Identifying Key Areas of Green Space for Ecological Restoration Based on Ecological Security Patterns in Fujian Province, China

Abstract

Urban expansion has resulted in the fragmentation of green spaces. Based on the concept of a living community that integrates mountains, rivers, forests, farmlands, lakes, and grasslands, the extraction of key elements in green spaces of regional ecosystems provides core scientific support for the ecological restoration of territorial spaces. According to the ecological service function importance and ecological sensitivity, the ecological sources were identified in this study. Furthermore, we distinguished the ecological corridors using the minimum cumulative resistance (MCR) model and identified the key areas of green spaces using the circuit theory model. The result showed that (1) 62 ecological sources were present with a total area of 4696 km², of which green space accounted for 98.19%; meanwhile, 151 ecological corridors (optimal path) were densely distributed in the southwest region around the Daimao and Bopingling mountains. (2) The key areas of ecological restoration in the study area included 17 key ecological sources and 19 key ecological corridors. The area covered by ecological pinch points was 1327 km², among which 77.54% of green space comprised forest area. The area of ecological barriers was 9647 km², and the forest area still accounted for the highest proportion (63.92%). (3) Based on a comprehensive analysis of the spatial distribution of key areas of ecological restoration and green spaces, we formulated classified ecological restoration measures. The study findings are expected to provide a reference for planning the ecological restoration of territorial spaces.

1. Introduction

As global ecological problems have become increasingly serious, China has proposed a strategy for the development of ecological civilization to substantially improve the urban and rural human settlement environments. In this context, the ecological and environmental pressures in China necessitate that urban development should be matched with the carrying capacity of resources and environment, open green space in cities should be strictly protected and further expanded, and the multi-function complex urban green space, based on ecological corridors, such as mountains, rivers, forests, and lakes, should be maintained to ensure the functional integrity of the ecosystem [1]. In 2016, the concept of a living community that integrates mountains, rivers, forests, fields, lakes, and grasslands was proposed to promote China′s ecological protection and restoration via effective planning of all factors, which has become the focus of many scholars [2]. As a basic urban element, green space promotes ecosystem services and can alleviate urban environmental problems, including the mitigation of the urban heat island effect, regulation of rainwater, carbon sequestration, and oxygen release. Green space can restore the urban ecosystem and relieve the urban ecological pressure [3,4,5].

The ecological security pattern is a potential ecosystem spatial pattern comprising key elements of different types of landscapes and their spatial locations, which is of great significance for the protection of biodiversity and the stability of ecosystem structure [6]. The ecological security pattern has become an effective way to ease ecological protection and economic development [7]. At present, the basic research framework of the “source identification-resistance surface construction-corridor extraction” has been adopted in many studies [8,9]. (1) An ecological source is a patch or ecological space that is important for the ecological security of a region; it is usually directly identified by nature reserves, large parks, and green spaces, or by quantitative assessment using a composite index system [10,11,12]. In the former approach, it is convenient to directly identify ecological patches, but the impact of ecological service is ignored. For example, some natural reserves have experienced degradation over time and failed to provide corresponding ecological services [13]. In the latter approach, the comprehensive index system usually includes ecological sensitivity, landscape connectivity, ecological service importance, etc. [14,15]. The construction of an ecological security pattern aims to improve the level of ecosystem services and promote human well-being. Therefore, the identification of ecological sources should mainly take the ability to provide effective services to humans into account [16]. Therefore, ecological importance indices are widely used [17]. (2) The concept of resistance surface was introduced from the transport geography field, aiming at measuring how landscape characteristics affect movement differentially and provide greater spatial complexity in connectivity modelling [18]. Most previous studies have constructed resistance surfaces by land-cover types that are assigned directly based on expert scoring [19]. This approach ignores the interaction between land-use and ecological processes, the complexity of land use patterns and the impacts of human activities [20]. In recent years, various spatial data are introduced to modify ecological resistance surfaces, such as Nighttime Light (NTL), traffic, and topography [21]. Compared with simple methods of constructing resistance surfaces by assigning different land cover values, these quantitative analysis methods are more accurate [16]. (3) The simulation of corridors based on the resistance surface is usually identified by the minimum cumulative resistance (MCR) model and circuit theory [22,23,24,25]. The former approach (MCR model) is used to simulate the minimum cost path by calculating the “cost” of species movement from the core areas when passing by landscapes, which is widely used in assessing landscape connectivity [26,27,28]. The latter is a relatively new approach for modelling functional connectivity in landscapes. It applies principles from electrical circuit theory, allowing for all available movement possibilities to be considered and mapped using resistance surfaces [29]. These surfaces (landscapes) are evaluated based on the cost of movement between nodes (core areas), and the lower the resistance, the higher the probability of moving between nodes. Linking nodes create a cost path that can be represented by cumulative resistance values or cost-weighted distance [29]. Therefore, it is possible to measure the probability of moving between two spatial locations, taking into account all other available routes [30].

