Quantitative Evaluation of Ecosystem Services of Urban Street Trees: A Case Study of Shengjing Historical and Cultural Block in Shenyang, China

Abstract

Urban street trees are of great significance to the sustainable development of human settlements, and are key factors to enhance the service value of the urban environmental ecosystem. In this regard, it is necessary to improve and balance the benefit distribution of urban street trees for promoting the environmental quality of cities. In order to make clear the urban street tree benefits in a city, this paper presents the study on the ecosystem services from street trees in Shengjing Historical and Cultural Block (SHCB), Shenyang, China. By conducting a field survey on 1968 street trees and using the i-Tree model and the ENVI-met model to quantify the original data, this paper evaluated the ecosystem services distribution of eight streets and nine zones in the SHCB. The results showed that the co-creation annual ecological benefit and annual thermal comfort benefit of the street trees of SHCB were 163,965.62 and 233,533.48 dollars, respectively, totaling 397,499.10 dollars. It is obvious that the thermal comfort benefit is higher than the ecological benefit. Meanwhile, urban streets with high ecological benefit may not necessarily produce high thermal comfort benefit. Therefore, on the basis of ecological benefit analysis, the ecosystem services can be reflected more accurately by superimposing the thermal comfort benefit. The quantitative assessment system obtained in this study can provide a reference for future block planning and urban street tree allocation of SHCB and other regions in similar areas.

1. Introduction

Ecosystem service is an environmental service function directly related to human well-being [1,2], and it is also an important asset derived from urban ecological community, which is of great significance for improving the quality of the human settlement environment in a city [3,4,5]. As a type of tree commonly seen on both sides of urban streets, street trees not only have a certain scale of planting area in urban greening but also provide practical contributions to the daily life of urban residents, such as shade, cooling, and streetscape beautification. It is therefore regarded as an important source of ecosystem services [4,6]. In the context of world urbanization, governments around the world have strengthened the dominant position of urban street trees in block greening through material incentives, policy promotion, and other incentive measures. At the same time, it also significantly improved the urban ecosystem services generated by street trees, and finally helped urban residents to obtain good entertainment, travel, office, and living experience [7,8,9,10,11]. Wu et al. (2022), Prigioniero et al. (2022), and Gillerot et al. (2022) identified that urban street trees can not only directly affect the ecological benefits of human settlements by reducing environmental pollution, improving soil erosion and air quality, but also improve the thermal comfort of the environment within a limited range, and finally achieve the purpose of optimizing the urban ecological environment [12,13,14]. However, with the world population increasing, the continuous expansion of human settlement areas has directly led to the decline of urban street trees volume year by year. In this context, adverse factors such as air pollution, the heat island effect, soil erosion, energy consumption, and dust diseases have become significant challenges for urban residents [15,16,17,18,19,20,21]. Therefore, whether it is a new large-scale urban community or a long-established urban settlement, it is necessary to establish a set of ecological service evaluation systems with individual street trees as units of measurement. So as to achieve its own planning and construction with the environmental carrying capacity of the region, and finally, create a win-win situation of ecological environment and urban development.

In the late 20th century, the concept of ecosystem services was first proposed by Costanza et al. (1997) [1]. Urban trees have gradually become the main assessment objects for quantifying ecosystem services in many countries [9,10]. With the continuous innovation of traditional measurement methods, the research on urban trees in various countries has gradually extended from the macro-region to the micro individual trees [22]. In this context, governments and relevant groups put forward different strategies for covering the ecological service of urban trees. For example, European Commission established European Green Capital in 2010, aiming to recognize and reward local contributions to the sustainable development of urban street trees [23]. In 2012, a System of Environmental-Economic Accounting (SEEA) for dynamic monitoring of urban trees ecosystem was launched, led by the United Nations Statistics Programme, the OECD, and the World Bank. It provides a basis for the evaluation of social service value for the accounting of the ecological service function of urban street trees [24]. Chinese local government also issued policy documents on urban street tree construction in 2016, proposing the establishment of an urban dynamic management perception system to enhance the management and perception ability of urban components including street trees [25]. From the perspective of management mode, this will help to improve the assessment of urban street trees’ ecological service by block.

Due to the impact of urbanization, the existing non-quantitative urban tree assessment schemes in various countries have been unable to reflect the direct contribution of individual urban trees to these problems. Therefore, there are still key knowledge gaps in the basic research of urban green space ecosystems, and the basic quantitative data of urban street trees are rarely incorporated into urban design and planning [26,27]. In order to refine the specific contribution of urban street trees in coping with urban adverse environments and better understand the annual values of individual trees, an urban tree evaluation system based on data quantification has become a new way to build an urban ecological environment. At the end of the 20th century, the Council of Tree and Landscape Appraisers (CTLA) proposed and developed the earliest formula for quantifying the value of individual trees (Tree value = basic value × ground diameter area × tree species% × growth of plants% × area), this is also the development predecessor of CTLA quantification method since then [28]. In the 1990s, quantitative models of plant-air-fluid interactions in urban spaces of micro-scale began to become an important part of global circulation models (GCM). Through numerical simulation analysis of fluid dynamics and non-static forces acting in urban street trees, quantitative research based on human comfort is realized [29]. Whether it is the development of CTLA quantification or the birth of a quantitative model of urban microclimate, it directly reflects that the process of urban street tree research will tend to be refined and data-based. With the development of both, the i-Tree model to estimate the ecological benefits of urban trees and the ENVI-met model to quantify the thermal comfort environment of urban trees appearing one after the other.

