Calculation and Optimization of the Carbon Sink Benefits of Green Space Plants in Residential Areas: A Case Study of Suojin Village in Nanjing

Abstract

Objectives: In the “dual evaluation” of land space, the evaluation of the importance of ecosystem service functions and residential areas is important, playing a significant role in plants acting as carbon sinks and thereby achieving the transformation of low-carbon settlements. Methods: The paper compares and analyzes five models for quantifying carbon sink benefits and focuses on the national tree benefit calculator (NTBC) model, which is suitable for the carbon sequestration benefits of plants in residential areas, to (i) estimate and compare the economic benefits brought by trees and shrubs in residential areas, (ii) analyze the reasons for the differences between the current data and data for the next 20 years, and (iii) comprehensively evaluate the technical points related to the plant landscape in residential areas to assess whether they comply with the “Green Settlement Standard.” The index system was scored according to the standard. Result: The current data collected for existing trees and shrubs include the following: When the trees are in good condition, the order of the trees according to their economic benefits in the current year is Zelkova serrata > Cedrus deodara > Sapindus saponaria > Sophora japonica > Cinnamomum camphora > Prunus cerasifera > Magnolia grandiflora > Ulmus pumila > Acer L. > Lagerstroemia indica L. > Sapium sebiferum > Sabina > Punica granatum L. > Acer palmatum > Sapium sebiferum > Celtis sinensis Pers > Bambusa multiplex > Cycas > Melia azedarach L. > Pinus parviflora, and that of the trees in the next 20 years is Zelkova serrata > Cinnamomum camphora > Sophora japonica > Sapindus saponaria > Ulmus pumila > Cedrus deodara > Prunus cerasifera > Magnolia grandiflora > Acer L. > Sapium sebiferum > Cycas > Punica granatum L. > Lagerstroemia indica L. > Acer palmatum Thunb > Sabina > Bambusa multiplex > Broussonetia papyrifera > Celtis sinensis Pers > Melia azedarach L. > Pinus parviflora. The order of shrubs according to their economic gain in the current year is Photinia beauverdiana > Pittosporum tobira > Ligustrum lucidum > Viburnum odoratissimum > Buxus cephalantha, and that of the shrubs in the next 20 years is Ligustrum lucidum > Photinia beauverdiana > Pittosporum tobira > Buxus cephalantha > Viburnum odoratissimum. Conclusion: Using plants, the construction ideas, community structure and landscape maintenance of the carbon sink estimation system of residential areas should be updated according to three aspects to promote the quantification of the carbon sink benefits of green areas in urban settlements and the development of low-carbon settlements in China.

1. Introduction

At the 75th United Nations General Assembly, General Secretary Xi proposed that China intends to achieve carbon peaking by 2030 and carbon neutrality by 2060. Since the First Industrial Revolution, and as climate change has caused increasingly severe environmental problems for human activity and life, the global average temperature has increased by 1.1 °C [1]. In its report, the Intergovernmental Panel on Climate Change (IPCC) stated that 90% of today’s global warming is caused by human activities [2]. Atmospheric CO₂ monitoring data show that the average annual carbon absorbed by China’s terrestrial biosphere from 2010 to 2016 was 1.11 ± 0.38 Pg, which was estimated to be equivalent to 45% of China’s annual anthropogenic emissions during the same period [3]. Settlement plant communities are complex and diverse, and vegetation fragmentation is high, so using plants’ own carbon sequestration capacity and ecological role to slow down climate deterioration in urban areas and enhance the carbon sequestration capacity of the green spaces of settlements has become a key means of achieving the strategic goals of carbon peaking and carbon neutrality.