The landscape ecology, based on the coupling of patterns and processes, can help clarify the complex relationships between the elements of landscape, mountains, rivers, forests, fields, lakes, and grasslands to provide a theoretical basis for implementing the ecological restoration of an entire area and all of its processes [31]. An ecological security pattern is a form of spatial simulation for landscape pattern optimization that can promote the concept of a living community, which aims to integrate mountains, rivers, forests, lakes, and grasslands based on global connectivity [32]. However, further identifying the key areas based on the identified ecological security pattern, especially for the green space areas requiring ecological restoration, has become a key issue for ensuring ecological security. Currently, the main targets of ecological restoration are rivers, lakes, wetlands, mining wastelands, and other single types of landscapes on a small scale [33,34,35,36]. This approach is aimed at a single element and ignores the integrity and systematic nature of the ecological process, while ecological restoration on a macroscopic scale can account for the integrity of the ecosystem. Therefore, the identification of key areas restricting the ecological service function aids in the accurate identification of the ecological breaking point to recover and enhance the ecological service function and realize the maximization of ecological objectives. McRae et al. proposed a connectivity restoration method for identifying biological habitats, such as the ecological pinch points and barrier areas, which can achieve the greatest ecological restoration effect with the smallest area, namely, optimal ecological restoration [37,38]. Previous studies have seldom identified and mapped the key areas of green space systematically. Hence, our study focuses on the green space with ecological advantages and aims to identify the key areas of green space, including urban landscapes, natural forests, urban farmland, wetlands, and other spaces with compound functions, including life support, ecological regulation, and environmental protection [39]. The meaning of green space is broad and complex. Usually, green space consists of vegetation and natural elements, and it may also refer to open space accessible to low-mobility residents, requiring paths, and flat surfaces [40]. Though it is unable to provide clear and unifying descriptions of green space, there are two possible explanations for green space, which offer a better functional understanding when a definition is provided. The first is that a macro understanding of greenspace is provided, which refers to areas covered with vegetation in a landscape, such as forests and wilderness areas, street trees and parks, gardens, and backyards in urban and rural areas [41]. The second interpretation is that green space is confined to urban areas and described as a subset of open space, representing a vegetated variant of open space, such as parks, gardens, yards, and urban forest farms [40]. According to the need for integrated regional development and coordinated urban-rural development in the context of territorial space planning, combined with the land resources classification system of China [42], this study provides the macro meaning of green space to classify land-cover types in Fujian Province. In ARCGIS10.3, the land-cover types of Fujian Province were reclassified into six types: grassland, forestland, cultivated land, wetland and water area, developed land and bare land, among which the green space consisted of grassland, forestland, cultivated land, wetland and water area.

The Fujian province has complex landforms, a good ecological foundation, the highest forest coverage rate in China, abundant species diversity, and is an important ecological region in the south of China. However, Fujian province has experienced serious ecological security problems, such as biodiversity degradation and forest, soil, and coastal erosion [43,44,45,46,47]. Therefore, our study regarded Fujian province as the study area and identified the ecological source areas by evaluating the ecological service function importance and ecological sensitivity and constructed ecological corridors by combining the MCR model and circuit theory. On this basis, the ecological pinch points and the barrier areas were identified, and the key areas of green space in the ecological spaces in Fujian province were evaluated as prioritized areas for ecological restoration. Furthermore, we also proposed suggestions for ecological restoration. Our study can provide a reference for decision-making for the overall planning of ecological protection and restoration in Fujian province.

2. Materials and Methods

2.1. Study Area

Fujian Province is located in the southeast region of China (23°33′ N–28°20′ N, 115°50′ E–120°40′ E), with a total land area of 124,000 km² (Figure 1). It experiences a subtropical monsoon climate with abundant rainfall. The annual precipitation is about 1700 mm. The majority of the areas in Fujian are mountainous and hilly, accounting for more than 80% of the total land area. Overall, the terrain is high in the northwest and low in the southeast. The cross-section is slightly saddle-shaped, forming the western Fujian alpine belt (Wuyi Mountain) and the central Fujian alpine belt (Jiufeng-Daiyun-Bopingling Mountain).

