Research on Promoting Carbon Sequestration of Urban Green Space Distribution Characteristics and Planting Design Models in Xi’an

Abstract

Urban green space is considered to reduce the concentration of forcing factors of climate change such as the carbon dioxide in the atmosphere. Promoting carbon sequestration efficiency within a limited urban green space has become a practical challenge that must be faced in urban sustainability. This study proposed three design models and a list of high carbon sequestration plants. Based on the research on the distribution and change in carbon sequestration in urban green spaces, combined with field surveys and remote sensing images, this study analyzed the main factors affecting carbon sequestration. The results showed that the carbon sequestration capacity in urban green space tends to decrease gradually along with the change in forest structure in a time series of the years 2000, 2007, 2014, and 2019, and this trend was mainly related to the characteristic factors of plant communities in urban green spaces: the carbon sequestration of plants was significantly positively correlated with DBH (diameter at breast height) and community density; positively correlated with hierarchical structure. In addition, we put forward a list of plants with high carbon sequestration, including Styphnolobium japonicum, Salix babylonica, Pittosporum tobira, Spiraea salicifolia, and Iris pseudacorus, proposed three planting design models for different green spaces and habitats to improve the efficiency of carbon sequestration in urban green spaces, and established the community structure models of high carbon-fixing plants which can be directly applied to practical projects. It also explored the sustainable design approach of ecological processes in low-carbon cities.

1. Introduction

At present, global warming attracts widespread attention, the main cause of its occurrence—CO₂ emission—is gradually becoming the focus of the world’s attention, and “carbon emission peak and carbon neutrality” is becoming an important research topic. According to the comprehensive assessment report of the Intergovernmental Panel on Climate Change (IPCC), anthropogenic greenhouse gas emission from the use of fossil fuels is one of the main causes of global warming. Thomas et al. and Root et al. integrated various climate prediction models and predicted that the global temperature will rise by about 1.4~5.8 °C in the next 100 years, and this trend of climate change will be further intensified [1,2]. Cities are the places where human activity has the biggest impact on the environment in terms of carbon emissions [3,4,5]. Studies have shown that cities are the main sources of carbon emissions [6], which account for about 90% of the country’s carbon emissions. Urban greenhouse gas sources in China mainly include four parts: production (44.5%), construction (19.8%), transportation (17.5%), and forest reduction (18.2%) [7]. There are two ways to reduce greenhouse gases: reducing carbon emissions and increasing carbon sequestration. Urban green spaces play an irreplaceable role in low-carbon cities. Through rational and scientific planning, urban green space can not only indirectly reduce carbon emissions, but also greatly increase carbon sequestration and affect the microclimate of the surrounding environment within a certain range [8].

As an important part of the urban ecosystem, urban green space plays an indispensable role in regulating the climate of the urban built environment [9,10,11]. First, the urban green space is the only natural carbon sequestration in urban areas, and planting trees is the only carbon sequestration method that does not consume energy [12,13]. Therefore, promoting carbon sequestration by urban plants is an important way to alleviate the urban heat island effect [14]. Studies have shown that Beijing’s forests and garden plants can absorb 71.308 million tons of carbon dioxide every year, accounting for 53.2% of Beijing’s total carbon dioxide emissions [15]. Carbon sequestration by urban plants accounts for 1.2% to 11.9% of Finland’s total CO₂ emissions [16]. Hangzhou’s urban forests offset 18.57% of the carbon emissions of industrial enterprises annually through sequestration [17]. Street trees on the boulevard can offset the CO₂ emissions of about 14 residents from traffic-related activities [18]. In addition, there are differences in the amount of carbon dioxide absorbed by different plant communities, so it is very necessary to study the carbon sequestration of different plant communities [19]. Based on the previous research on carbon sequestration by plants, how to transform the ecological research of high carbon sequestration plant communities into design language through scientific planning and design, use planting design to improve and adjust the urban microclimate, and provide a scientific basis for green space planting design in low-carbon cities have become practical problems to be solved urgently.

Secondly, urban green spaces can reduce emissions indirectly. Through the reasonable planning of the urban green space system, the overall energy consumption of the city can be reduced to achieve the effect of emission reduction [18,20,21]. Studies have shown that plants in urban green spaces can absorb and store CO₂ produced by nearby buildings and traffic [22,23]. In addition, the rational layout of green space makes life more convenient for residents, reducing the use of private cars and using more public transportation or walking can also greatly reduce carbon emissions. A reasonable urban transportation layout is an important way to realize low-carbon cities, and the construction of low-carbon cities should pay attention to promoting the “green transportation system”—establishing a traffic organization model that focuses on walking and riding, public transport, and limited cars. At present, there are many studies on the carbon sequestration capacity of urban green spaces [24], but the scales and evaluation indicators in these studies are also different [25]. In order to quantify and increase the carbon fixation efficiency of plants in urban green space, quantitative research using software and technologies, such as ArcGIS, Envi, and i-Tree, has become the current trend in this field [26,27]. However, in the current urban built environment, urban green spaces are mostly distributed in small patches and challenges such as unreasonable planting design make it impossible to exert their original carbon sequestration value. Therefore, how to rationally distribute green spaces within the limited urban scope and improve their carbon sequestration efficiency and ecological value has become the focus and difficulty of current research.

