A Review of the Effects of Urban and Green Space Forms on the Carbon Budget Using a Landscape Sustainability Framework

Abstract

Urban areas and green spaces are significant atmospheric carbon sources and sinks. Spatial planning is crucial for improving the urban carbon budget. However, there are many uncertainties due to the diversity and complexity of the effects of urban and green space forms on the carbon budget. As a result, the role of urban areas and green spaces in emission reduction and carbon sink increases remains ambiguous. We use a landscape sustainability framework and systematically review the literature from 2002 to 2022 to elucidate the interaction between urban and green space forms and the carbon budget. We focus on regional and landscape scales. Nine landscape planning indicators affecting urban area carbon emissions, four indicators affecting green space carbon sinks, and three indicators affecting the urban–green space ecotonal relationship and the carbon budget are derived. We analyze the causes of the differences between the studies and discuss the influences of the indicators on emission reduction, carbon sink increases, and sustainable development. We summarize the design and research of urban and green spaces and the urban–green space ecotone and provide suggestions for carbon emission reduction, carbon sink increases, and research directions for future studies.

1. Introduction

Global climate change has become the most significant challenge faced by humankind since the start of the 21st century. It is mainly characterized by temperature rise, glacier melting, and sea level rise [1,2]. Large-scale, extreme droughts and floods, rising sea levels, strong hurricanes, and other extreme weather events occur with an increasing frequency. Frequent climate disasters adversely affect agricultural output, increase the frequency and duration of strong haze events in winter, and harm species diversity, human life and health, and economic production, thereby hindering long-term sustainable development [3,4,5]. An important cause of global climate change is that of excess carbon emissions from the anthropogenic burning of fossil fuels [6], which have been exacerbated by the energy demands of the rapid urbanization and industrialization that have occurred since the Industrial Revolution. Effectively reducing the transfer of unsolidified carbon into the atmosphere has become the primary method of achieving carbon neutrality. Although technology researchers are actively exploring measures to improve non-point source carbon emissions, global energy-related CO₂ emissions were estimated to have reached 36.8 billion tons in 2022, with non-point source carbon emissions accounting for about 32% of total emissions [7]. In contrast, green spaces have the highest per unit carbon sequestration capacity [8], and the annual global net carbon sequestration capacity of green spaces is 7.6 billion tons [9]. In general, urban land is the highest carbon source, and green spaces are the areas with the highest carbon sequestration rate. Thus, these land-use types are critical for carbon budget research and policy making.

The morphology of urban areas affects non-point source carbon emissions. Carbon emission data for 2022 from the IEA [7] have shown that transportation sector emissions account for about 23% and that building emissions account for about 9% of total emissions. It is worth noting that the 2021 Global Covenant of Mayors Impact Report shows that the global share of emissions attributed to urban areas has increased from 62% in 2015 to 67–72% in 2020 [10]. The long-term optimization of the urban structure and pattern affects the energy demand of transportation and residential design as well as the control of non-point source carbon emissions [11,12,13]. Urban landscape designs indirectly affect carbon emissions by influencing urban residents’ travel modes [14,15,16]. The carbon sink capacity of green spaces can be improved by optimizing their morphology and reducing human interference and other indicators. It may also reduce the heat island effect and optimize the residents’ transportation modes and residential energy use, indirectly reducing urban carbon emissions [17,18,19]. Therefore, in addition to adjusting the industrial structure and reducing fossil fuel energy use, an appropriate design of urban areas and green spaces is critical for reducing emissions and increasing carbon sinks.

Since the 1990s, planning concepts such as new urbanism and ecological networks have guided the construction of urban and green spaces. From the perspective of landscape ecology, new urbanism advocates an urban form that has a high density, has low fragmentation, is compact, and employs mixed land use to increase the efficiency of the urban economy and transportation. Ecological network construction focuses on continuous, low-fragmentation green spaces to enhance the value of ecological services. However, in the context of climate change, the effects of these planning concepts on emission reduction and carbon sink improvement have been controversial. Thus, developing countries have difficulties choosing suitable urbanization approaches. For example, the Chinese government has abandoned the per capita construction land index for spatial planning and has included carbon peaking and the carbon budget in its work report to achieve a carbon budget balance while developing the economy through spatial planning. However, no guidelines for specific designs exist [20]. The reason for this is that several landscape morphology studies have provided contradictory results [21]. Therefore, there is an urgent need for a sustainable planning framework to analyze urban carbon budgets and ensure that urbanization, economic development, and ecological conservation go hand in hand with appropriate landscape designs [22].