The spatial distribution of carbon emissions shows that cities are relatively prominent. Eto et al. (2014) stated that the CO₂ emissions of large-scale production, living, transportation and other high-energy-consumption activities in urban areas account for 75% of total emissions globally [4]. Cai et al. (2017) concluded that the amount of CO₂ emissions in urban areas is even higher in China, reaching 85% [5]. Zhou et al. (2019) argued that cities are increasingly important for the development of ecological and economic benefits for society [6]. Therefore, cities, as a core part of the sustainable development of human civilization, are imperative in the aims of saving energy and reducing emissions to increase carbon sinks. Yang et al. (2022) pointed out that the focus of carbon sinks is to strengthen the construction and protection of urban ecological and green spaces [7]. Li Hui et al. [8], Wu Ziqi [9] and Ji Yuanyuan et al. [10] studied the interaction between landscape design of green spaces and carbon sinks in settlements from the perspectives of the carbon sequestration effect of green spaces, carbon sink capacity and comparison of carbon emissions from landscape operation and carbon sink of plants, respectively. In this paper, we focus on the carbon sink benefits of the green spaces of settlements and investigate the relationship between green spaces and carbon emissions.

2. Site Status

Nanjing, Jiangsu Province, is located in the lower reaches of the Yangtze River (31°14′~32°37′ N, 118°22′~119°14′ E) [11], 50–70 km from east to west and 150 km from north to south. Nanjing has a subtropical monsoon climate. With abundant rainfall, four distinct seasons, an average annual temperature of 15.4 °C and an average annual precipitation of 1106 mm, Nanjing is a blend of mountains, water, cities and forests, with a forest coverage rate of 26.4% and a green coverage rate of 45% in built-up areas [12]. The Xuanwu District is the largest central city in Nanjing, with an area of 80.97 km². The community of Suojin Village was established in 1984, covering an area of 7 km², an area with beautiful scenery and rich cultural heritage in the Xuanwu District (Figure 1 and Figure 2). As the “First Street of Happiness in Jinling,” Suojin Village creatively developed and implemented the “Harmonious Community Evaluation Guidelines,” which strongly promote harmonious community construction in Suojin Village, and created a harmonious community evaluation system at the street level in the country, which was highly valued and affirmed by the Ministry of Civil Affairs of China. “Suojin Sample,” a social construction brand, is well known nationwide.

The plant configuration and overall structure of the village are similar to those of other residential areas, with a rich variety of trees and shrubs planted in the residential areas.

The plant species in the sample plots were analyzed based on field studies (Table 1).

Table 1. Grouping of the sample plants.

Location	Plant Group
Suojin I Village entrance intersection	Magnolia grandiflora, Cedrus deodara, Ulmus pumila and Ilex chinensis Sims, etc. (Figure 3).
Both sides of the road in Suojin I Village	Cinnamomum camphora, Sapindus Saponaria, Eucommia ulmoides, Magnolia grandiflora and Sophora japonica Linn.
Technology Plaza	Buxus sinica, Aucuba chinensis Benth, Photinia beauverdiana, Cycas Linn, Bambusa multiplex, Prunus cerasifera, Sophora japonica ‘Winter Gold,’ Punica granatum and Osmanthus fragrans (Figure 4).
	Euonymus japonicus, Cycas Linn, Styphnolobium japonicum and Lagerstroemia indica L. (Figure 5).
	Sabina, Punica granatum and Cedrus deodara.
Entrance of Suojin II Village	Broussonetia papyrifera and Nandina domestica.
Both sides of the road in Suojin II Village	Punica granatum, Pinus massoniana Lamb, Cinnamomum camphora, Ulmus pumila L., Zelkova serrata, Acer L., Sophora japonica, Ligustrum lucidum, Euonvmus japonlcus, Zanthoxy lumpiperitum and Ophiopogon bodinieri.
Both sides of the road in Suojin IV Village	Punica granatum, Zelkova serrata, Cinnamomum camphora, Acer L., Sapindus Saponaria, Melia azedarach, Celtissinensis Pers, Euonvmus japonlcus and Ophiopogon bodinieri.
At the intersection of Suojin IV Village	Sapium sebiferum.

Table 1. Grouping of the sample plants.