Due to the lack of sustainable development awareness in the early urban planning process, the quality and quantity of urban green space are seriously insufficient at present. With the rapid progress of urbanization in Xi’an, urban ecological imbalance, the heat island effect, and other issues have become increasingly prominent. Therefore, promoting the carbon sequestration of urban green space and applying low-maintenance and sustainable planting strategies are still urgent practical problems. Therefore, this study simulated the distribution of carbon sinks in Xi’an in recent years through field measurements and remote sensing simulation methods and proposed planting design strategies suitable for different green spaces in low-carbon cities. The main contents are as follows: (1) Simulate the spatial and temporal distribution of carbon sinks in Xi’an in the recent 20 years, and analyze the reasons for changes; (2) quantitative study on carbon sequestration benefits of plant communities in urban green space, and analyze the relationship between growth indicators of plant communities and carbon sequestration capacity; (3) optimize the combination and configuration of plant communities in current urban green spaces, and propose planting design models and strategies to effectively improve the carbon sink capacity of green spaces. This study takes different green spaces in Xi’an as the object, and the results have guiding significance for the planning and design of other green spaces. It will provide theoretical support and technical guidance for urban green space planning, design, and ecological construction in Xi’an, and also provide new ideas for promoting the construction of low-carbon cities, alleviating the heat island effect.

2. Materials and Methods

2.1. Study Sites

Xi’an is an important city in northwest China. It is located in the middle of the Guanzhong Plain area, with a temperate semi-humid continental monsoon climate and four distinct seasons. It has an average annual frost-free period of 226 days. The average temperature in January is 0.4 °C, in July is 26.6 °C, and the annual average temperature is 13.3 °C. The annual average precipitation is 613.7 mm, and the annual average humidity is 6.96%. The altitude is 410–420 m. By December 2021, the administrative area of Xi’an was about 10,752 km², and the built-up area was 700.69 km².

2.2. Remote Sensing Data

According to the geographic strip number of Xi’an, the remote sensing images with a resolution of 30 m were obtained from China Geospatial Data Cloud (http://www.gscloud.cn/) (accessed on 21 May 2022). Then, we performed the radiometric calibration, atmospheric correction, image mosaic and clipping, and unsupervised classification on the images. Finally, we obtained four land-use types maps of Xi’an for the years 2000, 2007, 2014, and 2019 (Figure 1).

2.3. Simulation Method of Carbon Sequestration Data

After preprocessing the remote sensing images, InVEST models were used to simulate the temporal and spatial changes and trends of carbon sequestration in Xi’an in 2000, 2007, 2014, and 2019. The InVEST model (Integrated Valuation of Ecosystem Services and Trade-offs) is a comprehensive evaluation model of ecosystem service functions and trade-offs. In the InVEST model, the carbon storage of the ecosystem is divided into 4 basic carbon pools: the aboveground biological carbon pools (all the carbon that survives in plants above the soil), the underground biological carbon pools (the carbon that exists in the plant root system), soil carbon pools (organic carbon distributed in organic and mineral soils), and carbon pools in organic matter (carbon in the litter, inverted or standing dead trees). The total carbon storage (C total, kg·km⁻²) was calculated by the sum of all carbon pools by the area of each land type, as follows:

$$C_{totali}=(C_{abovei}+C_{belowi}+C_{soili}+C_{deadi})\times A_i$$

(1)

In Formula (1): i is the average carbon density of each land use, and A_i is the area of the land-use type (km²).

By simulating the carbon sequestration of plants and soil in different years, this study analyzed the trends and reasons for the temporal and spatial changes in carbon sinks in Xi’an. It found that the carbon sink changes most in the main urban area. Therefore, we selected green spaces within the main urban area for investigation and proposed a design method for improving the carbon sink of the plant community for the plant community with low carbon sink efficiency.