Landscape sustainability science can provide a framework that considers social development and the carbon budget to plan research and implement results. This framework can help to adopt suitable landscape designs at different scales and dimensions to obtain meaningful results. The core problem of the scientific research on landscape sustainability is that of the maintenance and improvement of the relationship between landscape ecological services and human welfare in the long term under internal and external disturbances. The sustainability of a landscape depends on its biophysical and socio-economic conditions and people’s values and perceptions of ecosystem services and human well-being [23]. According to Wu’s definition of sustainability, a dynamic balance exists between human development and environmental protection as well as between human needs and environmental integrity. The key to sustainability is complementarity rather than substitutability. Strong sustainability means balancing human well-being and ecosystem protection, creating a win-win situation [24]. Therefore, we use the scientific research framework of landscape sustainability to discuss the urban form indicators affecting urban carbon emissions and the landscape design indicators influencing the carbon sink capacity and carbon budget of green spaces. We discuss the ecosystem advantages resulting from the optimization of the urban and green space forms and the service functions they can provide for human welfare. Based on the results, we provide planning and governance suggestions for designers and policymakers to design more suitable urban and green space forms. We also provide a spatial development framework that balances social development with the carbon neutrality goal and is applicable to developing countries.

2. Carbon Budget Analysis Framework Based on Landscape Sustainability Science

2.1. Influence of Urban and Green Space Forms on the Carbon Budget

Reducing emissions and increasing the carbon sink to achieve carbon neutrality is an environmental issue but is also necessary for sustainable human development [25]. Some environmental pessimists previously believed in an irreconcilable contradiction between economic growth and environmental protection. It was believed that some approaches to slow down urbanization and economic development to reduce the carbon budget would result in zero capital growth and reduce human benefits to protect the environment and resources [26]. However, slowing down development or protecting green spaces is not conducive to balancing the protection of residents’ interests and environmental protection and does not necessarily improve human well-being. The scientific framework of landscape sustainability proposed by Wu Jianguo et al. (2013; 2021) includes all the interactions between human society, the environment, and ecosystems. It has four core systems as follows: human well-being, ecosystem services, ecosystem structure and function, and landscape planning and governance [23,24]. This framework is suitable for analyzing the effects of landscape morphology on environmental effects and considers human development and the protection of biodiversity and the environment. Therefore, it is highly suitable to clarify the relationship between urban and green space forms and the carbon budget as well as to determine landscape planning and governance strategies that balance human welfare and environmental protection at different scales.

We use the framework of landscape sustainability to analyze the impact of urban and green space forms on the carbon budget, human well-being, and ecosystem services (see Figure 1). The four sub-systems of the landscape sustainability framework are human well-being, green space services, green space, as well as landscape planning and governance. Human well-being refers to the economic, physical, mental, and social conditions that satisfy public health, safety, and social development. Green space services include the services and functions that internal and external green spaces provide for human society, such as the positive effects on the urban microclimate, biodiversity protection, and locations for residents’ leisure activities. Green space planning refers to the design of green spaces. Urban landscape planning and governance refers to urban design. In addition, the most important part of the framework is the configuration of the landscape pattern. The four subsystems are connected and have different scales. There are internal and external relationships and paths of influence affecting the coordination of the subsystems. Our framework is smaller than the existing landscape sustainability frameworks. First, due to limitations in landscape planning, it is difficult to reduce carbon emissions and sequester carbon in many natural ecosystems, such as wetlands, rivers, and oceans. Therefore, our framework has fewer elements than natural and social systems that can be transformed. Second, global-scale emission reduction requires broader international collaboration and research, which is beyond our landscape planning capabilities. The purpose of this paper is to provide more feasible and practical suggestions for landscape and regional planning and design. Therefore, we focused on the landscape and regional scales. The effect of the urban and green space forms on the carbon budget and the interactions and mechanisms of the subsystems are shown in Figure 1.

2.2. Shortcomings of Existing Studies

Few studies have used the scientific framework of landscape sustainability to analyze the relationship between landscape morphology and carbon sources and sinks. Most studies analyzed the effect of urban form on carbon emission reduction or of green spaces on increasing carbon sinks separately. Most cases would require large funds to change the landscape form to sequester carbon [27]. This type of research did not consider the interaction between urban areas and green spaces and human development and green space ecosystem health. We systematically review the literature on the effects of urban and green space forms on carbon emission reduction and carbon sink enhancement from a landscape sustainability perspective. We focus on the following two aspects often overlooked in existing studies:

• Despite substantial differences between urban and green space forms, a complex relationship exists between their effects on emission reduction and carbon sink increases [28,29]. Effective urban spatial planning involves the analysis of the interaction between urban form and land use, and appropriate green space planning must consider urban form indicators, which are often intertwined. However, most existing studies analyzed urban or green space indicators separately; therefore, we systematically review and analyze studies investigating the influence of the two factors on the carbon budget.
• Existing studies did not sufficiently consider sustainability for balancing the carbon budget. Limiting urban construction or expanding green spaces does not achieve a carbon budget balance without affecting human welfare and development. The impact of landscape pattern optimization on urban development should not be underestimated. For example, urban green space optimization improves urban development because of environmental optimization. However, there are also problems in landscape pattern optimization. Although the optimization can reduce carbon emissions, an excessive emphasis on the integrity and scale of green space during planning squeezes the space of urban construction [27,30,31]. Research on the effect of landscape planning on the carbon budget can be improved by considering landscape sustainability and comprehensively analyzing studies on human development and biodiversity conservation.