Location	Plant Group
Suojin I Village entrance intersection	Magnolia grandiflora, Cedrus deodara, Ulmus pumila and Ilex chinensis Sims, etc. (Figure 3).
Both sides of the road in Suojin I Village	Cinnamomum camphora, Sapindus Saponaria, Eucommia ulmoides, Magnolia grandiflora and Sophora japonica Linn.
Technology Plaza	Buxus sinica, Aucuba chinensis Benth, Photinia beauverdiana, Cycas Linn, Bambusa multiplex, Prunus cerasifera, Sophora japonica ‘Winter Gold,’ Punica granatum and Osmanthus fragrans (Figure 4).
	Euonymus japonicus, Cycas Linn, Styphnolobium japonicum and Lagerstroemia indica L. (Figure 5).
	Sabina, Punica granatum and Cedrus deodara.
Entrance of Suojin II Village	Broussonetia papyrifera and Nandina domestica.
Both sides of the road in Suojin II Village	Punica granatum, Pinus massoniana Lamb, Cinnamomum camphora, Ulmus pumila L., Zelkova serrata, Acer L., Sophora japonica, Ligustrum lucidum, Euonvmus japonlcus, Zanthoxy lumpiperitum and Ophiopogon bodinieri.
Both sides of the road in Suojin IV Village	Punica granatum, Zelkova serrata, Cinnamomum camphora, Acer L., Sapindus Saponaria, Melia azedarach, Celtissinensis Pers, Euonvmus japonlcus and Ophiopogon bodinieri.
At the intersection of Suojin IV Village	Sapium sebiferum.

3. Methodology and Applicability Evaluation

The method of using the carbon sink estimation system is different from the traditional biomass measurement method, which does not require measurement steps such as field survey, considerably saving manpower and material resources and making the calculation faster. The growth state of trees can be simulated through the attribute characteristics of vegetation itself, and accuracy can be enhanced from the perspective of estimation. The operation is simple, the survey data required are small and the professional requirements of the data collectors are low, which is suitable for the estimation of the carbon sink of urban green spaces to evaluate ecosystem service functions. This paper analyzed the feasibility of i-Tree, national tree benefit calculator, CITYgreen, Pathfinder and quantification model of the carbon sink of the entire lifecycle of the landscape, according to the regional characteristics of Suojin Village. We selected a suitable carbon sink estimation method for data statistics.

3.1. Methods

3.1.1. i-Tree

i-Tree provides the latest peer-reviewed urban forestry analysis and ecological benefit evaluation models of the USDA Forest Service to all types of users through free tools and web support [13]. To evaluate individual trees, My Tree, i-Tree Design and i-Tree Eco were used to derive estimates of individual tree benefits, and for canopy area assessment, Our Tree, i-Tree Landscape and i-Tree Canopy were used to estimate land and canopy cover as well as its benefits to facilitate the identification of landscape equity, climate priority plantings and protected areas. Graphs were produced regarding the structure, function and benefits of urban–rural vegetation, covering CO₂ sequestration and storage, stormwater mitigation, etc., as well as research data showing the impact of the study objects on building energy consumption and indirect reductions in CO₂ emissions and air pollution removal (Figure 6).

3.1.2. National Tree Benefit Calculator

A national tree benefit calculator (NTBC) is an easy-to-use calculator to assess the benefits of a single family of trees based on i-Tree data. The annual carbon sequestration of the subject is estimated by converting the aboveground biomass into carbon sequestration. The annual carbon sequestration in this model is actually the annual increment in carbon stored in the form of biomass [14,15]. NTBC can estimate the present-day plant carbon sink benefits and aggregate the carbon sink benefits over 20 years.