The i-Tree is an urban trees eco-efficiency assessment software developed by the Forestry Service of the United States Department of Agriculture (USDA) in 2006, which is different from the previous quantitative model of urban tree populations. The i-Tree model can be used as a basis for monetization value analysis by using individual trees as the basis for research [30]. The i-Tree model contains many analysis modules, such as energy-saving benefits, aesthetic benefits, air quality improvement, carbon dioxide absorption, fixation, and storm-water interception. The above-quantified contents were considered as the beneficial impact of the ecosystem on the human living environment, which is formulated based on the law of ecological balance, so it is also called ecological benefit [22]. Based on the field collection of basic data, the i-Tree model can realize the monetized value comparison of ecological benefits in different study areas. From the perspective of usage method and calculation results, the overall quantification process of the i-Tree model is more convenient and accurate than the CITYgreen model based on “3S” technology (GIS: Geographic Information System; RS: Remote Sensing Technology; GPS: Global Positioning System). After years of continuous development, i-Tree has been widely used in the world, especially in Europe and the United States. The studies from Michael et al. (2021), Charity et al. (2019), Nicholas et al. (2012), and Emily et al. (2020) have focused on estimating the ecological value of regional ecosystems [31,32,33,34]. Therefore, this study uses the i-Tree model as the quantification method of ecological benefits of urban street trees.

ENVI-met is a high-resolution CFD model based on fluid mechanics and non-statics developed by Michael Bruse and his team at the University of Mainz in Germany in 1998. It can be used to quantify the interaction between “solid surface-plant-air” in the scale of urban blocks, and realize the characterization of three-dimensional patterns at the block scale and the simulation of the atmospheric environment [29]. By simulating various influencing factors including spatial layout, ground surface material, water body, and vegetation coverage, environmental indicators of various microclimates such as temperature and humidity, wind speed, and wind direction of urban blocks are quantified, and the dynamic coupling analysis of human settlement climate is realized. After several years of mature application and continuous verification, the research on the ENVI-met has been applied to the comfort simulation design of the urban micro-environment [35,36,37,38,39]. In this study, the ENVI-met model was selected to quantify the urban thermal environment influenced by street trees.

Fang et al. (2022) and Suchismita et al. (2021) have summarized the ecological benefits of urban trees and the thermal comfort environment it creates in the ecosystem services of urban trees [40,41], but the results of both assessments are rarely mentioned together. Therefore, using the i-Tree model and ENVI-met model to quantitatively evaluate the current value of street trees in the Shengjing historical and cultural block (SHCB). This study aims to establish an assessment system of ecosystem service for urban street trees. At the same time, through the SHCB case, this study intends to solve the following research questions: (1) How much ecological benefit and thermal comfort benefit are generated by SHCB’s urban street trees? (2) What are the spatial distribution characteristics of urban street tree benefit in SHCB? (3) What factors affecting the distribution of urban street trees benefit SHCB? (4) In order to balance the distribution of urban street tree benefits, what strategies should urban planners adopt in future block planning? Based on the answers to the above questions, this study can realize the monetized value assessment of the ecosystem service of urban street trees, which is extremely important for improving the environmental carrying capacity of future blocks and promoting the sustainable development of urban street trees.

2. Materials and Methods

2.1. Study Site

Shenyang City is located in the south of northeast China, in the middle of the Liaohe Plain, with a total area of about 12,860 km² (Figure 1). Affected by the northern temperate monsoon, the climate of Shenyang varies significantly throughout the year, with an average annual temperature of 6.8–8.0 °C. The summer duration is short and the climate is hot and rainy, the maximum temperature exceeds 30 °C, and the winter is long and the climate is cold and dry, with the lowest temperature below −20 °C. The average annual precipitation in Shenyang is about 650 mm, and the annual frost-free period is about 150 d, which is a typical cold area in China [42].

Shengjing Historical and Cultural Block (SHCB, 41°79′62″ N, 123°45′9″ E) is located in the middle of Shenhe District in Shenyang. SHCB is surrounded by Dongshuncheng Street, Xishuncheng Street, Nanshuncheng Street, and Beishuncheng Street, with a total area of about 1.7 km². The total length of the four streets is about 1300 m. There are four streets inside of the SHCB: Zhongjie Street, Shenyang Street, Zhengyang Street, and Chaoyang Street (Figure 1). There are 16 cultural relics protection units in SHCB, such as Shenyang Imperial Palace, Commander Zhang’s Mansion, and Middle Street Pedestrian Street. Relying on rich cultural tourism resources, it has become the area with the most concentrated historical and cultural relics in Northeast China [43]. Since 2005, the SHCB has been planned and renovated five times, but it has always been difficult to resolve the contradiction between protection and development. With the continuous settlement of large-scale commercial centers and low-end commodity retailers, the current commercial land area of the SHCB has accounted for as high as 41%. It directly leads to a serious shortage of environmental comprehensive supporting facilities, compresses the amount of urban street trees, and further affects the environmental quality of the block [43,44].