2.4. Quadrat Selection and Data Estimation

The study selected Xingqing Park (Site A) in Xi’an and the green space of Kangdingheyuan residential area (Site B) in Fengxi New City as sample plots, which are located in the old and new urban areas of Xi’an, respectively. In this study, in Site A and Site B, 20 × 20 m tree, shrub, and grass quadrats and 1 × 1 m ground cover plants quadrats were selected for recording (Figure 2). Based on the systematic investigation, this study monitored 64 representative plant individuals of different types in Xi’an urban green space. We surveyed each quadrat from September 2020 to September 2021 and measured the plant growth data of all quadrats once in each season. The time was, respectively: 17–19 September in 2020, 19–21 December in 2020, 15–17 April in 2021, and 6–8 July in 2021. Measurements were taken every 2 h from 8:00 a.m. to 6:00 p.m. on a sunny, windless, or breezy day. We selected 5 leaves of similar size in each plant, which were located in the middle canopy and grew toward the sunny side, used LI-6400 as Portable Photosynthesis System (Licor, Lincoln, Nebraska), and collected 3–6 instantaneous photosynthetic rate data for each leaf every 2 h [28]. Other measurements were taken on each plant of the plot, once during the day of ground measurements: leaf area by using the Portable Leaf area meter YMJ-B (Zhejiang Topu Yunnong Technology Co., Ltd., Hangzhou, China) on 3–6 leaves per plant, leaf area index by using the TOP-1200 Plant Canopy Analyzer (Zhejiang Topu Yunnong Technology Co., Ltd., China), diameter at breast height (DBH) of the tree, and height of the shrub by using a measure tape.

Plant carbon sequestration was calculated by multiplying the instant value of net photosynthesis by the canopy leaf area index and time, assuming that the average daily value of photosynthesis was representative of the whole canopy and along the season [29]. Since this study evaluates the annual carbon sink benefits of plant communities, the calculation of photosynthesis of plants requires the removal of rainy days. According to the analysis of meteorological data in Xi’an, the effective days of photosynthesis of plants in spring, summer, autumn, and winter are 67.8 days, 64 days, 64.4 days, and 78.6 days, respectively [30]. Therefore, the number of days for photosynthesis of evergreen plants and deciduous plants is, respectively, about 274.8 days/year and 196.2 days/year. The annual carbon sequestration was obtained by multiplying the daily net photosynthetic rate data of the investigated plants by the number of days.

The daily net assimilation, daily average photosynthetic rate, and daily carbon sequestration of the tested plants were calculated by the following formulas.

$$\begin{array}{c}P={\displaystyle {\displaystyle \sum }_{i=1}^i}\left[\left(p_{i+1}+p_i\right)/2\times \left(t_{i+1}-t_i\right)\times 3600/1000\right]\end{array}$$

(2)

In Formula (2), P is the daily assimilation amount per unit area (mmol/m²); P_i is the instantaneous photosynthetic rate of the initial measurement point (μmol/m²/s); P_i+1 is the instantaneous photosynthetic rate of the next measurement point (μmol/m²/s); t_i is the time of the initial measurement point(h); t_i+1 is the time of the next measurement point (h).

$$w_{co2}=P\times 44/1000$$

(3)

In Formula (3), 44 is the molar mass of CO₂; w_CO2 is the fixed mass of CO₂ per unit area of the blade (g/m²/d). The formula for calculating the carbon fixation of the whole plant is:

$$W_{co2}=w_{co2}\times LAI\times C$$

(4)

In Formula (4), w_CO2 is the daily carbon fixation per unit area; W_CO2 is the daily carbon fixation of the whole plant (g/d); LAI is the leaf area index; C is the crown area (m²).

3. Results

3.1. Distribution and Variation Characteristics of Carbon Sequestration in Urban Green Space in the Study Area

The research shown in Figure 3 shows that: first, the amount of carbon sequestered by plants showed an overall upward trend from 2000 to 2019, but the carbon sequestration density decreased year by year. The carbon sequestration density of plants reached the maximum value in 2000 and the minimum value in 2019. At the same time, the woodland area in these four years was 50.91 ha, 50.20 ha, 48.73 ha, and 47.36 ha, respectively. Therefore, there was a significant correlation between the amount of carbon sequestered by plants and area of the woodland—the amount of carbon sequestration of plants decreases with the reduction in the area of woodland. Second, according to the research results, soil carbon sinks are closely related to land-use types. The changing trend of soil carbon sequestration is: rapid growth in 2000–2007 and slow growth in 2007–2019. The area of construction land increased rapidly from 2014 to 2019, while the area of woodland and cultivated land and the carbon fixation efficiency of soil decreased significantly. Third, the changing trend of total carbon sequestration is similar to that of soil carbon sequestration. The distribution of total carbon sequestration has changed greatly in the past 20 years. The carbon sequestration density in the main urban area of Xi’an decreased with the increase in construction land, while the carbon sequestration of woodland in the south of the Qinling Mountains did not change significantly, and it was still a good natural carbon pool in Xi’an.