3.1.3. CITYgreen

CITYgreen is an ArcView-based model for evaluating the ecological benefits of green spaces, not only for simulating the growth of vegetation, but also for evaluating vegetation-specific ecosystem services, such as carbon sequestration, transpiration, reduction in air pollutants, reduction in storm water runoff and shade and energy conservation [16] (Figure 7). Information on 300 species is included in this database, which can be used for eco-efficiency analysis. Updating details such as leaf density, diameter at breast height growth rate, height growth rate and crown shape of trees suitable for domestic studies, the system finds matches in the database to estimate the eco-efficiency of each tree [17]. In the latest version of CITYgreen (5.0), a new piece of Regional Ecosystem Analysis (REA) was added to combine remote sensing data to assess the function and structure of urban green spaces and to calculate ecological service value changes in urban green spaces in urban areas [17] (

Table 2. CITYgreen database parameter data [13].

	Carbon Storage Factor	Carbon Absorption Factor
Young	0.322	0.007
Middle Age	0.442	0.001
Mixed	0.539	0.002
Average type	0.430	0.003

).

The formulae are as follows:

Carbon storage (t) = carbon storage factor ∗ vegetation cover ∗area of study area [18].

Carbon uptake (t) = carbon uptake factor ∗vegetation cover ∗area of study area [18].

3.1.4. Pathfinder

Pathfinder is a US-developed urban landscape carbon calculation system in the form of a web page through which designers can introduce specific carbon emission targets for climate improvement [13].

Referring to the basic information on carbon sources, sinks and costs provided by the Athena Sustainable Materials Institute of Canada, the U.S. Forest Service and the U.S. Conservation Service, it can be concluded that 80 different materials used in landscaping projects, which are extracted, manufactured, transported, installed, used, maintained and replaced in paving, walls, fences, etc., generate carbon emissions associated with the process; trees, shrubs and grasses absorb CO₂ from the atmosphere and fix it into the soil; and emissions from pruning trees and shrubs occur periodically over the life of the project and are often referred to as “carbon from operations.”

3.1.5. Quantification Model of the Carbon Sink of the Entire Lifecycle of a Landscape

(1)

Research scope: 50 years as a lifecycle research threshold [19];

(2)

Calculation model: carbon sink C_SCO₂ (in the entire lifecycle of the landscape) = carbon sink Cs1 (of green plants) + indirect carbon sink Cs2 [20]

C_SCO₂ = Cs1 + Cs2

where Cs1 represents the direct carbon sink of plants in the landscape and Cs2 represents the indirect carbon sink of other elements in the landscape.

3.1.6. Greening Three-Dimensional Volume Method

The greening 3D volume method can derive the leaf area of individual trees and shrubs based on crown diameter [21]. For a tree species, its crown diameter (also called crown width) and crown height always have a certain statistical correlation. By field collecting the plant crown diameter, crown height, canopy morphology and other sample data and using regression analysis to establish the relevant “diameter–height equation,” the crown height can be obtained based on the crown diameter and using the volume of the canopy morphology. Finally, the leaf area corresponding to the unit volume of green volume can be obtained from the model forest parameters.

The three-dimensional greening method uses the method of “simulating a three-dimensional volume with flat volume,” and the crown height is derived from the crown diameter. Then, the green volume is obtained, so it is possible to simplify the complex three-dimensional measurement problem to a flat solution.

3.2. Applicability Evaluation

There are two major problems in estimating the carbon sink of vegetation in urban green spaces. One is the acquisition of data and the other is the selection of the applicable method. In the case that the information has been determined, the carbon sink estimation method chosen for an urban green space needs to be evaluated for applicability first (

Table 3. Comparison of the characteristics of various carbon sink estimation systems.

