The Ordinary Nature of Urban Trees by Yang Ou by yazid

The Ordinary nature of how urban treesâ&#x20AC;&#x2122; shade functions in Melbourne

s3556790 Yang Ou 1

CONTENTS

1. INTRODUCTION

2. PROBLEM OF HEAT ISLAND

3. URBAN SHADES STRUCTURE

4. SPECIES CHANGES

5. UTILITY MEASURES

6. SELECTION OF TREE SPECIES

7. LONG-TERM IMPACT ON URBAN LIFE

8. CONCLUSION

9. REFERENCES

Figure 1. Lineage Map

1. INTRODUCTION In recent years, with the change of urban climate and the aggravation of environmental pollution, the urban heat island effect, as a global urban climate problem, makes the botanists and urban planners in Melbourne gradually realize that urban plants may play a role in the city. According to the research (Frank et al., 2006), compared with foreign tree species, native tree species have better environment and climate adaptability in urban greening. But large-scale urban construction has led to the demise of a large number of native plants, which are now grown at just 16% in Melbourne. So, in Melbourne, where the soil is poor and the climate is dry and water is scarce, it is important to allocate and select street trees to achieve sustainability in the urban environment and in residents’ activities. It contributes to the formation of the ultimate heat island effect in the urban forest response. In the center of Melbourne, the Yarra River, also known as Melbourne's Mother River, runs through the heart of Melbourne. So, Swanston Street and St Kilda Road on the south bank of Yarra River are connected to form the central axis of Melbourne city. Since Swanston Street has a long history and a good commercial and cultural interface among many north-south streets, this study selected Swanston Street as the main research object, hoping to explain how urban trees provide shade areas to influence outdoor activities of citizens in Melbourne. From the point of view of street greening, at the same time, a number of streets will be selected as a comparison to perfect the demonstration. The strategy in this research is through observing from big scale to small scale areas in the city of Melbourne (Figure 2), try to find issues and existing conditions of tree canopy coverage among some specific streets, and try to think useful methods of increasing tree canopy in the next 10 years. The method that I use to find the existing tree canopy in Melbourne city through using ArcMap to create hotspot maps basing on the data of building outline, café, street trees, and tree canopy coverage which provided by “Melbourne Data Website”. The next step is to analyze the objected street which was already found in hotspot maps. In this process, the Google Earth and 3D cloud compare are used to create the cloud points of the site and 3D model of the site. In terms of sections, the Revit is used to create sections of the specific area, which could help to consider about the relationship between different elements in the place. Throughout the whole process of research, the real information is the fundamental method to observe the site from big scale to small scale and analyzing existing data.

Figure 2. Claming Ground. The map from big scale zoom into the specific area.

2. PROBLEM OF HEAT ISLAND

Figure 3. Site map of Swanston Street

The 20th century has witnessed the process of rapid urbanization of the global population, and people are embarking on a road of development that pursues higher levels of life, high production, high consumption and high abandonment. It has caused great damage to the ecological environment on which humans depend. Global environmental problems and regional environmental problems are mainly reflected in two aspects, one is the global climate problems across the ocean, such as global warming. As figure 10 shows, the 20th century begins to face this problem. On the other hand, this kind of problem in the regional environment is concentrated in the urban heat island, also including acid rain, soil, air pollution (Camilloni & Barros, 1997). The heat island effect caused by the acceleration of urbanization has caused great harm to the urban environment, the residents' life and the urban ecological structure: it has caused the accumulation of pollutants over the city; The heat emitted from the urban surface forms a near-surface warm air mass, and the smoke and dust in the city are shrouded in the nearground heating cluster and cannot be spread in time, thus harmful to the human body (Liang, 2007). In addition, the related pollutants will form other environmental problems such as acid rain. And accelerate global warming, causing a vicious circle of environmental issues. Therefore, from a larger scope and strategic point of view, the mitigation of the urban heat island effect can alleviate a series of urban environmental problems. It is said that the 21st century is the century of the city (Gaffin et al, 2015), which means people in the city will live and work in the city. The problems faced by the space of recreation and transportation are restored to the activities in the city, and the livable urban environment should become the minimum treatment of the urban people, so alleviating the urban heat island effect is in fact closely related to people's lives. This relevance is not only reflected in the strategic global human living environment, but also an improvement in the daily experience of people on a small scale, such as street activities (Pitman et al., 2015). These help residents participate in urban public activities and reduces their exposure to the sun and boredom in the city.