2.2. Study Area Division

The SHCB has 8 main streets, arranged in an approximately parallel or perpendicular manner to each other. In order to reduce the difficulty of field measurement and refine the measurement results, a modular measurement method was used to define the intersection of eight streets as the endpoints of street segments. In this way, a “short street” can be formed between every two endpoints (Figure 2). After division, the block has a total number of 24 “short streets”. This study takes short streets as an independent measurement project. Through field measurement of three short streets in the same direction, the data calculated by the software were summarized into the quantitative results of one main street. In addition, another advantage is that the quantitative data of each zone can be compared and analyzed at the same time. It is not difficult to find that the 9 zones of a-i are surrounded by four short streets in the east, west, south, and north, which include almost all the greenery inside the block. Therefore, this study analyzes the street tree quantification results of 8 main streets and 9 zones simultaneously through the modular measurement method.

2.3. Field Measurement

Field measurement is a common collection of basic data in the quantification of urban street tree value. The measurement date was selected as a typical weather day in summer when street trees grow densely, sunny, cloudless, and free of extreme weather. The field survey completed the measurement of 8 main streets in the SHCB on 6 July 2022. By identifying the species, and measuring diameter at breast height (DBH), height, crown width, and health status of 1968 street trees, the basic data collection work in the early stage of the i-Tree model is realized.

On the basis of this, record the type of land used around the street, building height, street width, wall materials, underlay surface paving, and the preliminary investigation of model establishment in ENVI-met are completed. In order to ensure that the error between the simulated environment and the actual environment of ENVI-met is within a controllable range, meteorological parameters including temperature, humidity, wind speed, and wind direction should be collected [45]. The experimental point is located at the end and midpoint of the “short street”, and the measurement period is 8:00–18:00, a total of 10 h. It is determined by the number of pedestrians in the surrounding area and the opening hours of the scenic spots in the neighborhood. Field survey collected hourly measurements on 8 July 2022 using the Kestrel 5500 hand-held weather station. On this basis, the error between the measured value and the model simulation value can be compared to verify whether the simulation result is in an allowable range. The test index RMSE is the root mean square error, the smaller the RMSE value, the higher the measurement accuracy. The test index MAPE is the mean absolute percentage error, and it is generally considered that the value is less than 10% as a trusted value [46,47]:

$$\mathrm{Root} \mathrm{mean} \mathrm{square} \mathrm{error} \mathrm{RMSE} =\sqrt{\frac{1}{n}{{\displaystyle \sum }}_{i=1}^n{\left({y^{\prime }}_i-y_i\right)}^2}$$

(1)

$$\mathrm{Mean} \mathrm{absolute} \mathrm{percentage} \mathrm{error} \mathrm{MAPE} =\frac{1}{n}{{\displaystyle \sum }}_{i=2}^n\frac{|y{\prime }_i-y_i|}{y{\prime }_i}\times 100\%$$

(2)

where n is the measured times, y’_i is the analog value, and y_i is the measured value.

2.4. Sample Model Establishment

In this study, 1968 street trees, 24 short streets, and block buildings in the SHCB were investigated, and the i-Tree model and ENVI-met model were established based on the field measurement results. Due to CY-3, ZJ-1, and ZJ-2 short streets lacking a planting environment for street trees, the results of this study were derived from other 21 short streets. Based on this, the quantitative results of ecological value and thermal comfort value generated by the influence of street trees were calculated. By transforming the ecosystem services of urban street trees into economic benefits (Figure 3), this study can provide an evaluation system with the same quantitative unit for ecological value and thermal comfort value in an urban environment [48].

This study uses i-Tree Streets 5.1.7 to create a new i-Tree Street work project. According to the annual average temperature, annual precipitation, regional environmental conditions, and other factors, the Northeast climate region in the United States was selected, which is similar to the local climatic conditions of Shenyang in the model [49]. At the same time, in order to ensure the accuracy of the economic parameters of the software, data correction should be carried out. Specifically, it includes: (1) Electricity price: 0.62 RMB/kw·h (Shenyang); (2) Gas price: 4.74 RMB/m³ (Shenyang); (3) According to the carbon emission standard tax rate in Swedish, the CO₂ absorption value of street trees is set at 1.20 RMB/kg (Shenyang). In addition, other corresponding values, such as air quality improvement, are based on the default values set by the software [50,51,52]. The measurement data of street trees is transferred to the MDB format file through the Access database and imported into the i-Tree to complete the establishment of the i-Tree model. Because the quantitative output of the i-Tree model itself is presented in the form of monetization, there is no need for a separate “ecological value-monetization” transformation.