Measurement Method	Main Features	Application
i-Tree	1. Reliable source of basic data. 2. Comprehensive estimation of vegetation carbon sinks (including carbon sinks of aboveground and belowground parts).	Not suitable for the precise assessment of the carbon sinks of single trees, but suitable for the estimation of the carbon sinks of large-scale urban green areas, such as urban forests.
National tree benefit calculator	1. Simplified on the basis of i-Tree, in the form of web pages, and easy to operate.	Only the carbon sink of a single plant can be estimated.
CITYgreen	1. Less data required. 2. Quick estimation. 3. Focus on the indirect carbon sink benefits of plants.	Carbon sink estimation for single family street trees and urban forests.
Pathfinder	1. Carbon calculation is comprehensive, including carbon source, carbon sink and carbon cost. 2. No need for parameter correction. 3. Web-based and easy to operate.	Not suitable for large-scale carbon sink estimation due to the need of entering specific information on carbon sink elements within the site.
Quantification of carbon sinks of the entire lifecycle of a landscape	1. Determines the area or proportion of area corresponding to different planting types, often using climate zone divisions to determine the parameters, with a large relative error.	It is suitable for green areas with known planting types and their corresponding planting areas, and is a rougher calculation method. It is also suitable for the carbon stock measurement of green areas under conditions of lack of planting information.
Greening three-dimensional volume method	1. Determines the area or area proportion corresponding to different planting types. 2. Often determines parameters by climate zone division, with a large relative error.	It is suitable for green areas with known planting types and their corresponding planting areas, and is a rougher calculation method. It is also suitable for the carbon stock measurement of green areas under conditions of lack of planting information.

), and then a suitable method is selected in combination with the basic information of the residential area.

It was found that the vegetation species in the village are limited, and, as the purpose of this study was to estimate the carbon sink benefits of plants in the village, the most suitable model for calculating the carbon sink benefits of individual plants in the village is the National tree benefit calculator. The carbon sink benefits of trees and shrubs were calculated separately for different planting sites and distances from surrounding buildings.

4. Results and Analysis

Based on the adaptability evaluation of the above methods, the NTBC carbon sink estimation research method was selected for the residential area of Suojin Village to quantify the carbon sink benefits of its trees, shrubs and ground cover. The specific operational framework is as follows (Figure 8):

4.1. Calculation of the Annual Carbon Sequestration of a Single Tree

Step 1: First, the growth status of the trees and the environment were measured in the field (Figure 9).

Step 2: Data for the current year were derived using the NTBC model (Figure 10).

Step 3: Data for the next 20 years were derived using the NTBC model (Figure 11).

In the case of trees in generally good condition, the order of the trees according to their economic benefits in the current year is Zelkova serrata > Cedrus deodara > Sapindus saponaria > Sophora japonica > Cinnamomum camphora > Prunus cerasifera > Magnolia grandiflora > Ulmus pumila > Acer L. > Lagerstroemia indica L. > Sapium sebiferum > Sabina > Punica granatum L. > Acer palmatum > Sapium sebiferum > Celtis sinensis Pers > Bambusa multiplex > Cycas > Melia azedarach L. > Pinus parviflora, and that of the trees in the next 20 years is Zelkova serrata > Cinnamomum camphora > Sophora japonica > Sapindus saponaria > Ulmus pumila > Cedrus deodara > Prunus cerasifera >Magnolia grandiflora > Acer L. > Sapium sebiferum > Cycas >Punica granatum L. > Lagerstroemia indica L. > Acer palmatum Thunb > Sabina > Bambusa multiplex > Broussonetia papyrifera > Celtis sinensis Pers > Melia azedarach L. > Pinus parviflora. It can be seen that, in light of the ranking change of Cinnamomum camphora, Ulmus pumila, Cedrus deodara and Cycas Linn, combined with the characteristics of the plants’ own properties, Cinnamomum camphora has a well-developed root system, is more resistant to strong winds, has a slower growth rate and has a longer life span. If only analyzing the current year’s income, that of Cinnamomum camphora is not as good as that of acacias, but in 20~50 years, the economic benefits brought by camphor far exceed those of other existing tree species. Ulmus pumila is a masculine tree, has a fast growth rate and has a well-developed root system, with the former carbon sink benefits being average. The carbon sink benefits at an early planting stage are average, but with time, and considering that Ulmus pumila grows fast, the economic benefits can even surpass those of Cedrus deodara after 20 years. Cedrus deodara is an ornamental tree with a strong ability for dust and noise reduction due to its tall body and beautiful shape, so its carbon sink ability at an early planting stage is high. At a later stage, it is also in the middle to upper levels and can be planted in the main nodes or on both sides of a road, offering a spectacular display. The growth cycle of Cycas Linn is long, so there are carbon sink benefits at the beginning. The carbon sink benefits of Zelkova serrata are very long, so it is in the middle of the list after 20 years.