Overall, woodland, shrub, and cultivated land were able to store more CO₂, while water, construction land, and bare land had lower carbon sinks (Figure 1 and Figure 3). Combined with the land-use types map, it can be seen that the carbon sequestration within the urban built environment of Xi’an has changed significantly (the construction land in the south of the city), while the carbon sequestration in the woodland and cropland have no obvious changes (most areas in the north of the city). The changes in aboveground carbon sequestration and underground carbon sequestration are similar; the carbon sinks of organic matter are mostly distributed in woodland and cropland. Carbon sink changes with forest land, so forest land is the most important source of carbon fixation. In addition, it can be found that the carbon sequestration of plants was the highest in 2000 and the lowest in 2019. The overall carbon sequestration showed a decreasing trend, and the change in land-use type had a direct impact on the change in carbon sequestration.

3.2. Heterogeneity of Plants and Habitats in the Study Area

Based on the previous research on urban scale carbon sequestration, four types of green spaces were selected in the study area: dense woods (tree–grass plant community), open woods (tree–shrub–grass plant community), open green space (shrub–grass plant community), and square green space (ground cover plant community). The sites were distributed in the main urban area and the new urban area (Site A is located in the Beilin district, and Site B is located in Xixian New District) to try to study the key factors affecting carbon sequestration.

3.2.1. Type and Size of Plants under Investigation

The species and stems of sixty-four plants belonging to fifty-eight genera in thirty-six families were investigated (Figure 4), including fourteen species of deciduous trees, eight species of evergreen trees, ten species of deciduous shrubs, ten species of evergreen shrubs, and twenty-two species of herbs. The dominant species of deciduous trees in the study area are Styphnolobium japonicum, Koelreuteria paniculata, Ulmus pumila, and Ginkgo biloba; the evergreen dominant species are Ligustrum lucidum, Pinus tabuliformis, and Pinus bungeana; the shrub dominant species are Nandina domestica, Pittosporum tobira, and Buxus megistophylla; the herb dominant species are Ophiopogon japonicus, Oxalis corniculate, and Iris tectorum. The results show that the most common species in the study area are Styphnolobium japonicum ‘Pendula’, Ulmus pumila, Paulownia fortune, and Styphnolobium japonicum, and they are the dominant species. These are common afforestation tree species in Xi’an, including native tree species and introduced tree species.

3.2.2. Analysis of Differences in Carbon Sequestration of Investigated Plants

Table 1. Carbon sequestration of trees and shrubs in the study area.

Plant Species	Annual Carbon Sequestration (kg/km²)	Plant Species	Annual Carbon Sequestration (kg/km²)
Deciduous Plants		Evergreens
Trees
Platanus orientalis	17,800	Photinia serratifolia	12,519
Salix babylonica	14,107	Magnolia grandiflora	8074
Koelreuteria paniculata	12,111	Ligustrum lucidum	6486
Melia azedarach	11,476	Juniperus chinensis ‘Kaizuca’	5534
Ulmus pumila	10,070	Pinus tabuliformis	4808
Celtis sinensis	9616	Osmanthus fragrans	3039
Styphnolobium japonicum	6940	Pinus bungeana	1950
Acer pictum subsp. mono	3078	Eriobotrya japonica	759
Acer buergerianum	2634
Punica granatum	2631
Prunus cerasifera ‘Atropurpurea’	1134
Acer palmatum ‘Atropurpureum’	771
Prunus davidiana	635
Acer palmatum	260
Mean value	6662	Mean value	5396
Shrubs
Lonicera maackii	1673	Rosa (Floribundas Group)	2349
Rosa xanthina	1108	Nandina domestica	1765
Kerria japonica	1058	Photinia × fraseri	1412
Spiraea salicifolia	1029	Pittosporum tobira	1085
Chimonanthus praecox	873	Ligustrum quihoui	1005
Forsythia suspensa	733	Buxus bodinieri	972
Ligustrum sinense	626	Buxus sinica var. parvifolia	639
Jasminum nudiflorum	611	Buxus megistophylla	534
Amorpha fruticosa	118	Juniperus procumbens	30
Weigela florida	62	Pyracantha fortuneana	25
Mean value	789	Mean value	897

and

Table 2. Carbon sequestration of herbaceous plants in the study area.