The thermal comfort model was completed by ENVI-met 5.0.2. In this study, the 21 short streets were taken as individual projects to establish a separate analysis environment. As the same as the prophase data calibration of the i-Tree model, the study site of the newly built ENVI-met project was changed to Shenyang, and the time region was changed simultaneously. Due to the difference in length and the distance between different streets, the area of the study varies from 180 m × 400 m to 180 m × 600 m, and the grid cell resolution is 5 m × 5 m × 5 m. Furthermore, in order to eliminate the influence of the top boundary effect on the accuracy of the model, it needs to ensure that the height of the Z axis in the modeling area is greater than 2 times the highest building [53]. Referring to the measurements of individual street trees in the i-Tree, modify the plant model parameters in the Albero plate to satisfy the description of the current situation. Finally, put in the simulation date, measurement period, temperature, humidity, and other information in the ENVI-guide section to complete the establishment of the ENVI-met model. It should be noted that, since there are 21 “short streets” with street trees in the block, a total of 21 project files simulating the distribution of the thermal environment are output from SHCB. By quantifying the street temperature, humidity, and wind speed corresponding to each project file with separate calculation results, and classifying each type of output as no street tree planting scenes, current street tree scenes, and difference scenes comparing the two, a total of 189 quantitative distribution maps of thermal comfort were output.

2.5. Monetization Evaluation of Thermal Comfort

Taking the environmental temperature variable of the short street model as an example, in order to quantify the environmental temperature change by the influence of street trees, the following operations should be carried out on each of the 21 short streets. Since the grid accuracy of ENVI-met was set to 5 m × 5 m × 5 m in the previous model construction process, the quantitative information in the street temperature distribution in the quantitative image is actually composed of 5 m × 5 m square pixel space. The Z-axis height of the pixel is designed based on the average height of an adult male of 1.72 m [54]. At the same time, comparing the temperature difference legend in the quantitative image, the temperature changes caused by planting street trees in a space of 5 m × 5 m × 1.72 m can be clearly seen. By using Adobe Photoshop software to calculate the proportion of square pixels, the overall spatial temperature difference of short streets is quantified. Based on the collation of the above results, the calculated results are introduced into the formula of unit space heat, which can realize the unit conversion of temperature difference of short street environment:

$$Q=C\times \rho \times V\times \left(T_2-T_1\right)$$

(3)

where C is the average temperature-specific heat capacity of the day, ρ is the air density at the average temperature of the day, V is the volume of air, T₂ is the temperature in the street tree scenes, and T₁ is the temperature in the non-street tree scenes. Similarly, by using the calculation formula of humidification capacity and the fresh air volume in the quantitative distribution of humidity and wind speed of short street, the quantification of thermal comfort is achieved:

$$RH(\%)=\frac{d_1}{d_2}\times 100\%$$

(4)

where RH (%) is the relative humidity in the air, d₁ is the actual density of water vapor contained in the pixel block space, d₂ is the saturated water vapor density at the same temperature, combined with the actual street space, the actual humidification of the street trees are calculated.

$$L=F\times V\times 3600$$

(5)

where L is the fresh air volume within one hour, V is the wind speed simulated in the pixel block, and F is the air inlet area. If the height of an adult male is taken as an example, the air intake area per pixel block space is 5 m × 1.72 m. Combined with the actual street space, the change in air volume affected by the street trees is calculated.

In order to unify the units of cooling energy saving, humidification, and fresh air volume, this study refers to the specific parameters of industrial electrical appliances (Figure 4) and calculates the actual power consumption required to create the same comfortable environment in the same volume space according to their power levels. It inducts a local commercial electricity price same as the i-Tree model, and finally realizes the monetization results of the three types of thermal comfort environmental benefits of temperature, humidity, and air volume, so as to complete the unit unification of this quantitative study. It should be noted that as of the completion of the quantitative statistics of thermal comfort in this study, the exchange rate of RMB against the US dollar is 0.1389, and its fluctuations will cause small errors in the results of the study.

3. Results

3.1. Quantification of Ecological Benefits

Referring to the original data recorded in the field survey before, i-Tree Street sorted out the relevant data about the morphological characteristics of street trees for this study. Taking the proportion of DBH among different tree species as an example, the DBH of street trees represented by Plum, Ash, and Chinese Juniper is mostly 7.6–15.2 cm, while that of Chinese elm, White Poplar, and other tree species is mostly 15.2–30.5 cm. These results will help researchers to improve the control of the overall growth state of street trees in SHCB. On the other hand, these data will directly support the ecological benefits created by street trees (Figure 5).

Based on the parametric simulation of 1968 street trees in the SHCB, the i-Tree model quantifies and outputs many analysis results from different research perspectives. In terms of tree species, the results showed that White poplar, Chinese elm, and Plum were the main street tree species in the whole block, accounting for 43.45%, 24.34%, and 16.36% of the number of street trees. Among them, the ecological value of the tree is different. The three most valuable tree species in the SHCB are White poplar (128.26 dollars per tree), Chinese elm (94.38 dollars per tree), and Pagoda tree (67.45 dollars per tree), and all of them are deciduous broad-leaved trees (

Table 1. Average annual ecological benefit value of street trees (dollars per tree).