Figure 4. Photo of tree canopies in Swanston Street

3. URBAN SHADES STRUCTURE

From the general plan, the center of Melbourne, capital of Victoria, Australia, has a rectangular chessboard-shaped distribution of nine vertical and nine horizontal roads. There is an organized traffic isolation zone in the center of many wider roads, on which the trees are usually planted at equal intervals, and the street trees on both sides of the sidewalk form a tree-lined boulevard (Figure 5), which is spectacular and has a good shading effect. In the view of the whole district, in order to ensure the visual landscape along Yarra River, and limited by the structure, there are not too many trees on the north bank of the river. On the contrary, there is a higher density of street trees in the northern city of Melbourne. Pedestrians feel comfortable walking on the street in winter and summer. Under the condition of limited ground space in the city, it is a relatively effective and reasonable form of greening.

Figure 5. Distribution of street trees in city of Melbourne

Figure 6. Useful lifetime expectancy of trees in Melbourne city

However, due to age and long-term drought, combined with extreme heat and water limits, the lifespan of many trees in Melbourne is coming to end (Figure 6). The useful lifetime expectancy of tree planting in central city is shorter than that in the city as a whole, and will face major changes or tree replanting in the next 10 years. The scope is mainly limited to the area among La Trobe Street, Exhibition Street, Spenser Street and the Yarra River.

In addition, according to the hotspot map of tree canopies and surface temperature in Melbourne city (Figure 7, 8 & 9), roads and streets are still regions where the urban heat is concentrated. Several north-south streets, including Swanston Street, do not have very high heat levels in all roads, and there does not exist nodes where the heat is significantly prominent, while east-west streets are more likely to show up in this aspect. On the contrary, the relative temperature is lower in the areas with higher shade intensity, such as the layout of parks and green spaces, that is, the influence of the heat island effect is weaker. This central Melbourne thermal image was taken at late night. It shows how the uncovered surface of Melbourne stores the heat generated by solar radiation and lead to higher temperatures in urban areas by storing it for several hours. Due to the research street is in the central city, it tends to retain the traditional urban texture and the continuation of the historical context in the region, so it is impossible to additionally create large green space to alleviate the heat island effect through the large demolition and construction. Therefore, road greening is more important in the process of reducing heat and creating urban landscape. In addition, respect for history does not mean isolation, but rather leading to people's activities to bring vitality to the region.

Figure 7. Canopy Cover and biodiversity Outcomes. From City of Melbourne, 2013.Central City Urban Precinct Plan 2013 â&#x20AC;&#x201C; 2023

Figure 8. Hotspot map of tree canopies in Melbourne CBD

Figure 9. Hotspot map of surface temperature degree. From City of Melbourne, 2013. Central City Urban Precinct Plan 2013 â&#x20AC;&#x201C; 2023