3. Literature Selection Method

This paper analyzes the impacts of urban and green morphology and their interactions on the carbon budget using a landscape sustainability framework by conducting a literature review. We extracted relevant peer-reviewed articles using the following criteria:

• We searched the most authoritative database, Web of Science, due to the interdisciplinary nature of this study. The search terms (topic words) regarding the relationship between urban form and carbon emissions were “#1:TS = urban form, urban pattern, urban spatial structure, urban structure, urban morphology” and “#2:TS = carbon emission, carbon neutrality”. The search terms (topic words) for the relationship between green space forms and carbon sink were “#1:TS = urban vegetation, urban landscape, urban forest, forest structure, urban greenspace, landscape design, landscape pattern” and “#2:TS = carbon sink, carbon uptake, carbon sequestration, and carbon neutrality.” The search terms (topic words) regarding the relationship between the carbon budget and the urban–green space ecotonal relationship included urban green space, rural green space, green space–urban edge effect, cooling effect, carbon sink, and carbon emission. We prioritized articles published in core journals and checked titles, abstracts, and subject headings to remove publications irrelevant to the study. We screened the records to exclude duplicates and irrelevant documents. Finally, we reviewed the references cited in the articles to extract and integrate the necessary information.
• We narrowed the preliminary search results by selecting articles matching our research objectives. The first concern was the academic quality of the articles; therefore, we focused on peer-reviewed articles published in academic journals. Second, we selected empirical studies investigating the correlation between carbon emissions and urban form indicators, carbon sinks, and green space form indicators, as well as the carbon budget and the interaction indicators between urban and green spaces. We focused on the following contents consistent with the theme of our review: the research field, urban planning indicators, carbon dioxide emissions, carbon sources, green space planning indicators, carbon sinks (capacity), interactions between urban and green space landscape planning, research objectives, research methods, and research results.
• We evaluated the degree of landscape sustainability in the publications based on optimized urban planning, green space landscape design, and the interaction between urban and green spaces. We tabulated the search results of the second step and the sustainable methods and systematically analyzed them. In addition, the potential relationship between the influencing indicators and their mechanisms was analyzed.
• We employed bibliometric analysis, which is a quantitative research technique that uses textual data and indicators to determine the characteristics and trends of a topic or study. We used VOSviewer and CiteSpace for bibliometric analysis. Citespace and Vosview were used to process the publications and visualize the results. After analyzing the literature, we used Citespace to generate knowledge maps to assess and visualize key articles and research interests in urban form, carbon emissions, green space form, and carbon sinks.

Our screening process is shown in Figure 2.

4. Results

We selected the top 250 articles published from 2002 to 2022 with the highest correlations between urban form and carbon emissions. We used VOSviewer (v 1.6.18) to create a keyword cluster map (Figure 3) and CiteSpace (5.7 r5) to create a keyword citation burst table to analyze research hotspots and trends (Figure 4).

Second, we selected the top 250 articles with the highest correlations between green space forms and carbon sink capacity. The keyword cluster map is shown in Figure 5, and the keywords with the strongest citation bursts are shown in Figure 6.

We analyzed the charts, screened the articles, and summarized the results. We selected 160 empirical studies (from 2002 to 2022) on the effects of urban and green space forms on emission reduction. Among them, 82 articles described the impact of urban landscape design on carbon emissions, 60 articles focused on the impact of landscape design on carbon sinks, and 26 articles described the topological relationship between green space and urban on the carbon budget. The dependent variables in these studies (carbon emissions and carbon sinks) were measured directly and indirectly, i.e., some studies measured carbon concentrations, whereas others analyzed the influence mechanism on carbon emissions or carbon sinks by measuring representative variables such as the transport distance or energy use of air conditioning. Some studies analyzed the relationships between landscape morphology indicators and carbon emission and carbon sink indices using regression analysis, correlation analysis, scenario simulations, and comparative analysis.

Following Wu [24,32], we divided the studies into local (landscape) and regional studies. Landscape-scale or regional-scale studies are typically used to evaluate sustainability because they represent the scale of interaction between humans and the environment. The regional scale in urban form studies includes the country, province, and multiple cities, whereas the landscape level focuses on the city level and below. Regional-scale studies on green space and the interaction between urban and green spaces focus on large, contiguous green spaces, whereas landscape-scale studies consider relatively small green spaces. We divided our study subjects into three categories according to the impact of human activities as follows: urban form, green space form, as well as the urban–green space ecotonal relationship (inclusion—green space within the city limits; adjacency—suburb green space near the city’s edge; separation—rural green space relatively far from the city). The reason for this division is that the green space’s location relative to the city affects the city’s carbon absorption and carbon emission efficiencies [33,34]. We want to study the relationship between the landscape form and carbon budget from different angles and, on the basis of morphological analysis, supplement the study on the influence of the relative location of cities and green spaces on carbon sink. Our classification of the urban–green space ecotonal relationship is shown in Figure 7.