Plant Species	Annual Carbon Sequestration (kg/km²)	Plant Species	Annual Carbon Sequestration (kg/km²)
Iris pseudacorus	1013	Miscanthus sinensis	63
Physostegia virginiana	432	Ophiopogon japonicus	52
Carex giraldiana Kukenth	262	Coreopsis basalis	38
Zephyranthes candida	256	Lolium perenne	32
Stipa lessingiana	230	Fatsia japonica	27
Iris tectorum	137	Dianthus plumarius	25
Miscanthus sinensis ‘Gracillimus’	123	Ophiopogon bodinieri	23
Pennisetum alopecuroides	112	Pennisetum alopecuroides ‘Little Bunny’	20
Oxalis corymbosa	102	Arundo donax ‘Versicolor’	11
Juncus effusus	91	Cynodon dactylon	9
Hedera nepalensis var. sinensis	86	Poa annua	4
Mean value 143

show the species and annual carbon sequestration (kg/km²) of each measured plant and the average values of different plant categories. It can be seen that different plants in the study area have different carbon sequestration capacities. The carbon sequestration of plants in the study area varies greatly, and the order of carbon sequestration capacity of urban green space plants in the study area is: deciduous trees > evergreen shrubs > deciduous shrubs > herbaceous plants. According to calculations, the annual average carbon sequestration of trees is about 6029 kg/km², that of shrubs is about 843 kg/km², and that of herbaceous plants is 143 kg/km². The carbon sequestration of trees is much larger than that of shrubs and ground cover plants. In addition, there are differences in carbon sequestration between evergreen and deciduous plants due to differences in leaf area index and photosynthetic rate. Among the trees, Platanus orientalis, Acer pictum subsp. mono, Acer buergerianum, and Styphnolobium japonicum have higher carbon sequestration capacity; among the shrubs, Photinia × fraseri, Amorpha fruticosa, Pittosporum tobira, Nandina domestica, and Ligustrum quihoui have higher carbon sequestration capacity; among the ground cover plants, Coreopsis basalis, Miscanthus sinensis ‘Gracillimus’, Oxalis corymbosa, and Iris tectorum have higher carbon sequestration capacity.

3.2.3. Analysis of Differences in Carbon Sequestration of Investigated Plants Communities

According to the field monitoring data, the plants in quadrat A1 and A5 have high carbon sequestration, and the annual carbon sequestration of plant communities in the quadrat is 49,925 kg/km² and 49,759 kg/km². The factors that lead to the large difference in carbon sequestration among plant communities mainly include plant species and plant hierarchical structure. For example, there are many plants with high carbon sequestration capacity in the quadrat A1 (Salix babylonica, Acer pictum subsp. mono, Kerria japonica), and there are multi-layer plant communities with high three-dimensional green amount, so quadrat A1 has the highest amount of annual carbon fixation. In contrast, although there are multi-layer plant communities, quadrat A2 has a low total carbon sink due to the low carbon sequestration capacity of the plants in the quadrat (

Table 3. Carbon sequestration of green space plant communities in the study area.

Research Site	Quadrat	Plant Community (the Number of Plants)	Annual Carbon Sequestration (kg/km²)
Site A	A1	Salix babylonica (4) + Ligustrum lucidum (4) + Acer pictum subsp. Mono (3) + Cupressus funebris + Punica granatum (3) + Prunus persica (1) − Photinia × fraseri (1) + Amorpha fruticosa (3) + Ligustrum quihoui (1) + Spiraea salicifolia (1) + Lonicera maackii + Pyracantha fortuneana + Fatsia japonica + Kerria japonica	49,925
	A2	Salix babylonica (4) + Acer buergerianum (4) − Pittosporum tobira (6) + Nandina domestica (8) + Photinia × fraseri (4) − Oxalis corymbosa + Iris tectorum + Ophiopogon bodinieri	21,195
	A3	Pinus bungeana (7) + Ligustrum lucidum (2) + Ulmus pumila (3) + Styphnolobium japonicum ‘Pendula ’(1) − Chimonanthus praecox (1) + Pyracantha fortuneana (2) + Spiraea salicifolia − Phyllostachys sulphurea var. viridis + Ophiopogon bodinieri	22,593
	A4	Pinus tabuliformis (5) + Ligustrum lucidum (3) − Lonicera maackii (3) + Chimonanthus praecox (1) + Ligustrum quihoui (1) + Photinia × fraseri (2) + Weigela florida (3) + Pittosporum tobira − Ophiopogon bodinieri	17,427
	A5	Pinus bungeana (6) + Celtis sinensis (2) + Punica granatum (1) + Photinia × fraseri (2) + Ligustrum lucidum (3) + Styphnolobium japonicum (1) + Pinus tabuliformis (3) − Spiraea salicifolia (2) + Lonicera maackii + Buxus bodinieri + Rosa xanthina − Fatsia japonica	49,759
	A6	Salix babylonica (4) + Punica granatum (1) + Prunus cerasifera ‘Atropurpurea’ (6) + Styphnolobium japonicum (2) + Ligustrum lucidum (4) − Pyracantha fortuneana (3) + Juniperus chinensis ‘Kaizuca’	36,857
Site B	B1	Styphnolobium japonicum (5) − Ilex chinensis (3) + Nandina domestica (2) + Rosa (Floribundas Group) (8) + Photinia × fraseri (2) − Iris pseudacorus + Carex giraldiana Kukenth + Miscanthus sinensis + Physostegia virginiana + Iris tectorum + Pennisetum alopecuroides + Poa annua	1665
	B2	Ilex chinensis (5) + Nandina domestica (3) + Pittosporum tobira (6) + Photinia × fraseri (6) − Coreopsis basalis + Miscanthus sinensis ‘Gracillimus’ + Stipa lessingiana + Lolium perenne	6308