2.2. Data Sources

Land-cover data (2020) were obtained from the GlobeLand30 dataset [48] (http://globallandcover.com/, accessed on 11 March 2022) with a resolution of 30 m. The green space in the current study included cultivated land, forest land, grassland, wetland, and water [42,49]. In 2018, fractional vegetation cover data were obtained from the National Earth System Science Data Sharing Infrastructure and National Science and Technology Infrastructure of China [50] (http://www.geodata.cn, accessed on 11 March 2022), with a resolution of 500 m. Net primary productivity (NPP) data were obtained from the NASA MODIS satellite products (https://modis.gsfc.nasa.gov, accessed on 11 March 2022) with a resolution of 500 m. Digital elevation model (DEM) data adopted the GDEMV2 digital elevation data with a resolution of 30 m, which were obtained from the Geospatial Data Cloud (http://www.gscloud.cn, accessed on 11 March 2022). Precipitation and temperature data were obtained from the National Earth System Science Data Sharing Infrastructure and National Science & Technology Infrastructure of China (http://www.geodata.cn, accessed on 12 March 2022), with a resolution of 1000 m. Rainfall erosivity and soil erodibility data with resolutions of 250 and 30 m, respectively, were obtained from the Loess Plateau Data Center, National Earth System Science Data Center, and National Science & Technology Infrastructure [51,52] (http://loess.geodata.cn, accessed on 19 October 2021). Slope gradient and length data with a resolution of 1000 m were obtained from the National Earth System Science Data Sharing Infrastructure and National Science & Technology Infrastructure of China (http://www.geodata.cn, accessed on 19 October 2021). Soil data with a resolution of 1000 m were obtained from the Soil Sub Center, National Earth System Science Data Center, and National Science & Technology Infrastructure (http://soil.geodata.cn, accessed on 19 October 2021). Nature reserve data were obtained from the China nature reserve specimen resource sharing platform (http://www.papc.cn/, accessed on 19 October 2021). The basic geographic data and spatial administrative boundary vector data were obtained from the National Basic Geographic Information System database (http://www.ngcc.cn/ngcc/, accessed on 13 April 2021).

2.3. Methods

Firstly, based on the framework of the “ecological source-resistance surface-ecological corridor-ecological node,” we identified the ecological source by analyzing the ecological service function (water conservation, soil and water conservation, windbreak and sand fixation, and biodiversity) importance and ecological sensitivity (soil erosion; Figure 2). Secondly, considering human disturbance and its impact on the natural environment, we selected three factors, namely, land-cover types, terrain, and distance to roads, to construct the minimum cumulative cost resistance surface. Subsequently, the ecological corridors and ecological nodes were extracted by the MCR model and circuit theory models. Finally, the natural breakpoint method was used to classify the ecological sources, ecological corridors, and ecological nodes as the key areas of ecological restoration [53].

2.3.1. Ecological Sources Extraction

Ecological sources are important for species migration and energy flow. They can provide important ecosystem services and must be protected [8,54]. In our study, the ecological service function importance and ecological sensitivity were comprehensively evaluated to identify the ecological sources [55].

(1) Evaluation of the Ecological Service Function Importance

To some extent, there are relationships between ecosystem services, such as the services of water yield, soil conservation, soil erosion by wind, and net primary productivity (NPP) [56]. For example, Hao et al. found that improving NPP decreases water yield in arid and semiarid areas [57]. For example, Costanza et al. found that there is a strong positive correlation, in certain temperature ranges, between biodiversity and NPP, so changes in biodiversity are correlated to changes in net primary productivity [58]. Meanwhile, in view of the “Fujian province Ecological Protection Redline Delineation Results Adjustment” program implemented by the Ministry of Ecology and Environment of China and the actual ecological status of the study area, we selected water conservation (WR), soil and water conservation (S_pro), windbreak and sand fixation (S_ws), and biodiversity (S_bio) to evaluate the ecological service function importance, which was calculated using the quantitative index method [59].

The formula of water conservation (WR) is as follows:

$$WR=NP{P}_{mean}\times F_{sic}\times F_{pre}\times \left(1-F_{slo}\right)$$

(1)

where WR is the ecosystem water conservation capacity index, NPP_mean is the annual average NPP of vegetation (2000–2015), and F_sic is the soil percolation factor obtained by dividing the T_USDA_TEX into the global soil data (HWSD) by 13. According to the soil texture classification of the United States Department of Agriculture (USDA), the values of 13 soil texture types were equally assigned between 0 and 1; clay (heavy): 1/13, silty clay: 2/13, …, and sand: 1. F_pre was calculated using the average rainfall from 2000–2020 and F_slo is the slope factor.

The formula of biodiversity is expressed as follows:

$$S_{bio}=NP{P}_{mean}\times F_{pre}\times F_{tem}\times \left(1-F_{alt}\right)$$

(2)

where S_bio is the index of biodiversity maintenance service capacity, NPP_mean is the annual average NPP of vegetation (2001–2015), F_pre is the index of annual average precipitation (2001–2020), F_tem is the index of annual average temperature (2001–2020), and F_alt is the index of elevation.

The formula for soil and water conservation is as follows:

$$S_{pro}=NP{P}_{mean}\times \left(1-K\right)\times \left(1-F_{slo}\right)$$

(3)

where S_pro is the index of soil and water conservation service capacity, NPP_mean is the annual average NPP of vegetation (2001–2015), F_slo is the slope factor, and K is the soil erodibility.