Species	Energy	CO2	Air Quality	Storm Water	Aesthetic/Other	Total
Chinese elm	5.30	16.91	0.16	4.66	67.36	94.38
White poplar	11.09	29.08	0.30	6.94	80.84	128.26
Plum	2.03	6.05	0.06	1.17	9.75	19.07
Pagoda tree	4.35	12.00	0.06	4.29	46.74	67.45
Ginkgo	3.61	9.54	0.06	2.26	22.56	38.02
Ash	3.56	8.92	0.11	3.21	37.54	53.34
Pine	4.10	10.18	−0.66	5.07	24.77	43.47
Spruce	4.10	10.18	−0.66	5.07	24.77	43.47
Chinese Juniper	1.07	3.88	0.04	0.71	12.45	18.15

).

In addition, by comparing the value types, it is found that except for Pine and Spruce, the ecological value types created by the other seven tree species are listed in order of value quantity: aesthetic, absorption and fixation of CO₂, energy-saving, retained storm water, and air quality improvement. This is consistent with the previous quantitative assessment of the ecological value of urban street trees in a similar climatic region. The calculated results of 21 short streets by i-Tree are based on an annual cycle, the result of quantification are output as the average value of individual street trees (Figure 6) and the total value of ecological benefits for different streets (Figure 7).

The results show that the street trees can create an ecological value of 83.32 dollars per tree annually in the SHCB. Among them, the aesthetic value is about 55.75 dollars per tree; the value of CO₂ absorption and fixation is about 17.01 dollars per tree; the energy-saving value of the building is about 5.94 dollars per tree; the value of stormwater retention is about 4.47 dollars per tree and the air quality improvement value is about 0.14 dollars per tree (

Table 2. Annual mean and total value of ecological benefit of street in SHCB.

	Energy	CO2	Air Quality	Storm Water	Aesthetic/Other	Total
Street trees average ($)	5.94	17.01	0.14	4.47	55.75	83.32
Block value ($)	11,691.03	33,471.25	274.71	8804.67	109,723.96	163,965.62

). In the 21 short streets surveyed, there are 14 short streets that reached the average annual ecological benefit of street trees in total, namely NS-1, NS-2, NS-3, DS-1, DS-2, DS-3, BS-1, BS-2, BS-3, ZY-1, ZY-3, CY-1, CY-2, and ZJ-3. Including 7 east-west streets and 7 north-south streets, the main street tree species are White poplar, Chinese elm, and Ginkgo. At the same time, the overall ecological benefit value created by the street trees in the block is 163,965.62 dollars per year, which is specifically expressed as an aesthetic value of about 109,723.96 dollars; the value of CO₂ absorption and fixation is about 33,471.25 dollars; the value of building energy saving is about 11,691.03 dollars; The value of retained storm water is about 8804.67 dollars and the value of air quality improvement is about 274.71 dollars (Table 2). In addition, the total annual ecological value of each street is shown in

Table 3. Annual ecological benefit value of each short street in SHCB.

Short street	NS-1	NS-2	NS-3	DS-1	DS-2	DS-3	XS-1
Total($)	14,146.63	22,176.55	21,481.55	12,115.78	10,548.34	2235.36	6795.05
% of Total	8.63	13.53	13.1	7.39	6.43	1.36	4.14
Short street	XS-2	XS-3	BS-1	BS-2	BS-3	SY-1	SY-2
Total($)	1821.24	2170.23	5021.11	14,659.05	13,284.93	1344.32	3981.5
% of Total	1.11	1.32	3.06	8.94	8.1	0.82	2.43
Short street	SY-3	ZY-1	ZY-2	ZY-3	CY-1	CY-2	ZJ-3
Total($)	3507.04	8207.47	8634.28	1002.81	7438.59	2817.51	576.29
% of Total	2.14	5.01	5.27	0.61	4.54	1.72	0.35

.

3.2. Quantification of Thermal Comfort Benefits

In this study, the thermal comfort environment simulated by ENVI-met was verified hour by hour. The results show that the root-mean-square error (RMSE) and mean absolute percentage error (MAPE) between simulated meteorological data and measured values of SHCB block represented by Beishuncheng Street (BS-1, BS-2, BS-3) are between 3–5% (Figure 8), which is far less than the verification standards of similar studies [46,53]. Therefore, this study has confirmed that this ENVI-met model is successful in restoring the real environment of the street.

The environmental quantification results of the ENVI-met model are based on the Leonardo module as the output way, and the simulation time is taken as the variable to show the distribution effect of the thermal environment of the scene. Taking the quantitative distribution of extreme temperature in “Short street BS-3” as an example, the street temperature distribution shown in Figure 9 is the real-time environment at 16:00 on 8 July 2022, and compared with the temperature distribution results at other times, it is confirmed that 16:00 is the most unfavorable moment (the highest temperature) of the day. Compared with the simulated environment without street trees (Figure 9a), the real-time temperature of the current street planting environment (Figure 9b) is 0.2–0.3 °C lower, which reflects the direct contribution of street trees to the thermal environment of short streets. Figure 9c shows the temperature difference between the two in the form of graphic quantification, which provides a reference form for the output of the temperature difference, humidity difference, and air volume difference of 21 short streets in SHCB.

Formulas (3)–(5) realize the transformation of the benefit value of street space comfort changed by street trees from the perspectives of cooling, humidification, and change of fresh air volume respectively (

Table 4. Improvement of thermal comfort of each short street in SHCB.