).

3.3. Influence of Plant Community Characteristic Factors on Carbon Sequestration of Urban Green Spaces

The results showed that: DBH and community density are positively correlated with plant carbon sequestration (

Table 4. Correlation analysis between characteristic factors of plant communities and carbon sequestration.

Characteristic Factors of Plant Communities	Canopy Density	DBH	Hierarchical Structure	Community Density	Carbon Fixation
Canopy density	1
DBH	0.828 *	1
Hierarchical structure	0.483	0.657	1
Community density	0.820 *	0.953 **	0.57	1
Carbon fixation	0.621	0.859 **	0.747 *	0.889 **	1

Note: the symbols * and ** represent significant correlation at the level of 0.05 and 0.01, respectively.

). The analysis of the results is as follows.

• Hierarchical structure of plant community

In the field investigation of the quadrat, it was found that the hierarchical structure of the plant community in the quadrature mainly includes three types: tree–shrub–grass type, shrub–grass type, and herb type. Among them, the tree–shrub–grass structure has the greatest impact on carbon sequestration. For example, the plant community in the A1 and A5 plots are the tree–shrub–grass type, and the annual carbon sequestration is as high as 40,000 kg/km².

2.

Canopy density of the plant community

The higher the canopy density, the greater the carbon sequestration of the plant community (Figure 5a). Comparing the carbon sequestration efficiencies of different types of plant communities shows that, the canopy density of the tree–shrub–grass plant community is recommended to be 40–70%, and the canopy density of the tree–grass plant community is recommended to be above 85%. The average canopy density of the plant community in Site A was 59.2%, the highest canopy closure was 75% of the A1 quadrat, and its carbon sequestration was also the largest. The community canopy density in Site B quadrature was 33.75% on average; the highest was 37% of B1.

3.

Diameter at breast height of plants

The diameter class structure of the study area was quite different (Figure 5b). The larger the diameter of the plant, the larger the carbon sequestration. When the diameters of plants in urban green spaces are in the range of 10–20 cm, the carbon sequestration and efficiency of plants are larger. The average diameter of plants with the largest diameter at the breast height of the A1 quadrat was 30 cm, and the annual carbon sequestration of the quadrat was about 49,925 kg/km².

4.

Community density of plant community

The study shows that the A1 plot has the highest community density of 2500 plants/ha (Figure 5c), and its carbon sequestration is also the largest, while the B2 plot has the smallest community density of 600 plants/ha, and carbon sequestration is minimal.

4. Discussion

4.1. The Relationship between the Distribution of Carbon Sequestration in Urban Green Space and Land-Use Change

The research shows that from 2000 to 2019, the structure and distribution of urban carbon sequestration changed greatly. Previous studies have shown that changes in urban carbon sinks are mainly affected by changes in urban land-use types [31,32,33]. Changes in land-use types have a great impact on the material cycle and energy flow of the urban ecosystem, which changes the structure, process, and function of the ecosystem, and then significantly affects the carbon allocation of each part of the ecosystem [34,35,36,37]. The impact of land-use type change on the carbon cycle of terrestrial ecosystems depends on the type of ecosystem and the way of land-use changes, so the land-use change may bring both carbon emissions and carbon sequestration [38,39,40]. Consistent with previous research results, changes in urban land-use types are the main reasons for changes in urban carbon sequestration distribution, which are mainly reflected in the following aspects: first, different land-use types have different carbon sequestration capacities; compared with urban green space and cultivated land, woodland has the largest carbon sequestration [41]. Second, changes in land-use types have a great impact on urban carbon sequestration [42]. For example, from 2000 to 2014, the area of cultivated land in Xi’an decreased significantly, the area of construction land increased significantly, and carbon sequestration showed a sharp downward trend. Finally, the structure of land use also affects urban carbon sequestration [43].