The formula for windbreak and sand fixation is as follows:

$$S_{ws}=NP{P}_{mean}\times K\times F_q\times D$$

(4)

where S_ws is the index of windbreak and sand fixation service capacity, NPP_mean is the annual average NPP of vegetation (2001–2015), K is the soil erodibility, F_q is the index of multi-year average climate erosivity, and D is the index of surface roughness.

(2) Ecological Sensitivity

According to the “Fujian province Ecological Protection Redline Delineation Results Adjustment” program and the “National Ecological Protection Red Line—Technical Guidelines for the Delineation of Ecological Function Red Lines (Trial),” the soil erosion index (SS_i) was used to evaluate the ecological sensitivity. The formula is as follows:

$$S{S}_i=\sqrt[4]{R_i\times K_i\times L{S}_i\times C_i}$$

(5)

where SS_i is the soil erosion index, R_i is the index of annual average rainfall erosivity (2001–2015), K_i is the soil erodibility index, LS_i is the index of slope gradient and length, and C_i is the fractional vegetation cover index.

Based on the comprehensive evaluation results of the ecological service function importance and ecological sensitivity, the results were corrected by selecting the national nature reserves in the study area. Subsequently, the evaluation results were divided into three grades by the natural breakpoint method: “high,” “medium,” and “low”. We selected the “high” grade areas and filtered patches with areas more than 10 km² as ecological sources [60]. The resolution of the output models is 1000 m.

2.3.2. Resistance Surface Construction

Species migration and energy flow need to overcome resistance in the corridors. Previous studies directly regarded the land-use type data as the resistance surface [41]. However, differences exist between the physical geography and human disturbance that affect the types of ecological processes [60,61,62]. As for land cover types, forestland and grassland cover some ecological resources with a small movement cost relative to the most suitable core habitats, so they were assigned slightly higher resistance values [63]. Water areas were selected as unsuitable, given Tremblay and St. Clair’s findings of the intense reluctance of woodland birds to cross it [64]. Developed land, including paved surfaces and buildings, was the least suitable base land cover class due to the lack of habitat amenities and their presence as physical barriers to movement in many cases [63]. There is observed reluctance to crossroads with heavy vehicle traffic, so major roads should be considered when considering factors of movement between nodes or core areas in our study areas [64]. In addition, Fullman et al. examined the relationship between relative habitat suitability and landscape resistance directly by analyzing five resistance scenarios and found that caribou avoided rugged terrain [65]. It means that terrain has potential influences on the movement of species. Hence, the terrain is considered in our study to calculate the cost between nodes. In view of the above descriptions, the actual ecological status of the study area, and the availability of data, three factors were considered, namely, the land-cover types, relief degree of the land surface, and distance to roads, to determine the resistance grades and weights (

Table 1. Factor grades and weights in the ecological resistance analysis.

Resistance Factor	Weight	Resistance Grade	Resistance Value
Land-Cover Type	0.4	Forest land	1
		Grassland	10
		Cultivated land	30
		Wetland and water	50
		Developed land	500
		Bare land	300
The Relief Degree of Land Surface (°)	0.3	h < 15	1
		15 < h < 30	20
		30 < h < 60	60
		60 < h < 90	80
		>90	100
Distance to Railway (m)	0.3	d < 1000	100
		1000 < d < 2000	80
		2000 < d < 3000	60
		3000 < d < 5000	20
		d > 5000	1
Distance to Freeway (m)		d1 < 1000	100
		1000 < d1 < 2000	80
		2000 < d1 < 3000	60
		3000 < d1 < 5000	20
		d1 > 5000	1
Distance to Roads (m)		d2 < 1000	100
		1000 < d2 < 1500	80
		1500 < d2 < 2000	60
		2000 < d2 < 2500	20
		d2 > 2500	1

) [43,44,66,67]. The resolution of the output models is 1000 m.

2.3.3. Construction of Ecological Corridors

Based on the sources and comprehensive resistance surfaces, the ecological flow channels for species migration were identified in the Linkage Mapper tool to obtain a minimum-cost path [68]. The ecological flow channel refers to potential ecological corridors with a certain width, including the minimum cost path. The MCR model was used to calculate the minimum cumulative distance from one source to another in the spatial grid [69]. The formula is expressed as follows:

$$MCR=f_{min}{\displaystyle \sum }_{j=n}^{i=m}{D}_{ij}\times R_i$$

(6)

where MCR is the MCR value, f_min is the positive correlation between the MCR and ecological process, D_ij is the spatial distance of ecological elements from source j to landscape unit i, and R_i is the resistance of landscape element i to the movement of an ecological element. The resolution of the output models is 1000 m.

2.3.4. Identification of Key Areas

We used circuitscape 4.0 to calculate connectivity between nodes. Circuitscape coupled graph and electrical circuit theory to map connectivity corridors as connections of an electrical circuit. To accomplish the circuit model, two rasters are required [70]. The first is that the core areas of suitable and potential habitats are represented to be connected (i.e., the nodes) and the other is the resistance values for the landscape (as introduced in Section 2.3.2). Finally, connectivity corridors are then identified with higher current values between nodes.