4. SPECIES CHANGES

City streets in the central Melbourne are most horizontal and vertical, medium-distance with wide green space, both sides of the road planted with trees, from which you can experience a traditional atmosphere. Melbourneâ&#x20AC;&#x2122;s urban history is not long. With the arrival of European immigrants in the mid-19th century, Melbourne and its surrounding native plants were basically removed to provide land and space for the development and construction of new cities. At the same time, Europeans have brought their familiar tree species to local cultivation to create a similar living environment to their hometowns. As figure 10 shows, plane trees appeared at that time. Although the natural habits of these exotic plants differ greatly from Melbourne's environmental characteristics, species such as Leucaena and Eucalyptus have the natural properties of rapid growth and extreme cold and humidity in winter and extreme drought and heat in summer (City of Melbourne, 2011). It has gradually become the dominant tree species in the area, and it has been planted as a border tree on the Swanston Street of Yarra River basin for a long time. In addition, silver birch, American white wax, New Zealand pine and other typical Melbourne landscape tree species, are widely planted in Victoria nature reserves, schools, churches, cemeteries and gardens (City of Melbourne, 2011). Meanwhile, Melbourne's native plants suffered a great deal of man-made damage in the mid-19th century, as a result of the removal of a large number of native trees and shrubs from Melbourne's metropolitan areas for the development of Australia's emerging agriculture.

Figure 10. Temporal evolution of Melbourneâ&#x20AC;&#x2122;s urban forest. From City of Melbourne, 2013. Central City Urban Precinct Plan 2013 2023

The concept of tree species diversity conservation has not been formed yet. In the late 19th century, with the rapid development of urban scale, the winter was able to provide sunlight, and the deciduous trees such as elms, which provided good shade in summer, gradually became popular in Melbourne and were widely used in roads and urban public spaces such as parks. By the 20th century, elm, poplars, oak, sunflower, chestnut, Canadian jujube, banyan, acacia, and bottle-dried trees had become the main tree species for greening boulevards and parks of Melbourne city forest (City of Melbourne, 2011). Rich native and exotic plants play an important role in Melbourne's construction of a high-quality livable urban environment. In recent years, with the change of climate and the aggravation of environmental pollution, botanists in Melbourne have gradually realized that native tree species have better environmental and climatic adaptability than foreign species. But large-scale urban construction has led to the demise of a large number of native plants, which are now grown at just 16% in Melbourne (Lindenmayer & Gibbons, 2012). In the center of Melbourne, the tree population now is dominated by three species: elms, planes and River Red Gums. This dominance is part of what creates the respected and unique character of Melbourneâ&#x20AC;&#x2122;s urban forest. As figure 11 shows, on Swanston Street, plane trees are the majority of street trees. This situation resembles in Collins and Elizabeth Street as a typical part in streets with wide footpaths. The most widely used species are London Plane, relatively suitable at present. Actually, Plane trees make up 75% of the trees within the central city. (City of Melbourne, 2013)

Figure 11. Distribution of tree species in Melbourne CBD. From City of Melbourne, 2013. Central City Urban Precinct Plan 2013 â&#x20AC;&#x201C; 2023.

5. UTILITY MEASURES

6. SELECTION OF TREE SPECIES

It is known that the ecological functions of urban plants are remarkable, but they still need to be measured to determine the intuitiveness of their utility. One way of quantification is to translate its role into value. It is selected a formula based on factors including tree condition, species type and growth rate, aesthetics value and locality values (Zhou, 2017). Through a tool called i-Tree Eco, we can measure the ecological value of the corresponding trees to the environment (TERASHIMA, 2004). From 982 trees of street such as Swanston, it was calculated that the funds needed to purify air pollutants, store carbon, convert carbon and other functions to solve the corresponding problems were much larger than the funds needed for planting.