From a realistic perspective, changes in urban carbon sequestration are mainly affected by major urban policies and activities. With the continuous acceleration of urbanization in Xi’an, the area of urban construction land has increased rapidly in recent years, while the area of woodland and cultivated land has continued to decrease, resulting in gradual changes in the structure of urban carbon sequestration [44,45]. Forests accounted for nearly 50% of the city’s total land area in 2000, which slowly decreased by 117.3 km² from 2000 to 2007, but from 2007 to 2019, the forest area increased. This may be related to the formulation of relevant policies. In recent years, the Xi’an Municipal Government has attached great importance to the construction of urban green space. The area of urban green space in Xi’an increased from 45.02 km² in 2004 to 121.4 km² in 2010, and the green coverage rate in built-up areas increased from 30.06% in 2004 to 37.50% in 2010.

4.2. Changes in Carbon Sequestration with the Growth of Plant Biomass

Although many studies have shown that plants with the larger DBH and biomass have a higher carbon sequestration capacity, this trend is not linear. With the growth of plants, the carbon sequestration efficiency of large trees will grow slowly, so in the planting design of green space in low-carbon cities, it is not necessary to focus only on the carbon sequestration brought by planting large trees, but the carbon sequestration provided by planting medium-sized plants is equally important in the long term.

In general, the higher the canopy density and hierarchical structure, the higher the carbon sequestration capacity of the plant community [46], but its landscape function needs to be considered in the design of the green space. There is evidence that dense plant communities have high concentrations of pollutant uptake capacity and high allergenicity [47,48]. Therefore, it is recommended to choose a suitable density for plant growth, which not only attracts residents and provides multifunctional space, but also promotes carbon sequestration in the long term.

4.3. Implication for Planting Design for Promoting Carbon Sequestration in Urban Green Space

4.3.1. Suggestions for Low-Carbon Green Space Planting Design

The study summarizes trees with a high carbon sequestration capacity: Platanus orientalis, Salix babylonica, Koelreuteria paniculata, and Melia azedarach, among which Salix babylonica, Koelreuteria paniculate, and Melia azedarach are native plants in Xi’an. Shrubs with high carbon sequestration include Lonicera maackii, Nandina domestica, Rosa (Floribundas Group), and Rosa xanthina. Herbs with high carbon sequestration are Iris pseudacorus, Zephyranthes candida, and Stipa lessingiana.

The urban green space in Xi’an is being vigorously constructed, among which the urban green space of different sizes radiating outward from the ring park is the skeleton of the entire urban ecosystem. In the urban built environment, the area of urban green space cannot be expanded indefinitely, so the planting design for the improvement of carbon sequestration is very important for the sustainable development of the city.

In order to maximize the improvement of urban carbon sequestration, the relationship between low-carbon green space and planting design should be considered before making decisions. This paper reveals the relationship between planting design and urban carbon sequestration and concludes that it can be directly applied to the planting design. In the process of low-carbon planting design, enriching plant species with high carbon sequestration efficiency and low carbon emission should be selected. Research shows that although plants can absorb CO₂ through photosynthesis, the consumption and maintenance of plants cause carbon emissions. Grasslands produce about three times as much carbon emissions as woodlands. Among them, the carbon emission of herb layers mainly comes from irrigation, the carbon emission of shrub layers mainly comes from waste transportation, and the carbon emission of woodland and ground cover comes from fertilization [12]. In terms of plant species, the carbon sequestration of deciduous trees is greater than that of evergreen trees, and the carbon sequestration of deciduous shrubs is greater than that of evergreen shrubs. Therefore, in the planting design, an appropriate proportion of evergreen plants and deciduous plants can be selected to play a good ecological benefit to the surrounding environment.

4.3.2. Extraction of Planting Design Models on Promoting Carbon Sequestration

As a place to serve human beings, urban green space is mainly used for leisure, recreation, ornamental, and fitness activities, and plant design should combine the main functions of urban green space. We cannot just emphasize planting more plants and increasing the area of urban green space [49,50]. Therefore, based on the actual project application, this paper proposes three different plant community configuration design modes to provide a scientific basis for planting design (

Table 5. The design model of high carbon sequestration plant community in urban green space.

Types of Plant Communities	Types of Application Sites	Layout Models	Vertical Layout Models
Tree–shrub–grass plant community	Park green space, residential green space, etc.
Shrub–grass plant community	Square green space, road green space, etc.
Ground cover plant communities	Road green space, roof greening, bioretention facilities, etc.

).

