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.
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.