157	6	Average
2016	0.3302	0.2390	0.5692	5	Average
2017	0.3091	0.2725	0.5816	4	Average
2018	0.3208	0.3236	0.6444	3	Good
2019	0.3066	0.3757	0.6823	2	Good
2020	0.3854	0.4178	0.8032	1	Excellent

. In addition, during 2011–2015 and 2018–2020, the comprehensive evaluation score of the social and economic subsystem of UGSs in Nanjing City is higher than that of the natural environment subsystem.

4.2. Assessing Coupling and Harmony Degrees to Evaluate the Ecological Benefits of UGSs

Between 2011 and 2020, the coupling degree between the UGS natural environment subsystem and the social–economic subsystem in Nanjing City ranged from 0.9212 to 0.9993. The overall level remained relatively stable and consistently in a high-level coupling stage, as shown in

Table 5. Coupling degree and harmonious degree of urban green space in Nanjing City from 2011 to 2020.

Year	Coupling Degree	Coupling Stage	Harmony Degree	Harmony Level
2011	0.9969	High-level coupling stage	0.6266	Basic harmony type
2012	0.9842	High-level coupling stage	0.5325	Basic harmony type
2013	0.9212	High-level coupling stage	0.4904	Imminent imbalance type
2014	0.9887	High-level coupling stage	0.5829	Basic harmony type
2015	0.9978	High-level coupling stage	0.7192	Basic harmony type
2016	0.9719	High-level coupling stage	0.7507	Basic harmony type
2017	0.9901	High-level coupling stage	0.7630	Basic harmony type
2018	0.9973	High-level coupling stage	0.8040	High-quality harmony type
2019	0.9997	High-level coupling stage	0.8252	High-quality harmony type
2020	0.9993	High-level coupling stage	0.8972	High-quality harmony type

. Unlike the coupling degree, the harmony degree between the UGS natural environment and social and economic subsystems in Nanjing City showed a more obvious fluctuating upward trend between 2011 and 2020, as depicted in Table 2. The harmony degree value was as low as 0.4904 in 2013, which was on the verge of imbalance. After 2018, the harmony degree value was higher than 0.8 and remained at the high-quality harmonious stage.

4.3. Test of Ecological Benefit of Urban Green Space in Nanjing

4.3.1. Stability Test of Evaluation Results

The Robust regression analysis was performed by using SPSSAU software to verify the evaluation results of ecological benefits of UGSs in Nanjing City and the results of the coupling harmonious degree among the subsystems (

Table 6. Robust regression analysis results.

Variable	Regression Coefficient	Standard Error	t	p	95% CI	Adjusted R2
X: Year Y: Comprehensive scoring of ecological benefit of UGSs	0.059	0.007	8.202	0.000 **	0.045~0.073	0.834
X: Year Y: Coupling harmonious degree among the subsystems	0.042	0.006	7.647	0.000 **	0.032~0.053	0.788
X: Coupling harmonious degree among the subsystems Y: Comprehensive scoring of ecological benefit of UGSs	1.326	0.043	30.698	0.000 **	1.241~1.411	0.986

Note: ** p < 0.01; X: the independent variable; Y: the bit-dependent variable.

). The results showed the following: (i) Taking the year as the independent variable and the comprehensive scoring of ecological benefits of UGSs as the dependent variable, the regression coefficient was 0.059 (t = 8.202; p = 0.000 < 0.001), indicating that the ecological benefit of UGSs in Nanjing had a significant positive effect from 2011 to 2020. (ii) Taking the year as the independent variable and the coupling harmonious degree among the subsystems as the dependent variable, the regression coefficient was 0.042 (t = 7.647; p = 0.000 < 0.001), indicating that the coupling harmonious degree among the subsystems in Nanjing had a significant positive effect during 2011–2020. (iii) Taking the comprehensive scoring of ecological benefits of UGS as the independent variable and the coupling harmonious degree among the subsystems as the dependent variable, the regression coefficient value was 1.326 (t = 30.698; p = 0.000 < 0.001), indicating that the coupling harmonious degree among the subsystems has a significant positive effect on the comprehensive scoring of ecological benefits of UGSs. The p values of the above regression results were all less than 0.001, indicating that the Robust regression could accurately evaluate the correlation between the above variables and verified the stability of the comprehensive evaluation results.

4.3.2. Correlation Analysis between Various Indicators

Figure 3 highlights a notable correlation between certain indicators in the natural environment subsystem, such as sulfur dioxide concentration, nitrogen dioxide concentration, inhalable particulate matter concentration, green coverage area, per capita public green-space area, and green coverage in built-up areas, and indicators in the socioeconomic subsystem, including sewage treatment rate, harmless treatment rate of domestic garbage and feces, per capita car ownership, per capita road area, per capita GDP, and the proportions of secondary and tertiary industries in the GDP.