Zelkova serrata has well-developed lateral roots and strong soil and wind resistance, and its carbon sink benefits at present and in the next 20 years exceed those of other species by a large margin. Sophora japonica has a well-developed root system and strong wind resistance, and it is resistant to toxic gases, such as sulfur dioxide, hydrogen fluoride, chlorine and soot. It can improve the microclimate, has strong wind and soil resistance, resists pollution and other ecological functions, and is an excellent tree for soil and water conservation. Sapindus saponaria is also deep-rooted, with strong wind resistance, fast growth and a long life. It is the first choice for the ecological greening of industrial cities. As it can mature in 5~6 years and has a strong resistance to sulfur dioxide, Sapindus saponaria ranks first at present, but it is gradually surpassed by Camphor and Acacia as time grows.

4.2. Calculation of Carbon Sequestration for Regional Areas

Step 1: First, the growth status of the trees and the environment were measured in the field (Figure 12).

Step 2: Data for the current year were derived using the NTBC model (Figure 13).

Step 3: Data for the next 20 years were obtained using the NTBC model (Figure 14).

In the case of equal tree conditions, the order of the trees according to their economic benefits in the current year is Photinia beauverdiana > Pittosporum tobira > Ligustrum lucidum > Viburnum odoratissimum > Buxus cephalantha, and that of trees according to their economic benefits for the next 20 years is Ligustrum lucidum > Photinia beauverdiana > Pittosporum tobira > Buxus cephalantha > Viburnum odoratissimumi. Ligustrum lucidum, as a deep-rooted species with strong resistance to atmospheric pollution and to gases such as sulfur dioxide, can also absorb hydrogen fluoride, chlorine and other toxic gases. Buxus cephalantha and Viburnum odoratissimum, widely used plant species, have a more prominent landscape effect and cultural value than their carbon sequestration effects.

4.3. Technical Requirements for Green Low-Carbon Settlements

The energy demand of residential areas, considering the benefits of plant carbon sinks, is much smaller than that of other communities, especially during occupancy periods. Therefore, it is necessary to consider the consumption of residential resources and energy. As far as plants are concerned, reducing carbon emissions can be prioritized by increasing the number of plants with high carbon sequestration benefits and prioritizing the development of landscape greening systems with strong carbon sink benefits. Through planning and design, the heat island effect can be reduced, greenery can be increased and the comfort of outdoor environments can be improved. The plan of residential buildings should be arranged reasonably, paying attention to the distance between plants and residences and the enclosing of the plants so as to improve natural ventilation. Local materials should be used as much as possible, selecting native tree species and maintaining the local ecology. Tree species with low resource consumption, low environmental impact and small losses during handling, planting and dismantling throughout the landscape lifecycle should be adopted [22].

The following is based on the quality evaluation index system of livability planning in the Green Settlement Standard (

Table 4. Green Settlement Standards.

Green Settlement Standards: Quality Evaluation Index System of Livability Planning
1	The green space is well configured and appropriately located, and the centralized green space is continuous with each other to form ecological corridors and combined with scattered green spaces.	11
2	Harmless tree species suitable for local growth are chosen and reasonably matched with trees, shrubs, grasses and flowers, with rich plant species.	11
3	Lighting facilities are set up in green areas or outdoor activity sites.	11
4	Green areas meet the technical requirements of sponge cities, and their activity sites use infiltration measures and are paved with 15–25% hard permeable bricks.	11
5	Outdoor noise control conforms to the provisions of the relevant national standards.	11
6	Outdoor sites meet the requirements of sunshine and shade, reduce the heat island effect, optimize the outdoor wind environment, and their centralized public activity sites, children’s activity sites and all-age sports fields are planned in conjunction with breezeways.	11

).