Short street	NS-1	NS-2	NS-3	DS-1	DS-2	DS-3	XS-1
Energy saved (kw·h)	96.11	135.12	131.69	47.47	25.25	5.59	200.04
Humidity increment (kg)	67.09	99.47	82.68	41.56	22.35	5.67	170.62
Air volume (m³)	2,032,200	4,356,000	1,967,400	13,869,000	8,670,600	3,324,600	44,546,400
Short street	XS-2	XS-3	BS-1	BS-2	BS-3	SY-1	SY-2
Energy saved (kw·h)	2.24	37.27	46.84	158.2	118.14	45.35	99.08
Humidity increment (kg)	20.96	29.62	37.14	107.97	72.96	21.42	54.79
Air volume (m³)	7,479,000	9,498,600	657,000	5,733,000	4,264,200	682,200	3,160,800
Short street	SY-3	ZY-1	ZY-2	ZY-3	CY-1	CY-2	ZJ-3
Energy saved (kw·h)	204.43	36.58	161.81	4.8	22.25	22.35	37.27
Humidity increment (kg)	121.06	39.42	150.61	2.84	17.24	18.49	19.64
Air volume (m³)	3,038,400	16,324,200	3,115,800	2,266,200	10,553,400	4,323,600	2,421,000

). According to the conversion results, SHCB saved a total of 1637.88 kw·h of electricity consumption for thermal comfort improvement in a period of 10 h, it provides 1203.60 kg of water for air humidification and alleviates the fresh air volume of 152,283,600 m³. Liu. (2018) and Gregory et al. (2022) identified that a price need set for each benefit through direct estimation of known benefits and implicit estimation of environmental externalities [22,30]. Therefore, the improvement effect of the thermal comfort of street trees can be simulated by using artificial electrical appliances according to the previous paper, and the specific results are shown in Figure 10.

In calculating the benefits of annual thermal comfort in the block, it is needed to understand that deciduous species cover different levels of canopy area in winter and summer due to their physiological characteristics. Referring to the study on the growth cycle of urban trees in various areas of Shenyang in the Land and Resources of Shenyang City, street trees can provide ecosystem services for 180 days every year [42]. The results show that the annual thermal benefit value of 21 streets in SHCB is 233,533.48 dollars, of which the value of changing temperature quantity is 25,389.18 dollars, the value of changing humidity is 54,879.98 dollars, and the value of changing air flow is 153,264.32 dollars. Among them, the 3 streets that maximize the benefits of creating thermal comfort are XS-1, ZY-1, and DS-1, and the main street tree species are Chinese elm and Ash. In addition, the annual total thermal comfort value of each street is shown in

Table 5. Annual thermal comfort benefit value of each short street in SHCB.

Short street	NS-1	NS-2	NS-3	DS-1	DS-2	DS-3	XS-1
Total($)	5956.04	9988.38	6746.92	19,142.01	12,707.30	5972.62	39,308.51
% of Total	2.55	4.28	2.89	8.20	5.44	2.56	16.83
Short street	XS-2	XS-3	BS-1	BS-2	BS-3	SY-1	SY-2
Total($)	11,631.35	14,679.68	3406.40	12,010.36	9646.27	3089.24	6958.50
% of Total	4.98	6.29	1.46	5.14	4.13	1.32	2.98
Short street	SY-3	ZY-1	ZY-2	ZY-3	CY-1	CY-2	ZJ-3
Total($)	8601.95	21,708.41	8545.53	5947.37	14,345.31	7388.98	5752.35
% of Total	3.68	9.30	3.66	2.55	6.14	3.16	2.46

.

3.3. Ecosystem Services of Street Trees

Based on the data statistics of short streets in SHCB, this study combined the ecological benefits of street trees on the natural environment and the thermal comfort benefits of human settlements into the framework of ecosystem services. By integrating the quantitative processing results of the i-Tree model and the ENVI-met model, the result outputs of the benefits of ecosystem services in the block are realized (

Table 6. Comprehensive benefit value of each short street in SHCB.

Short street	NS-1	NS-2	NS-3	DS-1	DS-2	DS-3	XS-1
Total($)	20,102.67	32,164.93	28,228.46	31,257.80	23,255.64	8207.99	46,103.56
% of Total	5.06	8.09	7.10	7.86	5.85	2.06	11.60
Short street	XS-2	XS-3	BS-1	BS-2	BS-3	SY-1	SY-2
Total($)	13,452.60	16,849.90	8427.51	26,669.41	22,931.20	4433.56	10,940.01
% of Total	3.38	4.24	2.12	6.71	5.77	1.12	2.75
Short street	SY-3	ZY-1	ZY-2	ZY-3	CY-1	CY-2	ZJ-3
Total($)	12,108.99	29,915.87	17,179.80	6950.17	21,783.90	10,206.49	6328.65
% of Total	3.05	7.53	4.32	1.75	5.48	2.57	1.59

).

4. Discussion

The research results show that the urban street trees within the scope of SHCB provide ecosystem services valued at 397,499.11 dollars in a year. It includes 163,965.62 dollars in ecological benefit value and 233,533.48 dollars in thermal comfort value. However, the actual comparison of the distribution of eight streets, nine zones, and two orientations in SHCB shows that there are still many realistic factors that are not conducive to the balanced distribution of ecosystem services.