It can be seen from Figure 12 that coffee shops in the central city are share a higher density because of the layout of economic function, so the activity skills of attracting local residents and tourists can promote the economic growth of the city and show the vitality of the street at the same time. Combined with the characteristics of the population, it is considered that the guarantee of feelings of privacy is stronger among young people, and the elderly are more likely to be able to see children's activities for psychological pleasure. Combined with the spatial characteristics of Swanston Street, more street furniture set outside needs to be added near restaurants. Small public rest areas may be set up at intervals to provide pedestriansâ&#x20AC;&#x2122; short break. In addition, places where there is a lot of traffic may set up street corner square to organize activities to further attract the flow of people. They will all serve as an opportunity to plant trees together. In Day (an architect)'s view (Dobbs et al., 2013), good urban planning is a respect for and emphasis on history and local traditions. He argues that the basic principles of the Melbourne Development Model are subtle but not complex, and that trees are planted on open streets that not only shade the streets, but also add character to the streets. It is the same on Swanston Street. In an introverted city (unlike Sydney, Hong Kong, San Francisco, etc.), Swanston Street with tram, bikes and pedestrians should be people oriented. On this street, flowers and trees, shops and city landmarks on the side of the street can be seen sometimes. Shop windows, coffee shops, hidden bars, and more detailed graffiti are likely to make interaction with pedestrians. For example, the shade outside coffee shops could provide customers a leisure outdoor environmental experience (Figure 12 & 13). There are several ways to use trees outside the space of coffee shops. First, it is to separate the public area from the shop space, providing a more private space for drinking coffee and chatting; The second is to enclose a small courtyard-like space for consumers and pedestrians to use as a small public space on the side of the street; Finally, it is fully open and decorated with small shrubs. The outer space of the coffee shop is integrated with the street as a fully open space, and at this point, the shade is not the main function.

Figure 12. Building outline and Distribution of cafĂŠ in city of Melbourne

Figure 13. Hotspot map of relationship between tree canopies and cafe in Melbourne CBD

In the middle of 19th, as a typical example, Swanston Street had an opportunity to widen its footpaths and led to a new role of plantings (City of Melbourne, 2013). Tram as the city's character, Swanston Street become a typical class of streets, connecting to the attractions of Melbourne's central area, is a point of commercial vitality. Part of the street was closed in the 1990s as a pedestrian street, while the entire street met a 5.4m-wide sidewalk. It is a scale that helps to satisfy the development of leisure and recreational activities for citizens. According to the planning requirements (City of Melbourne, 2013), large deciduous tree species should be planted on the main tram streets, including Collins, Bourke, Swanston, Elizabeth Streets and so on. Deciduous tree is a kind of border tree used in the whole line of Swanston Street at present, and it does not mean to change or replace in the planning documents. And on Elizabeth Street, there is a chance of some changes in its southern part, which may grow large deciduous trees in the future. Collins Street, on the other hand, is almost entirely replaced by large deciduous tree. First of all, the growth properties of the tree species are considered in the selection of urban street trees. Deciduous trees grow fast and can be achieved in 1-3 years. On the contrary, evergreen trees take a very slow setting time, small evergreen trees generally do not have a good viewing effect in 3-5 years (Salmond et al., 2016), and large tree maintenance costs are high, and the risk of transplanting is very high. In addition, the cost of deciduous trees is generally lower than that of evergreen trees. Ecologically speaking, deciduous trees have better shading ability in summer. Therefore, in order to create the effect of sun in cold and shade in hot, deciduous trees play a major role in urban walkway trees. But this can cause a lifeless problem in the streets of the cold season. Therefore, it is also necessary to plant evergreen trees or shrubs between deciduous trees, which helps to maintain the vitality of the urban landscape. The British sycamore has a longer life span, so if it is not necessary not to remove the Swanston Street trees, consider adding evergreen and other ornamental plants to corner squares and pocket parks. According to Williams`s research (2002), 192 residents in Melbourne were surveyed and they had exactly indicated about preference for mediumsized trees with a globular or oval form. Leaf texture is also an important factor, and people prefer trees with relatively wide or thick leaves. However, there is no general preference for natural or introduced tree. It gives people a more dense and shady feeling. People are more likely to take a cool and rest when they are planted on the street.

Figure 14. Short-term Impact

7. LONG-TERM IMPACT ON URBAN LIFE

Urban shade should first be able to imply the structure of the city. To some extent, it is a response to the functions of the city. Swanston Street, as the main axis of the city, links the northern CBD of Melbourne and is to the south towards the Yarra River. The whole street has a clear control of the height of the building, while organizing trams, bicycles, pedestrians and other forms of traffic on the street. As the main north-south street, it should be an important axis of the urban landscape, which can provide tourists and residents with sufficient urban characteristics. Then, it may have the function to ensure the safety of urban life. Elizabeth Street is a north-south road in the same area as Swanston Street. They are in the valley thus height control of the building is in accord with the traffic organization. However, due to the geographical characteristics of the city, Elizabeth Street is in the flood area. Then, we should select the tree species with strong resistance to waterlogging in the selection of trees. There is also a suggested open space along the street, so it is also necessary to take this factor into consideration.