The six items in the above table of “Green Settlement Standards” total 66 points, accounting for 44% of the total 150 points of the Quality Evaluation Index System of Livability Planning, on the basis of which experts can score and evaluate the Suojin Village subdistrict.

On the basis of the above, the green space within Suojin Village is well configured, ecological corridors are combined with green space, and the area of trees and shrubs used in the living area is large, totaling more than 10 species, with the top-ranked trees having good ecological functions and being resistant to polluting gases.

However, there are still some aspects in the district that are not perfect, such as the lack of flower planting, insufficient lighting facilities, and low visibility at night, and because there are not many large trees in the Suojin Village Science and Technology Plaza, they cannot meet the shade and cooling needs of residents in summer.

According to the above criteria, the final score of the Suojin Village district is 8 + 6 + 5 + 8 + 8 + 8 + 5 = 48. There is still room for improvement in terms of green plant configuration, plant species, lighting, shade requirements, etc. This paper proposes high carbon sequestration and high-efficiency residential renewal strategy, starting from the plant aspect.

5. Discussion and Conclusion

5.1. Construction Ideas for Low-Carbon Settlements

Based on our field research and correlation analysis, we can conclude that the green space system planning in Suojin Village is not perfect, especially considering the lack of low-carbon-related green space system planning. In previous research, the spatial elements affecting the carbon sink benefits of the settlement were screened out and the essence of each influencing element was analyzed. The important elements affecting the carbon emission of the settlement were summarized, and then five major low-carbon construction paths for the settlement to reduce carbon emissions and increase carbon sink were reviewed with the analysis of the carbon sink of the settlement, which involved the five aspects of energy and resource use, land use, road traffic, building construction and design, and green space environment. Using these five aspects, specific measures and control elements for the construction of low-carbon residential land were analyzed, low-carbon indicators were derived, and then low-carbon indicators for residential land were generated by combining the low-carbon indicator database and the traditional control plan framework [23].

5.2. Reasonable Community Structure

According to the analysis of the carbon sequestration capacity of plants, fewer ornamental tree species generally have a higher carbon sequestration capacity. According to the data of the Nanjing garden department, according to the garden city standard introduced by the state, Nanjing’s greening index far exceeds the national standard. The improvement of the total amount of plant carbon sink benefits can be optimized from four aspects: increasing the proportion of dominant species that contribute to carbon sequestration; increasing vegetation coverage; replacing some species that do not highly contribute to carbon sinks; and replacing missing trees and drawing a reasonable optimization design of the plant community from the ecological perspective of the plants themselves. Through the latter, the trees can coexist harmoniously, considering the plants’ own ecology, thus maximizing the carbon sequestration capacity of the plant community. In the vertical structure of plant communities, carbon storage mainly occurs at the tree, shrub and grass levels and there is some carbon storage in the soil. According to domestic and foreign studies, complex community structures, such as vegetation structures with three layers of trees, shrubs and grasses, usually have a stronger carbon sequestration capacity; mixed evergreen–deciduous forests have a stronger carbon sequestration capacity than simple evergreen forests or deciduous forests.

In terms of the planting density of different plants, trees and shrubs are also very different. In order to achieve the maximum growth of tree leaves and enhance their own carbon sequestration capacity, trees need to form a suitable degree of depression among them in order to have sufficient space for growth. The planting density should be controlled within the range of 250 trees/hm² to 450 trees/hm² [24], and should not be too dense. The existing arrangement prefers to choose one tree as the main body and the rest of the species as the auxiliary arrangement; the number of dominant species is much larger than the number of other species on a road, and there is almost no fixed matching pattern among auxiliary species.