Meanwhile, LinkageMapper tools, an ArcGIS extension, were used to analyse various wildlife habitat connectivity. As for circuitscape, LinkageMapper requires core areas and movement resistance rasters, using the least-cost path theory to extract corridors where habitat characteristics facilitate or hinder movement (pinchpoints or barriers) [26].

(1) Key areas of ecological sources

Based on the circuit theory, the importance of ecological sources was analyzed using the Centrality Mapper tool in ArcGIS, and the key areas of ecological sources were identified. In principle, the minimum cost model was used to calculate the cumulative path resistance between each ecological source, referring to the resistance of the corresponding corridor. Subsequently, the Centrality Mapper tool iterates through all source pairs, injecting 1 A of current into one source and setting the others to the ground. It sums up the results across all sources and links them to generate a centrality score, namely, the current flow centrality (CFC).

(2) Key areas of ecological corridors

The interaction strength between source sites, referred to as the relative importance of corridors, is calculated by the gravity model (GM). The formula is as follows:

$$\begin{array}{cc}{G}_{ab}& =\frac{N_a{N}_b}{{D_{ab}}^2}=\frac{\left(\frac{1}{P_a}\times \ln S_a\right)\left(\frac{1}{P_b}\times \ln S_b\right)}{{\left(L_{ab}/L_{max}\right)}^2}\\ & =\frac{{L_{max}}^2\ln S_a\ln S_b}{{L_{ab}}^2{P}_a{P}_b}\hfill \end{array}$$

(7)

where N_a and N_b are the weights of sources a and b, respectively, D_ab is the standardized value of resistance of the corridor between sources a and b, P_a and P_b are the resistance values of sources a and b, respectively, S_a and S_b are the areas of sources a and b, respectively, L_ab is the cumulative resistance of the corridor between sources a and b, and L_max is the maximum resistance of corridors in the study area.

(3) Key areas of ecological nodes

An ecological pinch point is an area representing a higher value of current density in different landscape types and is an ecological corridor that facilitates the easy migration of species and has a great impact on regional connectivity [31]. In our study, the Pinchpoint Mapper tool was used to simulate and map the ecological pinch points. The current density between patches was calculated using the circuitscape 4.0 tool to simulate an ecological pinch point. This implies the existence of a higher current density in the area and a higher probability of ecological migration. Meanwhile, it may be more vulnerable than other areas in the surroundings.

Ecological barriers, which are referred to as the obstructive areas in ecological corridors, have a passive effect on the passage of corridors. Restoration of ecological barriers enables the effective improvement of regional connectivity. In our study, the Barrier Mapper tool was used to search for barriers using a circular search window.

3. Results

3.1. Evaluation of the Ecological Service Function Importance and Ecological Sensitivity

According to the evaluation results of the ecological service function importance and ecological sensitivity in Fujian province (Figure 3), the spatial distributions of WR, S_bio, S_pro, and S_ws were different. (1) The most important areas of WR have a total value of 2634 km², accounting for 2.15% of the total area of Fujian province. They are mainly distributed in the northern Wuyi Mountain (Nanping), the southwest region of the Hawksbill Mountain (Sanming) and Bopingling Mountain (Zhangzhou), and the northeast region of the Jiufeng Mountain (Ningde). These areas are of great significance to regulate the runoff and make rational use of water resources. (2) The areas of S_pro have a total value of 2068 km², accounting for 1.69% of the total area of Fujian province. They are mainly distributed in the central Daiyun Mountain (the intersection of Fuzhou, Quanzhou, and Sanming), the Hawksbill (Longyan) and Bopingling mountains (Zhangzhou), and the Taimu Mountain (Ningde) in the southwest. (3) The most important areas of S_bio have a total value of 4731 km², accounting for 3.87% of the total area of Fujian province. They are mainly distributed in Wuyi Mountain (Longyan), Hawksbill Mountain (Longyan), Bopingling Mountain (Zhangzhou), and Taimu Mountain (Ningde). (4) The important areas of S_ws have a relatively small total value of 830 km², and most of them are distributed in coastal areas.

Based on the above evaluations of a single factor, we obtained the results of the evaluation of the ecological service function importance in Fujian province (Figure 3 and

Table 2. The statistics of the ecological service function importance and ecological sensitivity in Fujian.

Importance Grade	High		Medium		Low
	Area (km²)	Proportion (%)	Area (km²)	Proportion (%)	Area (km²)	Proportion (%)
WR	2634	2.15	14,042	11.48	105,609	86.37
Spro	2068	1.69	15,491	12.67	104,726	85.64
Sbio	4731	3.87	16,331	13.35	101,223	82.78
Sws	830	0.69	1190	0.97	120,265	98.35
Comprehensive ecological service function importance	5405	4.43	32,040	26.24	84,482	69.09
Comprehensive ecological Sensitivity	0	0	20,992	17.29	100,391	82.71

Note: water conservation (WR), soil and water conservation (S_pro), windbreak and sand fixation (S_ws), and biodiversity (S_bio).