4.1. Impacts of Urban Form Indicators on Carbon Emissions

Urban indicators with significant impacts on carbon emissions include the city size [35,36,37,38,39], urbanization level [40,41,42], economic level [43,44,45,46], population size [47,48,49], and building volume [50,51]. We included these indicators as control variables. The difference between the control variable and the urban form factor is that the former is a non-landscape form factor affecting total carbon emissions, whereas the latter is the focus of our research and may influence the emissions per unit area. The control variables are the carbon sources not the object of the urban landscape pattern study. This study focuses on improving the carbon emission efficiency per unit area by optimizing the urban form and not by limiting urban development to ensure human well-being. We found 83 papers analyzing the effects of different urban forms on carbon emissions. These publications were categorized into the four themes as follows: land use, road traffic network, urban pattern, and urban structure.

4.1.1. Regional-Level Studies at or above the Municipal Level

Studies on carbon emissions and urban form indicators at the regional level focused on several urban forms and relatively large scales (including national, provincial, and municipal levels). Four studies described land-use characteristics by measuring the degree of mixed land use. Sixteen articles on road traffic networks focused on three indicators as follows: traffic volume (twelve articles), connectivity (four articles), and distribution of public transport facilities (five articles). Fifty-four publications on urban patterns considered three indicators as follows: patch compactness (thirty-two articles), patch shape complexity (fifteen articles), and population density (thirty-four articles). Fourteen articles discussed the influence of urban spatial aggregation on carbon emissions, focusing on single (three articles) and multiple (eleven articles) centers (

Table 1. Impacts of urban form indicators on carbon emissions and sustainable human well-being.

Theme	Indicator	Metrics	Scale	Impact on Carbon Emissions	Articles	Impact on Sustainable Human Well-Being
Land use	Degree of mixed land use	Land use mix (LUM) Entropy type land use mix (ELUM) Residential-to-employment area ratio Public service accessibility Distance from work to residence Distance traveled for non-work Land use diversity	Regional	−	[43,52,53,54]	Highly mixed land use has a positive impact on economic development and facilitates residents’ daily activities.
			Landscape	−	[14,15,55,56,57,58,59,60,61,62,63]
				/	[64]
Road network	Traffic volume	Road density Road length Per capita road area Paving rate	Regional	+	[38,42,65,66,67,68,69]	Improving the density and connectivity of the road network improves economic development and residents’ travel.
				−	[41,70]
				/	[71]
				*	[72]
				?	[73]
			Landscape	+	[74]
				−	[14,15,62,63]
	Connectivity	Coupling degree between urban spatial structure and road network Interblock connectivity	Regional	−	[41,45,70,75]
	Distribution of public transport facilities	Density of public facilities (subway stations, bus stations, parking facilities) Number of public transport routes Accessibility between work and public transport systems	Regional	+	[49,66]	A well-developed public transport system improves economic development and residents’ travel.
				−	[53,65,69]
			Landscape	+	[14]
				−	[15,55,56,58,60,63,74]
				?	[16]
Urban pattern	Patch shape complexity	Area-weighted mean shape index (AWMSI) Area-weighted mean patch fractal dimension (AWMPFD) Perimeter-to-area fractal dimension (PAFRAC) Mean perimeter–area ratio (PARA_MN) Landscape shape index (LSI) Edge density (ED)	Regional	+	[37,41,44,47,48,52,72,75,76,77,78,79,80,81]	The complexity of urban patch morphology has negative effects on economic development and residents’ daily activities.
				−	[82]
			Landscape	+	[83]
	Patch compactness	Patch cohesion Index (PCI) Aggregation index (AI) Normalized compactness index (NCI) Compactness index (CI) Compactness ratio (CR) Number of patches (NP) Patch density (PD) Euclidean nearest neighbor distance (ENN_MN) Patch relative density (PRD) SPLIT index (SPLIT)Landscape separation index Landscape division index (DIVISION) Commuting distance The distance to the city center Percentage of like adjacencies (PLADJ)	Regional	+	[82,84,85]	Compact urban form generally has positive effects on economic development, but a highly compact urban form adversely affects economic development and residents’ physical and mental health.
				−	[37,39,41,45,48,51,52,53,67,75,76,77,78,79,81,86,87,88,89,90,91,92]
				*	[47,80,93,94]
				?	[44,72,95]
			Landscape	−	[14,55,56,59,83,96,97]
				?	[15,98]
	Population density	Urban population density Urban residential density (RD) Employment densityResidential density	Regional	+	[41,49,68,69,70,99,100]	Urban population density generally has a positive impact on economic development, but a very high population density adversely affects the urban economy and residents’ physical and mental health.
				−	[36,42,43,51,52,65,66,67,71,80,84,85,88,90,92,101,102,103,104,105,106,107]
				*	[40,47,72,108]
				?	[73]
			Landscape	+	[61,64,109]
				−	[14,55,74]
				*	[16,96,110]
				?	[15]
Urban structure	Polycentric structure	Morphological polycentricity Functional polycentricity Polycentricity index	Regional	+	[88,111]	A polycentric structure is conducive to green economic development and facilitates residents’ commuting. However, a higher number of centers does not necessarily improve economic development.
				−	[84,102,112,113,114]
				/	[87]
				?	[86,91,115]
			Landscape	−	[116]
	Monocentric structure	Largest path index (LPI) Buffer compactness index (BCI)	Regional	+	[47,75]
				?	[48]
			Landscape	+	[83]

Note: +: positive impact on carbon emissions; −: negative impact on carbon emissions; /: the effect is not significant; ?: uncertain impact (need to consider various indicators of the city); *: nonlinear relationship.