Between 2011 and 2020, indicators such as sewage treatment rate, domestic garbage treatment rate, per capita car ownership, and the proportion of the tertiary industry all showed a highly significant negative correlation with air pollutants (nitrogen dioxide, sulfur dioxide, inhalable particulate matter). This shows that with the gradual increase of indicators such as sewage treatment rate, domestic garbage treatment rate, per capita car ownership, and the proportion of the tertiary industry, the concentration of air pollutants in Nanjing City decreased significantly. The main reason is that with technological innovation and structural industrial optimization and upgrading, the proportion of new energy vehicles in the city has gradually increased. On the other hand, a series of measures, such as scientific pollution control, precise pollution control, legal pollution control, and systematic pollution control, have achieved significant results. As a result, the concentration of air pollutants in Nanjing City has shown a downward trend. In addition, with the gradual increase of indicators such as urban green-coverage area, per capita park green-space area, and green coverage in built-up areas, pollution control, resource allocation, and economic development have shown a highly significant positive correlation with these indicators. This indicates that implementing UGS natural environment protection and improvement measures will greatly promote the high-quality development of society and the economy. In recent years, Nanjing City has focused on winning the battle to restore the ecology of the Yangtze River. It has implemented a series of environmental protection measures to attain blue sky, clear water, and clean land. These measures have played a positive role in improving the ecological benefits of UGSs in Nanjing City while promoting the modernization of the UGS governance system and governance capacity and continuously improving social and economic development.

5. Discussion

5.1. Analysis of Ecological Benefit of UGSs in Nanjing from 2011 to 2020

China was in an important period of the 12th Five-Year Plan from 2011 to 2015, during which industrialization and urbanization were rapidly advancing, the consumption structure of urban and rural residents was accelerating and improving, and the trend of globalization was gradually strengthening. Investment in UGS ecological construction was very limited during this period. At the same time, the development of strategic emerging industries led to the continuous expansion of the industrial production scale, resulting in increased emissions of air, waste, and other pollutants, which seriously damaged the UGS ecological environment. Therefore, the overall ecological benefit of UGSs is suboptimal. From 2016 to 2020, during the late period of the 12th Five-Year Plan, the Chinese government introduced several policies and regulations, such as the “Opinions of the State Council on Strengthening Urban Greening Work” (2014) and “Made in China 2025” (2015), to promote sustainable urban development and to address environmental challenges. The comprehensive evaluation score of ecological benefits from UGSs gradually increased. This was mainly attributed to the transformation of traditional industries, the full utilization of modern technological achievements, and the continuous optimization and upgrading of industrial structure through strategic planning in the later period, which effectively alleviated the ecological pressure on UGSs.

This substantial improvement during the 13th Five-Year Plan (2016–2020) can be attributed to the joint release of the “National Urban Ecological Protection and Construction Plan (2015–2020)” by the Ministry of Housing and Urban–Rural Development and the Ministry of Environmental Protection in 2016. In response, the Nanjing City government implemented various environmental protection policies and support measures, such as the “Nanjing City 13th Five-Year Plan for Ecological Environment Protection” (2016), “Nanjing City Environmental Protection Regulations” (2017), and “Nanjing City Permanent Green Space Management Regulations” (2018). These policies emphasized environmental protection as a crucial aspect of the city’s comprehensive and long-term development, effectively bolstering the ecological construction of UGSs.

On the one hand, Nanjing City must adhere to ecological priorities and strengthen UGS planning in the future. A green development concept should be established, with the ecological benefits of UGSs regarded as an important basis for measuring development. It is a necessary goal to construct high-quality UGSs. This city should strengthen the environmental governance, ecological protection, and ecological compensation of UGSs. Nanjing City needs to strengthen the awareness of relevant personnel responsibilities, make full use of market mechanisms, and comprehensively use various means to strengthen UGS planning, such as optimizing UGS layouts, building UGS monitoring networks and regular monitoring, and implementing management and supervision. On the other hand, it is essential to refine the industrial structure and to encourage iterative advancements. Over the past decade, Nanjing City has seen a considerable increase in the high input–output tertiary industry proportion, which has positively influenced the ecological benefits of UGSs. However, there is still room for adjustment in the industrial structure. The government should adjust the industry according to regional characteristics and resource endowments, cultivate emerging industries, and achieve the transformation and upgrading of traditional industries toward green development.

5.2. Assessing Coupling and Harmony Degrees to Evaluate the Ecological Benefits of UGSs

These data indicate a significant positive effect between the natural environment and social and economic factors in Nanjing City in different years and further demonstrate a strong mutual influence and interaction and a long-term benign resonance and coupling state. These findings also confirm the theoretical expectation of the academic community that the urban ecosystem is a highly harmonious composite ecosystem of nature, society, and the economy [63]. Natural environment protection and social and economic development are closely related to the construction degree of ecological benefits of UGSs, and a dynamic correlation exists between them that mutually promotes each other. From 2011 to 2020, natural environment protection in Nanjing City promoted social and economic development through top-level design, positive incentive mechanisms, and increasing public environmental awareness. Social and economic development also affected natural environment protection through technological innovation and industrial structure optimization.