). The areas of the most important region have a total value of 5405 km², accounting for 4.43% of the total area of Fujian province. They are mainly distributed in northern Fujian (Nanping), central Fujian (the confluence of Fuzhou, Putian, and Quanzhou), northeastern Fujian (Ningde), and southwestern Fujian (the confluence of Longyan and Zhangzhou). All these areas are distributed in large mountain belts, with high vegetation coverage and rich resources, which are of great protective value.

According to the evaluation results of the ecological sensitivity in Fujian province, we obtained the spatial distribution of SS_i (Figure 3 and Table 2); it indicated that the coastal area in the east was more sensitive than the inland area. The size of the medium-sensitive area was 20,992 km², accounting for 17.29% of the total area of the Fujian province. The high intensity of human activities in coastal areas has made water areas and their surroundings more sensitive and fragile, leading to environmental destruction.

3.1.1. Key Areas of Ecological Sources

Based on the comprehensive evaluation of the ecological service function importance and ecological sensitivity, the “high” grade areas were extracted and corrected by nature reserves as the ecological sources by eliminating the scattered small patches. Consequently, we identified 62 ecological sources with a total area of 4696 km² (Figure 4). The results showed that the patches were mainly distributed in western Fujian, including the Wuyi, Liangye, and Longqi mountains. Subsequently, the composition of green space in ecological sources was analyzed (

Table 3. Area proportion of the types of green spaces in the ecological sources of Fujian.

Source Type	Green Space				Non-Green Space	Sum
	Cultivated Land	Forest Land	Grassland	Wetland and Water	Developed Land
Area (km²)	462	3918	185	46	85	4696
Proportion (%)	9.84	83.43	3.94	0.98	1.81	100

). Notably, the area of forest land was the highest (83.43%), followed by that of cultivated land (9.84%). This indicates that forests act as important habitats for species survival and migration and should be considered important objects for ecological protection in Fujian province.

Subsequently, the centrality of current in each source was calculated using the Centrality Mapper tool, wherein a higher value indicated higher importance of the source, and the results were graded using the natural breakpoint method (Figure 4). There are 17 high-level ecological sources along the northeast-southwest direction, indicating that denser vegetation coverage in the mountain areas provides suitable ecological conditions for ecological migration. These ecological sources are vital for the connectivity of the entire ecological security pattern, and these areas should be prioritized for protection.

3.1.2. Key Areas of Ecological Corridors

The resistance surface of the minimum cumulative cost was calculated by the Linkage Mapper tool (Figure 5). The results show that high resistance areas were mainly distributed in the coastal areas, where the terrain was relatively flat with a relatively dense traffic network, thus hindering species migration and material and energy flow. The high-value areas in the northern Fujian province may be affected by the altitude and slope, thereby making it more difficult for organisms to spread between sources.

Moreover, based on the resistance surface of the minimum cumulative cost, we simulated the ecological flow channels (minimum cost distance channel) and extracted the optimal path (Figure 6). According to the identification results, 151 ecological corridors (optimal paths) were identified in the 62 ecological sources in Fujian province, forming a network loop, which is mainly distributed around the Hawksbill and Bopingling mountains in the southwest region of Fujian province. Meanwhile, we also analyzed the composition of green space in ecological corridors (

Table 4. Area proportion of the types of green spaces in ecological corridors of Fujian.

Corridor Type	Green Space				Non-Green Space		Sum
	Cultivated Land	Forest Land	Grassland	Wetland and Water	Developed Land	Bare Land
Area (km²)	14,514	52,986	6062	894	1073	41	75,570
Proportion (%)	19.21	70.12	8.02	1.19	1.41	0.05	100

). The results show that forest land was predominant, accounting for 70.12% of the total corridor area, followed by cultivated land, accounting for 19.21%. The proportion of developed land was 1.41%, which indicated that the ecological corridor was encroached by non-green space, leading to the disruption of ecological security and connectivity.

Finally, we calculated the important grade of corridors between sources based on the GM, and the ecological corridors were divided into three grades: G_ab ≥ 100 were classified as the most important ecological corridors, 10 ≤ G_ab < 100 as the moderately important ecological corridors, and G_ab < 10 as least important ecological corridors [43]. Notably, 19 key ecological corridors connect the more densely distributed ecological source areas in the southwest region (around the Hawksbill Mountain) and the northeastern Taimu Mountain (Figure 6). These ecological sources are small and in close proximity to each other, making them potential ecological transition points between other sources. There are 47 medium-grade ecological corridors and 85 low-grade ecological corridors, most of which are longer, indicating that the cumulative resistance of these ecological corridors is greater and enables species migration.