).

4.1.2. Landscape-Level Studies at the City Level

These studies focused on urban form indicators at the city level or below. Twelve articles analyzed land use. In 10 studies on road traffic networks, 5 papers considered the traffic volume and 9 papers investigated public transport facilities. Sixteen articles on urban patterns considered three indicators as follows: patch compactness (9 articles), patch shape complexity (1 article), and population density (10 articles). Two articles analyzed the urban structure; one focused on multiple centers and one on a single center (Table 1).

4.2. Influence of Green Space Forms on Carbon Sink

Green space indicators with significant impacts on carbon sink include the green space extent [117,118,119,120,121], forest cover [122,123,124,125], forest type [120,126,127], stand structure [118,128,129,130], tree species [34,131,132], and tree age [129,133,134]. These indicators significantly affect the carbon sink capacity. Generally, the higher the diameter at the breast height (DBH) of trees and the larger the leaf area the better the light absorption capacity and the carbon sink capacity [128]. Due to the influence of these green space non-landscape indicators, it is challenging to use landscape planning to improve the carbon sink capacity of green spaces and alleviate the contradiction between socio-economic development and sustainable protection. Therefore, we considered these indicators to be the control variables. Sixty studies investigated the impact of different green space forms on carbon sinks. We categorized these studies into green space pattern and vegetation community studies.

4.2.1. Regional-Level Studies of Large, Contiguous Green Spaces

Regional-level studies focused on large, contiguous green spaces, such as cross-administrative protected areas or large areas of natural forests. Eighteen articles investigated green space patterns, focusing on patch compactness (12 articles), stand density (4 articles), and patch shape complexity (10 articles). Patch compactness and green space connectivity were measured by various metrics. Patch shape complexity describes the fragmentation degree, which is related to the edge effect. The studies on stand density analyzed the influence of the vegetation community density on the carbon sink capacity. The vegetation community was based on the internal structure of the green space. Six studies focused on landscape richness, which analyzed the effects of biodiversity on carbon sinks. (

Table 2. Effects of green space form indicators on carbon sinks and ecosystem service sustainability.

Theme	Indicator	Metrics	Scale	Impact on Carbon Sinks	Article	Impact on the Green Space Ecosystem Service Sustainability
Vegetation community	Landscape richness	Shannon diversity Index (SHDI) Habitat diversity Homogeneity Biological diversity (species diversity, functional diversity, and functional dominance) Tree neighborhood diversity Tree diversity Species richness	Regional	+	[118,128,135,136,137,138]	The landscape richness, biodiversity, and biomass are higher in green spaces; therefore, these ecosystems should be protected.
			Landscape	+	[123,126,129,139,140,141,142,143,144,145,146,147,148,149]
				/	[124,125,150]
Green space pattern	Patch compactness	Patch cohesion index (COHESION) Aggregation index (AI) Vegetation landscape connectivity (VLC) Number of patches (NP)/number of fragments (NF)/patch number (N) Patch density (PD) Mean nearest neighbor distance in a few miles (ENN_MN) Patch relative density (PRD) Landscape separation index (DIVISION) Mean forest patch size/mean patch area (AREA_MN) Area-weighted mean contiguity index Distance to forest edge (m)	Regional	+	[128,151,152,153,154,155,156,157,158,159]	Highly connected green patches indicate high ecosystem diversity. The decrease in the number of fragmented patches is the result of management.
				−	[160]
				/	[161]
			Landscape	+	[119,162,163,164,165]
				−	[123,166,167,168]
	Stand density	Tree density (TD) The number of trees and shrubs per hectare Planting density Stem density	Regional	+	[118,128,169]	Within a certain range, the higher the tree density the greater the biomass. Reasonable, high-density planting maintains the woodland ecosystem’s health.
				*	[170]
			Landscape	+	[122,124,125,129,140,163,171,172,173,174]
				/	[121]
				*	[144,175]
	Patch shape complexity	Average perimeter area ratio (PARA_MN) Landscape shape index (LSI) Edge density (ED) Shape index mean value (SHAPE_MN) Edge effect	Regional	+	[160,161,176]	A larger area of forest land with an irregular perimeter is exposed to external disturbances. This may result in the growth of marginal vegetation due to high temperatures but is not conducive to ecosystem stability.
				−	[130,151,152,153,155,159,177]
			Landscape	+	[168,178,179]
				−	[119,164,180,181,182]
				/	[183]

Note: +: positive impact on carbon emissions; −: negative impact on carbon emissions; /: The effect is not significant; *: Nonlinear relationship.

).

4.2.2. Landscape-Level Studies of Small Green Patches

These studies focused on small green patches; 27 studies considered the green space form, including 9 on patch compactness, 13 on stand density, and 9 on patch shape complexity. There were 17 papers on vegetation community, including landscape richness. (Table 2)

4.3. Influence of Topological Relationship between Urban and Green Spaces on the Carbon Budget

Studies on the topological relationship between urban and green spaces considered the influence of the urban–green space ecotonal relationship on the carbon budget. We categorized the influencing indicators of the urban and green belt ecotone on the carbon budget into three themes as follows: adjacency, separation, and inclusion. Twenty-six publications focused on the regional and landscape levels.