Firstly, this study compared the ecosystem services of the eight main streets within SHCB over the course of a year. Among them, the four streets with the highest annual benefit value are Nanshuncheng Street (NS-1, NS-2, NS-3) with a value of 80,496.06 dollars; Xishuncheng Street (XS-1, XS-2, XS-3) with a value of 76,406.06 dollars; Dongshuncheng Street (DS-1, DS-2, DS-3) with a value of 62,721.43 dollars; Beishuncheng Street (BS-1, BS-2, BS-3) with a value of 58,028.12 dollars. To a certain extent, this reflects the actual situation that the distribution of urban street trees in the SHCB is greater than the inside of the block. At the same time, from the perspective of spatial orientation, the distribution of current urban street trees shows the distribution characteristics of the southwest side of the block being larger than the northeast side of the block. To a certain extent, this unequal distribution of resources will exacerbate the uncertainty and unequal benefit of pedestrians or residents in zones through urban street trees [55]. Therefore, how to make the 8 main streets create more ecosystem services of urban street trees and tend to achieve a dynamic balance will become a new challenge for block gardeners.

In addition, through the transformation and reorganization of the modular data results, the research data of 21 “short streets” are recombined into the quantitative results of a total of nine small zones a-i in the historical and cultural block of the Shengjing imperial city. The results showed that the 4 zones with the highest benefit value of urban street trees were zone-a (BS-3, ZJ-3, XS-1, ZY-1) with a value of 105,279.28 dollars; Zone-b (BS-2, ZY-1, CY-1) with a value of 78,369.18 dollars; Zone-g (SY-3, NS-3, XS-3, ZY-3) is valued at 64,137.52 dollars and zone-c (BS-1, CY-1, DS-1) is valued at 61,469.21 dollars. Except for zone-b, the other three zones are the top division of the block and are formed by two outside streets and two inner city streets. This is the same as the previous analysis of the 8 main streets in the block, that is, the value created by the streets outside the block is greater than that of the streets inside the block. However, the CY-3 street on the west side of zone-i (SY-1, NS-1, DS-3) has no street trees and the poor growth condition of the street trees is dominated by elm trees on the east side due to pest erosion. As a result, its ecosystem service value is lower than the inner zone of the block. This also once again confirms the unbalanced distribution of ecosystem services of urban street trees in the SHCB from another angle.

Finally, based on the orientation of each street in the SHCB, this study compared the annual ecosystem service value of urban street trees of eight streets, including north-south and east-west directions. In terms of the improvement of ecological benefits, the total annual income created by the four east-west streets is 100,178.97 dollars, much more than the 63,786.66 dollars created by the four north-south streets, which once again confirms the important impact of street trees on the block space in terms of quantity and volume [22]. In terms of the improvement of thermal comfort benefits, Lee et al. (2020) identified that in addition to the influence of the number and volume of street trees, the width/height ratio (w/h) of street canyons also has a profound effect on the thermal environment of blocks [36]. Under this effect, the total annual income of the four north-south streets is 161,377.54 dollars, much higher than the 72,156.41 dollars created by the four east-west streets. Therefore, in the SHCB, the ecological benefit value and thermal comfort benefit value created by the east-west street and the north-south street show an opposite output ratio, which will further lead to the unbalanced allocation of urban street trees in the block.

Policies and documents issued by Shenyang Municipal Committee, CPC have required Shenyang to complete the establishment of an urban dynamic management perception system, including landscaping, with the project cycle from 2016 to 2018 [25]. However, the current research results show that the ecosystem service value of urban street trees presented by SHCB has not fully reached the planning level proposed by the local government. Restricted by the property of commercial land, this means that the management of urban street trees in SHCB is relatively behind that in other blocks of Shenyang City. The unbalanced distribution of street trees has become the primary problem restricting the balanced development of the SHCB urban ecosystem.

To eliminate or alleviate this unbalance, many cities around the world have proposed new strategies for urban street tree management. Among them, New York, Chicago, Los Angeles, and other cities have carried out large-scale tree-planting actions [56,57,58], hoping to achieve a new balance by increasing the number of street trees. However, it should be noted that improving the long-term survival of existing street trees can also improve urban street tree cover and economic efficiency [32]. Studies from Indianapolis and Philadelphia show that the death of a single street tree costs the city 40–50 dollars in lost revenue, while a normally growing street tree only brings about 11.30 dollars in value per year. If the annual survival rate of street trees is higher than the expected 93%, the ecosystem services of urban street trees were expected to provide an increasing value year by year in the next 5–10 years, and on the contrary, it will suffer an annual loss [59]. Combined with costs of artificial maintenance and land use, which is very close to the ecological service value of a single street tree in the SHCB. To test the survival rate advantage brought by higher levels of maintenance, provided daily tasks such as watering and pit care for street trees in the block, and showed that the mortality rate of street trees without daily care was expected to increase three times [60]. By comparing the planting and maintenance costs of street trees under daily irrigated and non-irrigated, it is found that non-irrigated street trees would incur higher planting costs one year later due to mortality and other problems [61].