3.1.3. Key Areas of Ecological Nodes

Our study identified ecological pinch points with a total area of 1327 km² (Figure 7), which were mainly distributed in Hawksbill Mountain in the southwest, Bopingling Mountain (Longyan, Zhangzhou), and Jiufeng Mountain (Ningde) in the east of Fujian province. These areas were smaller and intensively distributed, surrounded by areas with high cumulative resistance values. By overlaying the MCR surface, we found that the ecological pinch points comprise regions with less resistance in the process of species migration, indicating that these regions play a connecting role in the ecosystem. Subsequently, the analysis of the green space composition of the ecological pinch points shows that the highest proportion of the pinch points is contributed by forest land (77.54%), indicating that forest land is a suitable space for the migration of most species in the ecosystem (

Table 5. Area proportion of types of green spaces in the ecological nodes of Fujian.

Node Type		Green Space				Non-Green Space		Sum
		Cultivated Land	Forest Land	Grassland	Wetland and Water	Developed Land	Bare Land
Ecological pinch point	Area (km²)	204	1029	84	10	0	0	1327
	Proportion (%)	15.37	77.54	6.33	0.76	0	0	100
Ecological barrier	Area (km²)	1901	6166	657	172	675	76	9647
	Proportion (%)	19.71	63.92	6.81	1.78	6.99	0.79	100

).

The ecological barrier had a total area of 9647 km² (Figure 7). The results show the presence of a coupling area between the ecological barrier areas and parts of the ecological corridors, which indicates that some space was taken over by human activity in the ecological corridor, such as building structures and grazing fields. Repairing the ecological barrier area can compensate for the breaking point of the ecological corridor and improve the connectivity of the area. The analysis of the green space composition of the ecological barrier area showed that the forest land was still predominant, accounting for 63.92% of the total area (Table 5). In contrast to the ecological pinch point, the proportion of developed land in the ecological barrier point was 6.99%. This indicates that the width of the ecological corridor was narrow, and this kind of area hindered the material and energy flow between the ecosystems.

4. Discussion

4.1. Application of Key Area Identification to Ecological Restoration Planning

Ecological restoration is an important measure to improve the ecological environment, including the kinds of available management and policies in China. Most of the previous studies only determined the spatial distributions of the ecological sources and ecological corridors, or only established the buffer zone of uniform width around the ecological sources or the corridors. This approach neither determines clear areas for restoration nor considers the landscape heterogeneity. Composed of forest land, grassland, cultivated land, wetland, and water area, green space has less resistance to species migration. Thus, the restoration of different types of green space is highly conducive to improving the connectivity between ecological sources to ultimately achieve the exchange of material and energy flow. Therefore, based on the established ecological security pattern, we analyzed the composition of green space and further extracted the key areas of ecological sources, ecological corridors, and ecological nodes as prioritized restoration areas. Compared with previous studies that figured out the identification of ecological sources on representative species or target group habitats [71,72], our study focused on the approach of multiple important ecological processes by assessing critical ecosystem services and internal landscape connectivity to identify key locations like barriers and pinchpoints. These key locations needed to be restored usually lies in corridors in our study and identifying key restoration areas enables us to provide effective planning guidelines for conserving biodiversity [73]. For example, there is more space for movements of species, if the area of key locations (pinchpoints with high current density) is expanded. Furthermore, these key sites are the most important areas among the identified patches or corridors and they should be given priory to be restored [43]. The reason is that they may become breakpoints if they could not accommodate such a number of species [73]. Moreover, ecological restoration is a long-term fundamental measure, which not only requires regional overall planning, but also requires specific arrangements that can be maintained for a long time, including the cost and efficiency of ecological restoration [74,75]. The green space ecosystem can undergo self-adjustment and restoration based on its own ecological advantages and avoid excessive artificial interference that can lead to low benefits from the ecological recovery and wastage of resources. Therefore, the results of the identification of key areas of ecological restoration help improve the disrupted ecological environment in cities and further improve the social-ecological benefits of ecological restoration, which has practical significance for planning ecological restoration activities. In the future, the practical application of the identification results of key green space areas should be considered for ecological restoration planning, and our study can be regarded as an important pilot project [76].