Figure 15. Long-term Impact

From the figure 16, it can also be realized that on Collins Street, Flinders Street and so on, it is planned to add some new areas for open space. When defining these regions, it is to ensure the livability of the city and the scale of human beings according to the planning. According to forecasts, Melbourne's relatively poor soil and dry climate are short of water, and climate change will reduce Melbourne's freshwater resources by about 20 percent by 2050. Compared with native plants, exotic species require higher conservation costs (such as water supply, fertilization, conservation and pest control).

Figure 16. Natural and Open space Context. From City of Melbourne, 2013.Central City Urban Precinct Plan 2013 - 2023

The shade of the existing streets can be evaluated from the aspects of urban functional requirements, ecological functions and landscape effects, and so on. According to the requirements of replanting trees in different streets in figure 17, it can be seen that Swanston Street does not actually require tree changes, and in some parts of Elizabeth Street, trees need to be replaced in the next 10 years. Collins street, on the other hand, needs a whole replacement over different periods of time within 10 years. Then when the tree species die, it can be planted according to the original pattern. Otherwise, it is a good choice to use other tree species to plant and increase the diversity of trees. Corylus colurna (trial), Turkish hazel, Fraxinus Americana, White ash are some good choices. As London Planes a species with longer life, Swanston Street will remain shaded after many years. Elizabeth Street will have a lot of changes in some parts. However, Collins Street will change greatly in the next decade, and it is more likely to plant new tree species, which is beneficial to the formation of a richer street landscape. (figure 18). Trees take different responsibility in different street. Taking sections of street as comparison, Swanston Street has a relatively suitable interface for human scale, and the skyline is smoother. People's walking feeling thus can be enriched with the help of greening. Collins Street and Elizabeth Street, on the other hand, need the penetration of certain nodes to help pedestrians recognize the landmark.

Figure 17. 10-Year Planting Plan. From City of Melbourne, 2013. Central City Urban Precinct Plan 2013 - 2023

Figure 18. Comparison of different changes of street trees in the next decade.

Figure 19. the expected increase of tree canopy coverage in Swanston Street

Figure 21. the expected increase of tree canopy coverage in Collins Street

Figure 20. the expected increase of tree canopy coverage in Elizabeth Street

8. CONCLUSION

Figure 22. Managing Asset

This essay starts with the analysis of the urban heat island effect, and selects the important role of the Melbourne city, solving the urban problem, urban trees, in order to take the important urban axis. According to the research (Pitman, 2015), the function of urban street trees is first embodied in ecological aspect. They can intercept, reflect and absorb solar radiation and heat gas through evaporation of trees, thus regulating the temperature. The air circulation brings cool air from the suburbs into the city. In addition, the trees on the streets also have aesthetic and social values, which help to organize traffic, induce sight, and bring conditions for leisure, entertainment and cultural education in social life. At the same time, the Swanston Street, which has a long history and important structural function, is taken as the object of this study. At the same time, the change of street and their trees, the selection of tree species, the emphasis of urban shade and so on are analyzed by comparing the similar streets like Elizabeth Street and Collins Street. Finally, the city shade function has got a certain summary. It is considered that urban green shade plays a structural role of urban style and features, and also helps to ensure urban safety and increase landscape effect. More importantly, it also helps people carry out activities in the streets. As the main tram street in the city, it is reasonable to adopt large deciduous trees in terms of driving safety and the overall characteristics of the city. In addition, according to the planning document of treeâ&#x20AC;&#x2122;s replacement and adjustment, more importantly, for urban plant diversity, the planting of other trees in this area ensures seasonal ornamental effect, especially on important streets.

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