The reasons for this are twofold. On the one hand, the harmony degree is an indicator that comprehensively considers the level of the natural environment and social and economic development and their coupling relationships. Before 2015, the comprehensive index of the two subsystems in Nanjing City was relatively low, and the harmony degree was affected at a low level. On the other hand, before 2015, the difference in comprehensive scores between the two subsystems was large, reaching 0.1219 in 2013, which affected the improvement of the harmony degree. Overall, in recent years, the harmony level between the UGS natural environment and social and economic subsystems in Nanjing City has steadily increased. The support of socioeconomic development for the natural environment is gradually deepening.

In the future, Nanjing City still needs to consider how to fully leverage the mutual promotion of natural environment protection and social and economic development while improving the level of both.

This city must compensate for regional shortcomings and achieve harmonious development. Nanjing City should analyze its location advantages, reasonably plan UGSs, develop differentiated development ideas, and highlight regional characteristics. Combined with the actual local situation, Nanjing City should rely on favorable regional environmental conditions to take active measures to govern UGSs. The development advantages and shortcomings should be carefully analyzed to achieve high-quality development of the ecological benefits of UGSs.

5.3. Shortcomings and Prospects

While the connotations of the ecological benefits of UGSs are abundant, there are limitations in this study. On the one hand, this study quantitatively evaluates the level of ecological efficiency construction of UGSs in Nanjing City, but it is subject to limitations which result from data availability and so on. The selection of indicators still needs to be further optimized and improved. On the other hand, obtaining relevant indicator data for each district and county in Nanjing City was difficult. This study only selected time-series data for econometric analysis, which did not fully reflect the spatial differences of the research questions.

With regard to the main research directions in the future: we should pay attention to research on UGS ecosystem management, so as to avoid the negative impact of UGSs on human beings and to be aware that human beings should coexist with the ecosystem in an equal and harmonious way. For example, urban green space management will cause soil acidification, pesticide pollution, and other problems unless sustainability practices are properly planned and executed. In addition, threshold rules exist in the evolution process of the urban green space ecosystem. In the future, the S-shaped curve threshold and the Panel-STAR model [64] can be used to further study the key thresholds of urban green space natural environment and socio-economic subsystems so as to provide theoretical support for the quantitative evaluation of ecological benefits, judgment of resource and environment carrying capacity, and ecological security monitoring and early warning.

6. Conclusions

In this study, by constructing a comprehensive evaluation system, the entropy method was utilized to evaluate the ecological benefits of UGSs in Nanjing from 2011 to 2020. The dynamic coordination between subsystems was analyzed by the coupled harmony model, and the evaluation results were verified by Robust regression analysis. The following main conclusions were drawn:

• From 2011 to 2020, the comprehensive evaluation score of ecological benefits of UGSs in Nanjing City showed a clear upward trend, with a significant increase in development speed after 2013, and the best level in 2020 during the 13th Five-Year Plan. However, the score of the natural environment subsystem is relatively low, based on the comparisons of the scores of each subsystem. This finding indicates that the ecological benefits of UGSs in Nanjing City are greatly constrained by the natural environment. Therefore, efforts should be made to construct UGSs in Nanjing City in the future. This city should further increase the natural environment management of UGSs.
• From 2011 to 2020, the coupling degree between the UGS natural environment and the socioeconomic subsystems in Nanjing City was between 0.9212 and 0.9993, indicating a high coupling level. This suggests that the two systems exhibit a strong mutual influence and interaction. The harmony degree, which was as low as 0.4904 in 2013, showed a more obvious fluctuating upward trend, which was on the verge of imbalance. After 2018, the harmony degree value was higher than 0.8 and remained at the high-quality harmonious stage. The improvement in the harmony degree is mainly attributed to the comprehensive evaluation score of ecological benefits from UGSs.
• With the improvement in the coupling harmonious degree among the subsystems of UGSs, the ecological benefit of UGSs will be significantly improved. According to the correlation analysis of various indicators, overall indicators such as urban green-coverage area, green coverage in built-up areas, and per capita park green-space area have a highly significant positive correlation with resource allocation and economic development indicators, and have a significant negative correlation with air pollutant indicators. The quality of the ecological environment is indicative of the regional level of social and economic development and can influence pollutant emissions.

The main contributions of this study are as follows: First, the empirical research verifies the theoretical expectation of the academic circle, that there is a dynamic correlation between the natural environment and the social economy, and that the coupling harmonious degree among the subsystems of UGSs will have a significant positive impact on the ecological benefits of UGSs. Second, it provides a reference for further research on the ecological benefits of UGSs in different regions and scales. Third, the research conclusion provides a decision basis for realizing the collaborative optimization of the natural environment and the social economy in Nanjing City, and further promotes the sustainable development of the UGS ecological environment.