• “Tree-shrub-grass” type plant community

The “tree-shrub-grass” type plant community is mainly used in park green space, residential green space, and other sites, and mainly plays the function of ecological benefit (Table 5). The following points are required in plant design. Tall deciduous broad-leaved trees should be selected for the upper layer of trees because the leaf area is large and the gap between the leaves is large so that the lower layer plants can fully absorb sunlight and rainwater. For the middle layer of large shrubs, plants with higher branches and leaves should be selected, such as Lonicera maackii and Chimonanthus praecox, and small shrubs should be selected with dense branches and leaves, such as Nandina domestica and Spiraea salicifolia. Ground cover plants should be resistant to drought and shade-tolerant plants, such plants mainly include Zephyranthes candida and Iris tectorum.

2.

“Shrub-grass” type plant community

The “shrub-grass” type plant community is mainly used in the square green space and road green space (Table 5). The main functions are ornamental, protection, and ecology. In the process of design, attention should be paid to the collocation and combination of shrubs and ground cover plants. Tree species with a height of more than 3 m and large and sparse leaves should be selected for the large shrub layer, such as Lonicera maackii and Cercis chinensis. The small shrub layer plants should be evergreen shrub hedges with a height of more than 1 m and small and dense leaves, such as Buxus megistophylla and Ligustrum quihoui. Ornamental herbs below 50 cm are generally selected for the ground cover. In addition, shade-tolerant, flood-tolerant, and drought-tolerant plants should be selected, respectively, to shade and moisture conditions, such as Oxalis corymbose and Miscanthus sinensis.

3.

Ground cover plant communities

Referring to Thomas Rainer and Claudia West’s community layered design model for the landscape of ground cover plants—”structural layer + seasonal theme layer + ground cover layer” [51]—the structural layer is composed of tall herbs with vertical structure, and the seasonal theme layer contains beautiful plants that bloom in fixed seasons. The ground cover layer is mainly some low-growing herbs, which play a role in soil and fertilizer conservation. Ground cover plant communities are mainly used in small green spaces, such as roof gardens, road green spaces, and bioretention facilities (Table 5). Among them, the structural layer plants should choose species with tall plants and sparse leaves, such as Penstemon digitalis and Chrysanthemum indicum. The seasonal theme layer plants should choose plants with a height of 30–60 cm, large crown width, and dense leaves, such as Coreopsis basalis and Saponaria officinalis. The ground cover plants should choose plants with low plants, which can cover the soil to avoid exposed topsoil, such as Festuca glauca and Sedum sarmentosum.

5. Limitations of the Study and Future Research Prospects

In this study, the carbon sequestration of common garden plants in Xi’an was monitored and evaluated. The monitoring time lasted only one year (from September 2020 to September 2021), so it is necessary to study the relationship between plants and carbon dioxide from a longer time dimension. While most of the carbon sequestration in urban green spaces is due to trees rather than shrubs and grasses, combining various plant species in urban green spaces can multiply carbon sequestration [52]. At present, the research on the design of low-carbon green space needs a lot of research on the design mode of high carbon sequestration plants, which can further support the application of practical projects. Due to the rapid development of the city, the planning of green space is fragmented, and the research on the carbon sequestration of green space still needs to study the spatial characteristics of green space. In addition to considering the carbon sequestration benefits of urban green space, we still recommend that stakeholders focus on balancing the basic functions of urban green space, such as the relationship between human behavior and needs and biodiversity, to become the focus of human sustainable development decision making, and to accelerate the realization of strategic goals for carbon neutrality and carbon peaking.

6. Conclusions

This study focused on the relationship between the carbon sequestration indicators of urban green spaces and planting design, and screened out plant community types with high carbon sequestration efficiencies. The results showed that the carbon sequestration of urban green space showed a decreasing trend year by year with the change in woodland, and this trend was mainly related to the characteristic factors of plant communities in urban green space: the carbon sequestration of plants was significantly positively correlated with DBH and community density; positively correlated with hierarchical structure. According to the research recommendations, the canopy density of the tree–shrub–grass plant community should be 40–70%, the canopy density of the tree–grass plant community should be above 85%, and the DBH of trees is preferably 10–20 cm. In addition, we put forward a list of plants with high carbon sequestration such as Styphnolobium japonicum, Salix babylonica, Pittosporum tobira, Spiraea salicifolia, and Iris pseudacorus, and summarized three planting design models suitable for different green spaces and habitats. The planting models proposed in this paper can provide theoretical guidance for the practice of planting design in low-carbon cities. It is suggested that in future studies, plant community indicators (such as DBH, community density, canopy cover, etc.) should be used as indicators for planting design in low-carbon cities, and appropriate plant community structure models should be established to provide a scientific basis for green space planning and design in low-carbon cities and explore sustainable design approaches for the ecological process of urbanization.