4.3.1. Regional-Level Studies of the Urban–Large Green Space Ecotonal Relationship

The main objective of the regional-level studies was that of studying the different positional relationships between urban areas and large, contiguous green spaces. One paper analyzed this topic, focusing on the carbon sequestration difference between urban and rural green spaces (

Table 3. Impacts of urban and green space disturbance effects on carbon budget, human well-being, and green space ecosystem service sustainability.

Group	Theme	Indicator	Metrics	Scale	Impact on Carbon Sinks	Articles	Impacts on Human Well-Being and Green Space Ecosystem Service Sustainability
Topological relationship between green space and urban areas	Adjacency	Green space–urban edge effect	Edge fragmentation Landscape shape index (LSI) Edge effect The gradient effect of urbanization	Landscape	+	[184,185]	Forests provide green products for residents. However, fragmentation caused by urbanization changes the growth conditions of forests. Although a fragmented forest edge can increase productivity under certain conditions, the heat stress at the forest edge can exceed a threshold, adversely affecting the green space ecosystem.
					−	[186,187,188]
					*	[166]
	Separation	Carbon sequestration difference between urban and rural green spaces	Distance between the green space and the city Natural forests and urban forests	Regional	The carbon sink capacity is higher outside of the city	[189]	Urban forests can provide more ecological services to residents, but urbanization and fragmentation of green spaces have a more adverse impact on humans in urban areas than those in rural areas.
				Landscape	The carbon sink capacity is higher in the city	[34,122,166,185,190]
					The carbon sink capacity is higher outside the city	[119,123,124,125,171,173,186,191,192,193,194,195]
	Inclusion	Cooling effect of green spaces on the city	Green cover Grass irrigation cover Shading coefficient Vegetation layout Vegetation aggregation Green index (quantity + structure)	Landscape	+	[33,196,197,198,199,200]	The cooling effect of urban forests reduces the urban heat island effect, improves the microclimate, and positively impacts the physical and mental health of residents. It reduces the heat stress response of plants; however, it results in an ecosystem that is fragile due to the size of the green space.

Note: +: positive impact on carbon sinks; −: negative impact on carbon sinks; *: nonlinear relationship.

).

4.3.2. Landscape-Level Studies of the Urban–Small Green Space Ecotone

These studies focused on the ecotonal relationship between urban areas and relatively small green spaces such as urban parks. Smaller green spaces have a higher fragmentation degree than large, contiguous green spaces due to human influence, thereby affecting their carbon sink capacity. These studies investigated the influence of high temperatures, dryness, high wind speed, human interference, and other indicators on the difference in the biomass or carbon sink capacity of the interior and edges of green belts.

We found 25 studies on this topic. Six studies focused on adjacency, i.e., the green space–urban edge effect, and seventeen studies on connection discussed differences in carbon sequestration between urban and rural green spaces. Six articles on inclusion considered the cooling effect of urban green spaces. They discussed the indirect effect of the size and layout of green spaces on urban carbon emissions by influencing the energy use of the housing sector (Table 3).

5. Discussion

5.1. Urban Landscape Design

(1)

Improvements in urban patch shape complexity, land use, and traffic connectivity can reduce carbon emissions and ensure sustainability.

Improvements in urban patch shape complexity, mixed land use, and road traffic network connectivity (better coupling between road traffic network and urban structure), which are related to the optimization of urban form, can reduce carbon emissions. An irregular city shape increases the distance between people’s homes and workplaces. The distance to urban centers affects land use patterns [201]. Complex urban forms require a higher transportation level, thereby resulting in higher energy consumption and carbon emissions [75] and reduced carbon emission efficiency [77]. Mixed land use reduces carbon emissions by reducing vehicle mileage, improving the accessibility of public service facilities, shortening travel distances, promoting public transport and low-carbon travel, and balancing traffic flow [53,202,203]. Its effect is strongly related to the landscape level [14,58,204,205]. Improving the coupling degree between the traffic network and the urban structure can also reduce carbon emissions. Ensuring that new roads are constructed close to residential and work areas can help reduce energy consumption [70]. Improving urban form and the traffic network specifically enables better traffic flow and improves the accessibility of traffic nodes and economic activity areas [206].

From the perspective of sustainable development, optimizing urban form improves urban function and structure and increases the connection between urban settlements, industries, and institutions [207]. It reduces transport-related carbon emissions [75], transportation time, and construction costs, thereby promoting sustainable urban development.

(2)

Although compact cities and centralized structures are hot topics in sustainability studies, it is unclear whether they reduce carbon emissions.