4.2. Strategy for Ecological Restoration

The objective of evaluating the regional ecological process and ecosystem service function is to construct a reasonable and stable ecological pattern [77]. Moreover, ecological restoration is the foundation for repairing natural resources in the core region. The restoration of key areas in the ecological security pattern of Fujian province can effectively and rationally allocate the natural resources of Fujian province and enhance the connectivity of green spaces to achieve the sustainable utilization of natural resources. Therefore, this study proposed the following suggestions for ecological restoration and spatial optimization:

(1) Restoration of ecological corridors

Although the ecological sources of Fujian province are connected by ecological corridors, some corridors were excessively long and less abundant than others, especially in the northern Wuyishan Mountain. The connectivity and stability of the ecological network require substantial improvements. Therefore, the land demand and buffer zone division of ecological corridors should be guaranteed to ensure that these ecological corridors are not disturbed and the connection between sources is not blocked. Additionally, by improving and optimizing the quality and structure of forests, the ecological sources can act as stepping stones between the original ecological sources and ensure the integrity of the ecological network structure [78]. Protective measures, mine rehabilitation, etc., should be considered to prevent natural disasters such as landslides and soil erosion. Besides, from the micro point of view, a high-volume, low-cost soil restoration method for the ecological corridor in areas with extensive soil damage can be provided, which is proved to be beneficial, when enough time is given. More carbon could be sequestered in the soil of the artificial forest or water areas in the ecological corridor in the long term [79].

(2) Restoration of ecological pinch points

As for natural cover in pinchpoints, vegetation protection, forestry eco-engineering, and matching of forest species should be strengthened to improve the community structure of forests in the area and provide space for species migration [80]. Moreover, the discharge of pollution into wetlands and water areas should be strictly forbidden, and appropriate measures should be taken to prevent pollution, thereby strengthening the stability of the water shoreline. Considering cultivated land, the forest belt can be developed around such land to reduce its ecological resistance and avoid the disruption of ecological processes.

(3) Restoration of ecological barriers

The resistance to the ecological process should be less, and forest land in the ecological barriers should be strictly improved with strict management. Meanwhile, forest land should be expanded, and its connectivity should be improved [81]. Considering wetland and water areas, corresponding remediation measures should be implemented, such as the control and interception of sewage sources, and the maintenance of water ecological function, to solve the specific problems of pollution, sediment accumulation, and the degradation of water ecological function. This can strengthen water runoff regulation, water supply, biodiversity protection, and other functions. Additionally, even small areas of developed land are a great threat to species migration. Small ecological patches can be developed around them to weaken their ecological resistance. In urban areas, more emphasis on the principles and management of landscape ecology should be placed on establishing a community that will conserve water, beautification of the residential environment, and the carrying capacity of the environment [82].

4.3. Limitations and Future Research

Coupling patterns and processes are the core of landscape ecology research, and the essence of constructing a regional ecological security pattern is to follow the mutual feedback relationship between patterns and functions. Focusing on the protection of landscape structure of specific patches and corridors enables the realization of specific or integrated ecological functions and restoration and conservation processes. However, in the construction of the regional ecological security pattern, different analysis methods of a single ecological process or multiple coupled processes may directly impact the final result of the regional ecological security pattern construction. In particular, we assumed that different ecological processes are compatible with each other, and there is no synergy or trade-off relationship between them in the construction of the regional ecological security pattern. Close interactions and influences exist among different ecological processes, and their coupling relationship with the overall ecological function may differ considerably. Our study only conducted a simple analysis based on various ecological processes but did not consider the balance and synergy between them. Therefore, future studies must explore the coupling of different ecological processes by integrating multiple types of data and disciplines. This would considerably aid in constructing a regional ecological security pattern oriented toward the integrated management of mountains, rivers, forests, fields, lakes, and grasslands. Additionally, the threshold of corridor width remains to be discussed in the future.

5. Conclusions

In this study, the ecological source of Fujian province was identified by comprehensively evaluating the ecological service function importance and ecological sensitivity. We simulated the ecological corridors, ecological barriers, and ecological pinch points using the MCR and circuit theory models and determined the key areas requiring ecological protection and restoration in Fujian province. This provided a new perspective and framework for ecological protection and restoration in Fujian province. The results were as follows:

(1) According to the evaluation of the ecological service function importance and ecological sensitivity, 62 ecological sources were identified in Fujian province, with a total area of 4696 km², of which 98.19% comprised green space; large patches were mainly distributed around the Wuyishan, Liangye and Longqi mountains in western Fujian. We identified 151 ecological corridors (optimal paths) that were densely distributed around the Hawksbill and Bopingling mountains in the southwest.

(2) The key areas of ecological restoration in Fujian province included 17 key ecological sources along the northeast-southwest direction and 19 key ecological corridors around the southwest ecological sources, such as the Hawksbill and Taimu mountains. Additionally, the ecological pinch area was 1327 km², which was mainly distributed in the Hawksbill and Bopingling mountains (Longyan, Zhangzhou) in the southwest and Jiufeng Mountain (Ningde) in the east of Fujian province; the forest land accounted for the highest proportion (77.54%) in our study. The area of ecological barriers was 9647 km², and the highest proportion was still contributed by the forest land (63.92%).

(3) In future, ecological restoration should enhance the protection of key areas of ecological sources and ecological corridors through spatial planning, including the implementation of differentiated supervision and prioritized restoration of ecological nodes and degraded ecological areas.