In view of the above considerations, the existing plants in the first and second villages and the science and technology square of Suojin Village were recombined and matched from the aspects of tree species selection, plant configuration and community structure, adjusting the ratio of deciduous, evergreen, broad-leaved and coniferous trees, and choosing garden species with a strong carbon sequestration capacity as the main and skeleton species, with plants in this landscape creation planted as far as possible from each other [23]. Preserving the existing large area of street trees, such as the original Cinnamomum camphora and Sapotaceae, ranks the carbon sink benefits of both Cinnamomum camphora and Sapotaceae in terms of current and future carbon sink benefits for 20 years. However, the current renewal rate of urban residential areas is fast, and a group of trees may be replaced in less than 20 years, so the carbon sink capacity of Sutra will be underexploited, and it is suggested to use trees that produce a quick effect. Considering plant coloring in the science and technology square, plants with colorful foliage that have a strong carbon sequestration capacity and high carbon sink benefits can be increased, such as Purple vetch, Prunus cerasifera and Acer palmatum. The number of acacia trees in the square can be increased. Long-clawed acacia is the bud variant of Chinese acacia, with a high carbon sink benefit; it covers a small area, and the general planting spacing is 1.5 m ∗ 2 m in patches, with an initial planting density of 222 trees/667 m².

Integrating the spatial and aesthetic characteristics of tree species, efforts were made to create a richly layered community structure [24], and in the vertical structure, a compound landscape structure of trees, shrubs and grasses was used. Trees can be selected from Zelkova serrata, Cinnamomum camphora, sapota, sebifer and long-clawed acacia; shrubs and herbaceous plants should be selected from species with reduced artificial intervention and a high carbon sequestration capacity, including woody plants such as maidenhair, heather, pearl plum and pavement Sabina, reducing the number of Melia azedarach L and five-needle pine in residential areas.

5.3. Reduced Manual Intervention for Landscape Maintenance

The pruning, fertilization and irrigation of plants generate carbon emissions, so the maintenance and management aspects of green spaces should also be considered regarding the carbon sink benefits of residential areas. Studies have shown that some landscape projects later require manual maintenance, such as hiring workers for shrub pruning, the use of non-organic fertilizers and the labor costs used for irrigation, all of which reduce the carbon sink benefits of vegetation to some extent [25] (

Table 5. Information on the annual carbon sink of lawns based on Pathfinder [25].

Type	Untrimmed	Minimum Management	Moderate Management	High-Intensity Management
Annual carbon sink (kg/m2)	0.794	−0.035	−0.049	−0.196
Fertilizer application	Organic fertilizer	Organic fertilizer	Non-organic fertilizer	Large amounts of organic fertilizer
Mowing	Unmanned pruning	Manned mowing	Manned prunning	Neatly pruned
Irrigation	Unattended irrigation	Unattended irrigation	Manned irrigation	Manned irrigation

). Therefore, the decarbonization and greening of the landscape should reduce the manual involvement as much as possible in maintenance and post-management processes, which can include unmanned lawn mowing, the use of organic fertilizers and natural ways of irrigating vegetation, such as rain gardens and permeable paving, that not only maintain the carbon sink benefits of vegetation, but also enable the residential areas to achieve the technical requirements for sponge cities in the Green Settlement Standard.

It can be seen that residential green areas are an important part of urban green areas and are closely related to people’s lives. The landscape construction of residential areas, such as the human activities that take place in them, generates a large amount of CO₂ in the entire lifecycle of a landscape. The carbon sink benefits of vegetation considered in this study aimed to provide new ideas for residential plant landscape configurations, and, according to the results, it can be seen that not all plants have positive benefits from a carbon neutrality perspective and may also generate economic burdens. By measuring and analyzing the plant carbon sink benefits of existing old residences, suitable plant groups were reselected.

Therefore, increasing the carbon sink benefits of settlements and maximizing the ecological benefits of settlements in urban ecological construction are key to the construction of low-carbon settlements and green settlements. Not only for residential areas but also for the whole city, urban green areas with reasonable planning and design can have high carbon sink benefits, which are of great significance to the green development of the city. Considering the renewal of residential areas from both reducing carbon sources and increasing carbon sinks, according to model calculations, not only can we enrich the plant database but also apply the low-carbon concept to the entire landscape design cycle, which can help to improve the carbon sink benefits of residential plants.