Compact cities are characterized by aggregated functions, a high density, and a strong transportation network [208]. Their advantages include short travel distances and travel times, low-carbon travel, mixed land use, the high accessibility of facilities, high population density and development, and an adequate public transport system [63,66,113]. Fewer private cars are used due to less urban sprawl and suburbanization, thereby reducing fossil energy consumption and vehicle emissions [56,97,209]. Increasing the population density, sharing infrastructure and resources, and concentrating the energy supply reduce power consumption and carbon emissions in the residential sector [36,51]. However, compact cities have disadvantages. At a certain compactness level, the positive effect of population density on carbon emission reduction begins to decrease [40,96]. Dense public transport facilities (bus stops, parking lots, etc.) may increase the probability of residents traveling and driving, and an extensive road transport network may lead to the excessive expansion of the city, thereby increasing vehicle emissions [42,66]. Although the degree of urban land aggregation is improved, unreasonable resource allocation may increase carbon emissions [47]. In addition, compact urban development plans differ significantly in their level of emission reduction due to different city sizes and economic levels [77,208]. Due to scale effects, compact large cities have lower per capita carbon emissions than compact small cities [77]. Some studies have shown that increasing urban density resulted in the highest carbon emission reductions in medium-sized cities [95]. Therefore, it is necessary to investigate the development status of different cities and formulate suitable plans for compact cities. However, there is no agreement on whether compact cities are more sustainable than non-compact cities. Some studies showed that compact cities ensured sustainable development due to a lower traffic volume and the protection of green space outside the city [210,211,212]. However, a high density can cause more road congestion and exacerbate the urban heat island effect, increasing electricity consumption. Therefore, some researchers believe that the construction of compact cities is not necessarily a sustainable way to reduce urban carbon emissions [213,214]. Meanwhile, overly compact cities adversely impact people’s well-being, resulting in their unsustainability [215].

In addition, the effect of multiple centers on emission reduction is controversial. The reason is that a city with multiple centers can reduce residents’ daily trips and vehicle emissions [84,113]. Since single-center compact cities can increase vehicle emissions due to high traffic congestion [75], the rational development of a compact, multi-center city can improve commuting efficiency and reduce carbon emissions [77,216]. However, some researchers believe that the city size affects the level of emission reduction, and the construction of multiple centers in small cities may not reduce emissions [87]. From the perspective of sustainable urban development, polycentric cities increase the chances of marginalized families obtaining employment and other opportunities [217]. They prevent excessive resource concentration and low resource efficiency and contribute to the sustainable development of the urban green economy. However, excessive multi-centralization may reduce economic efficiency and resource utilization; therefore, urban planners should limit the number of centers according to the city’s development level [91,218].

5.2. Green Landscape Design

(1)

Appropriate landscape richness can increase the carbon sink capacity and sustainability.

Optimizing mixed land use in green spaces can increase the carbon sink capacity. Increasing landscape richness in a limited space can improve the carbon sink capacity because high biodiversity allows more ecological niches to be occupied and ensures higher plant productivity [140]. Higher trait diversity reduces the vulnerability to external impacts [219], enhances niche differentiation and productivity [220], and ultimately increases carbon sink levels.

Green space construction affects human well-being, and a more complete and continuous green space has a positive impact on residents’ physical and mental recovery [221,222]. Therefore, strengthening the protection of the integrity and continuity of existing forest vegetation, timely reforestation, improving landscape richness, and reducing human disturbance can ensure species richness and promote sustainable urban development while maintaining ecosystem stability [223,224].

(2)

Although green patch complexity, compact form, and stand density are hot topics in sustainability studies, it is unclear whether these indicators improve the carbon sink level.

The effect of green patch complexity, compact form, and stand density on enhancing the carbon sink level remains controversial. Generally, increasing the patch shape complexity results in a higher edge effect. The intensity of the edge effect at the ecotone [225] and changes in the forest edge microenvironment affect community structure and composition and ecosystem functions and processes [176]. Since edge fragmentation affects ecosystem processes related to biomass dynamics, vegetation degradation increases tree mortality and changes the green space’s structure and composition [152,226]. Green patches with complex forms are more prone to external influences, which may not be conducive to carbon sequestration [119]. Thus, the mainstream academic view is that reducing the morphological complexity of green spaces can increase their carbon sink level. An ecosystem with more compact and intact patches is more stable and has higher productivity than one with fragmented patches [152,158]. Most studies suggest that increasing the stand density increases the ecological niches and the biomass per unit area, promoting carbon sequestration [140].

The opposing view is that patch complexity and fragmentation do not necessarily have negative effects on green spaces. Some research results have shown that tropical green spaces are more likely to decrease the carbon sink capacity due to patch fragmentation [151,153]. However, the edge fragmentation of temperate green spaces has no significant effect on carbon loss or may increase the carbon sink capacity of vegetation because the resilience of forest land differs in different climate zones [160,227]. Green spaces with complex shapes have more edges. More favorable light and temperature conditions at the edge of green spaces promote vegetation growth, improving the carbon sink capacity [176]. Theoretically, the higher the plant biomass, the higher the carbon sink, and the carbon sink benefit increases with an increase in the planting density. However, after reaching a certain density, the plant growth and carbon sink capacity decrease [228]. Therefore, trees should be planted at an appropriate density according to local conditions to improve the carbon sink level of green spaces and increase ecosystem sustainability [169].