In addition to the above influencing factors, the scientific collocation form and tree species selection are also the key factors affecting the ecosystem services of the urban street trees to achieve dynamic balance. Take some European countries as an example, extreme weather, diseases, and insect pests are the key causes of damage to urban street trees, and the large-scale use of de-icing salt also affects the normal growth of street trees [62,63]. This adverse environment is very similar to the surrounding environment of the SHCB. Except for the disadvantages caused by snowmelt in winter, the main street trees represented by Elm trees often suffer from the large-scale erosion of pests (Elm green leaf beetle) in summer. As a result, the ecosystem services of streets with Elm trees as the main street trees such as DS-1, DS-2, and DS-3 were seriously damaged, and even the ecosystem services of urban street trees in zone-i were not as good as other zones. In this regard, the overall planning of street trees of the block becomes another way to improve the imbalance of ecosystem services. By planting well-tolerated intermediate trees between street trees of a single species and regulating the straight-line distance between trees and between trees and buildings, the ecosystem services of urban street trees represented by street trees in Reykjavik have been significantly improved [31]. Therefore, in order to improve the uneven distribution of urban ecosystem services in the SHCB from multiple dimensions, the local government can refer to the quantitative data in this study to optimize the distribution of street trees in the block. Specific measures suggested include:

• Increase the number of street trees planted in the streets with low value of ecosystem services, and increase the coverage area of street trees. Increase the planting of street trees in the commercial area represented by Middle Street, properly handle the commercial development and ecological management of the city, and create an environmentally friendly block.
• Optimize the daily maintenance of street gardens and artificially reduce the unnatural mortality of street trees. Properly coordinate the responsibility distribution system and daily maintenance efforts of the government, especially the urban landscape department. From the perspective of urban street tree planning, the health status of street trees should be quantitatively managed from block to street and from street to individual.
• According to physiological characteristics of street trees, tree species suitable for local climate environments are selected to increase the richness of street trees. It is necessary for urban planners to refer to other cities and regions that belong to the same cold climate region, learn management experience conducive to the construction of local landscapes, optimize the reasonable collocation of exotic tree species and native tree species, and create a balanced and harmonious future community.
• Optimize the planting layout of existing street trees to avoid “fault” phenomena similar to CY-3, ZJ-1, ZJ-2, and other streets. Combined with the quantitative results of the ecosystem services of urban street trees and the current landscape distribution status of the block, the gap in the area without street trees should be filled and the environmental differences between streets and zones should be balanced.
• Optimize the planting collocation, and improve the landscape layout of streets by planting trees and trees and trees and shrubs at intervals. From the perspective of enhancing aesthetic value, it breaks through the current configuration status of street single-street tree species, and finally achieves the purpose of enriching the layering of the block landscape.

5. Conclusions

How to manage the allocation of urban street trees in city block has been a long-term research subject for countries all over the world. In this study, the ecosystem services of urban street trees were summarized as the ecological benefit which can improve the natural environment and the thermal comfort benefit which is conducive to human survival. Based on the i-Tree model and the ENVI-met model as quantitative evaluation methods, the paper simulated and analyzed the distribution of urban street trees ecosystem services in SHCB.

The results show that eight streets and nine zones in SHCB created a total ecological benefit value of 163,965.62 dollars and a combined thermal comfort benefit value of 233,533.48 dollars totaling 397,499.10 dollars in one year. However, the benefit is not equally distributed in the spatial sense. According to the quantitative results, the urban street trees benefit of the outer streets in SHCB is more than that of the inner streets. The benefit of urban street trees in the northwest zone is more than that in the southeast. In addition, the streets with high ecological benefit value may not have a high thermal comfort benefit value. For example, the ecological benefit value of the four east-west streets is 100,178.97 dollars and the thermal comfort benefit value is 72,156.41 dollars while the ecological benefit value of the four north-south streets is only 63,786.66 dollars but the high thermal comfort benefit value of 161,377.54 dollars. This contrast of values shows that the width/height ratio (w/h) of canyon street and the street direction is also the key factor affecting the total value. Together with the planning, volume, survival rate, and other factors of street trees, the distribution of ecosystem services of urban street trees in SHCB is unbalanced. Finally, in view of the disadvantages existing in SHCB, this study sorted out and put forward corresponding improvement strategies.

In an urban human settlement environment, street trees are not only the direct source of ecosystem services but also the key factor of harmonious coexistence between man and nature. From the macro perspective of urban development planning, this study utilized the composite evaluation system composed of the i-Tree model and ENVI-met model to complete the first coupling of urban street trees’ ecological benefits and thermal comfort benefits. Through the unified monetization conversion of different quantitative indicators, the study realized the ecosystem services of urban street trees evaluation of urban blocks in China’s cold regions. Because of the universality of the evaluation system of this study, researchers can establish the same research framework for other cities and regions in the future. For urban landscape planning and designers, this research method helps to achieve a dynamic balance of ecosystem services of urban street trees and provides an important reference value for future community construction. For the urban block residents, this method can provide a clear understanding of the pattern, and deepen the importance of street trees for daily life. Therefore, it is foreseeable that optimizing the allocation of urban street trees in human settlements is a shared responsibility from the top-up to policymakers and down to ordinary residents.