The complexity of green spaces should be reduced in landscape planning to ensure sustainable development. In some cases, edge complexity caused by fragmentation can improve the carbon sink capacity due to increased nitrogen deposition, improved light conditions, and altered microclimatic conditions near the edge [176]. However, excessive habitat fragmentation and a decrease in the number of habitats adversely affect reproductive capacity and species diversity [229,230]. Therefore, it is crucial to maintain the integrity and connectivity of habitat patches in accordance with local conditions. Moreover, a reasonable planting density can also increase biomass and promote ecosystem sustainability [231].

5.3. Urban–Green Space Ecotone Design

Although the urban–green space ecotone is a hot topic in sustainability studies, academic opinions differ on its impact on the carbon budget. The adjacency between green spaces and the city is a controversial topic in sustainable development. Studies on the topological relationship between cities and green spaces have focused on the influence of the edge effect on carbon sinks during urbanization. Some researchers argue that the edge effect of green spaces during urban development causes the degradation of forest ecosystems, reduces landscape functions and productivity, and is detrimental to carbon sinks [232]. The opposing view is that under certain conditions, tree growth at the edges of suburb green spaces is enhanced due to higher temperatures. Thus, fragmented forests may contribute to combating climate change [185]. Therefore, protecting the integrity of existing green spaces should be prioritized during urban development to reduce edge effects. When fragmentation and more green space edges are inevitable, researchers should consider limiting patch fragmentation in landscape design to increase the carbon sinks of the green spaces.

The comparison of the carbon sink capacity of green spaces in and outside of cities is also controversial. Many researchers believe that the carbon sink level is higher in rural areas or regions far from the city than in urban green spaces [233] because urbanization causes land use changes [201], and the negative impact of urban sprawl on forest carbon stocks decreases with the increasing distance from the urban core to rural areas [194]. In addition, lower rural temperatures may reduce the respiration of vegetation in green spaces outside of the city, increasing the carbon absorption rate [191]. A minority view is that urban green spaces have higher carbon sequestration capacity and productivity per unit area than rural areas due to human management, temperature, and other indicators [166,234]. Nevertheless, most of these studies underscore the importance of prioritizing the protection of rural green spaces.

Scholars studying the inclusion relationship between green spaces and the city have focused on analyzing the impact of green space cooling on urban carbon emissions. They suggested that the distribution of green spaces affected energy efficiency and reduced atmospheric carbon dioxide concentrations [199]. Some urban green spaces are wedge-shaped, resulting in urban ventilation corridors. When trees are planted close to buildings, they can mitigate the urban heat island effect by moderating the temperature and wind, reducing building energy use (e.g., air conditioning energy use) [196,200,235,236], indicating that the layout of green spaces in the city has a variable effect on emission reduction [33]. In addition, the size, structure, quantity, and type of vegetation in green spaces affect the cooling effect and carbon reduction. Many researchers believe that a larger area contains more plants, resulting in higher evapotranspiration rates, more shade, a better cooling effect, and higher amounts of carbon. Meanwhile, more complex shapes have a longer boundary, providing more heat relief [197]. For the same area, more small green areas performed better than fewer large green areas for carbon savings [197], which could be applied to urban green space construction. However, this does not mean that green spaces should be intentionally fragmented. Scholars have investigated various fragmentation configurations and shape complexities of green space to achieve the optimal cooling effect in urban environments [237,238]. More research should focus on shape complexity.

Regarding the sustainable development of human and ecological spaces, residents living near the urban–green ecotone have a higher quality of life [239]. However, they might have to bear the costs of urban green space construction [27]. Protecting the integrity and connectivity of urban green spaces is equally important because they enhance species richness and are critical for urban ecosystem stability [240]. In summary, the integrity and connectivity of existing green spaces should be protected, and human interference should be minimized. More small green spaces with simple shapes, rich structures, and high connectivity should be considered in urban design because they are crucial for maintaining ecological stability and improving human well-being [239]. Our main conclusions are summarized in Figure 8.

6. Conclusions

This paper used a landscape sustainability framework to investigate urban and green space form indicators affecting the carbon budget at different scales. We systematically reviewed the influencing indicators and their interactions on carbon emission reduction, carbon sink capacity, and sustainable development. In addition, we provided recommendations to achieve carbon neutrality in urban and green spaces and the urban–green space ecotone and outlined future directions for multi-factor landscape ecology to improve the carbon budget from the perspective of sustainable development. We provided practical suggestions for urban and green space forms.

We provide the following suggestions for future research: (1) Most papers lacked guidance on urban planning and construction and definitions of the level of urban development. Empirical research should provide more specific information on strategies suitable for different scales, economic levels, and regions. (2) There should be focus on topics that have resulted in contradicting results. For example, does the construction of green spaces increase carbon sinks and indirectly increase carbon emissions from transportation, thereby reducing economic well-being? (3) Universal optimization standards for urban and green space forms should be established to guide planning based on landscape sustainability. This is crucial for developing countries to develop widely applicable guidelines for urban and green landscape design. Meanwhile, the coordination and conflicts of various spatial planning strategies should be investigated, and quantitative research on sustainability and emission reduction related to landscape forms should be strengthened.