Category Archives: 2024

Cooling down under: Urban greening for climate resilience

Introduction

Australia is no stranger to the challenges posed by rising temperatures and heatwaves. As urbanisation continues to reshape the Australian landscape and the impacts of climate change are beginning to take effect, it becomes crucial for local councils to prioritise innovative and sustainable urban greening solutions. This blog post explores the significance of urban greening as a strategic approach to mitigate urban heat and enhance overall climate resilience. Practical insights and examples are provided to help Australian councils adapt to heatwaves and the changing climate.

Understanding urban heat impacts

Climate change affects the air temperature in two ways: it increases average temperatures and makes heat waves more frequent and longer lasting. These changes pose significant challenges for communities, ecosystems, and economies.

The high quantities of heat absorbing materials found in urban areas, such as asphalt and concrete, causes higher temperature build-up in urban areas compared with rural areas. This phenomenon is called the urban heat island (UHI) effect. Asphalt and concrete radiate heat even after the sun sets causing nighttime temperatures to remain elevated.

Prolonged exposure to surface radiation can intensify the health impacts of high air temperatures, as it disrupts the body’s ability to cool down.  Surface radiation, particularly the radiation emitted from impervious surfaces like roads and pavements, has emerged as a critical factor influencing heatwave-related health risks. While air temperatures are traditionally used to gauge the severity of heatwaves, recent studies indicate that mean surface temperatures can be an even more influential determinant of heat-related mortality.

Recognising the significance of both air and surface temperatures in heatwave-related health outcomes is vital for designing effective urban greening strategies that specifically target these effects, ultimately mitigating the risks associated with extreme heat events while delivering other benefits.

The role of urban greening

Urban greening involves strategically incorporating green spaces, vegetation, and sustainable infrastructure into urban planning. This multifaceted approach not only addresses urban heat effects but also offers numerous social, environmental, and economic benefits.

The integration of trees and green spaces within urban planning plays a pivotal role in regulating temperatures and radiation. By casting expansive shade, these natural elements effectively reduce surface temperatures and create a cooler microclimate in densely populated urban areas. In addition, the evapotranspiration taking place in plants cools the environment by dissipating heat through water vapour release.

Figure 1. A thermal image at Orbit Drive, Whittington, Victoria taken on 24 Feb 2015 at 4.20pm. The unshaded roadway is 15 degrees Celsius hotter than asphalt shaded by a tree. The numbers on this photo indicate degrees Celsius. (Image source: City of Greater Geelong, Urban Forest Strategy 2015-2025)

In addition to substantially reducing the radiant temperature of urban surfaces, the air temperature itself can be greatly impacted. A recent study evaluated the extent to which street trees can reduce sub-canopy air temperature relative to ambient conditions and how cooling potential relates to tree traits and microclimatic variables. Air temperature under the canopies of 10 species was recorded within residential areas in Western Sydney, Australia, during the hot, dry summer of 2019–2020. Species differed significantly in their air cooling values, with peak cooling at around −3.9 °C.

Urban greening not only enhances climate change adaptation by mitigating urban heat and storm impacts but also promotes biodiversity, improves air quality, and fosters community well-being.

Moreover, with storms predicted to increase with climate change, green infrastructure, including trees and vegetation, can play a key role in stormwater management. By absorbing and managing stormwater runoff, green spaces mitigate the risks of flooding and soil erosion in urban areas.

Practical strategies for Australian councils

Start with good governance

Adopting principles of good governance is essential for the success of urban greening initiatives, regardless of organisational capacity. Building consensus on objectives, taking a future-oriented view, aligning initiatives with broader council visions and frameworks, and emphasising evidence-based decisions can support long-term plans to fruition. Collaboration with internal and external stakeholders is vital, while transparency, accountability, and effective communication channels are key. For councils excelling in urban greening planning, a clear vision, integrated operational support, and empowered champions are all common elements. A structured management process and adequate resource allocation, encompassing people, time, and finances, solidify the authority to act and ensure sustainable urban forest growth and maintenance.

Establish a baseline and identify opportunities

To understand the local context, challenges, and opportunities for urban greening, councils should conduct a comprehensive assessment of their urban environment. This involves mapping and evaluating the current state of the urban landscape and if necessary, engaging experts in sustainability, urban forestry, and spatial mapping.

Utilising existing data from various sources and digital tools, such as drone sensing, GIS and LiDAR data, can greatly assist in creating useful maps. An example of baseline establishment is a project by the City of Parramatta to identify hotspots within the city. During a 3-day heat event, aerial thermal imaging was conducted across the local government area, producing visual maps of heat distribution during both day (Figure 3) and night. This mapping helped Council identify areas with limited green infrastructure and low tree canopy coverage.

Figure 2. Thermal heat imaging of Parramatta CBD during the day, blue and green temperatures range from 10 to 34°C, while orange, red and purple range from 36 to 53°C (Image source: Minimising the impacts of extreme heat: A guide for local government)

Another example of baseline establishment is tree canopy mapping. A mapping exercise completed by Wagga Wagga City Council revealed that canopy cover across the local government area varied between 2% to 16% across the urban area. This finding helped Council set a goal of increasing canopy cover to 20% by 2030. Overlaying canopy cover data with the Australian Bureau of Statistics’ Socio-Economic Indexes for Areas (SEIFA), provided Council with insights into communities that are most vulnerable to heat extremes.

Similarly, 100% Renewables recently measured the tree canopy coverage in a regional NSW town using i-Tree Canopy, an online canopy estimation tool. The result was a tree canopy coverage of 1 to 27% across the urban area.

When assessing baseline conditions, there are additional spatial factors to consider, including socioeconomic variation. According to the Cooperative Research Centre for Water Sensitive Cities, some neighbourhoods are much more prone to heat-related health risks than others. These are the areas where high heat exposure, high population vulnerability (such as elderly people or lower socioeconomic areas), and high outdoor activity coincide. The Trees for a Cool City guidelines suggests that planting trees can help cool down these areas effectively.

Once councils have established a baseline through on-ground assessments, tree canopy estimates, and hot spot mapping, the next crucial step involves leveraging this information to identify strategic opportunities for urban greening. Firstly, councils should conduct a comprehensive analysis of the identified areas with lower tree canopy coverage and higher heat concentrations. This analysis should consider factors such as population density, socio-economic conditions, and ecological sensitivity. By integrating these considerations, councils can prioritise areas where urban greening initiatives would yield the greatest benefits. Councils can also engage with local communities, seeking input and collaboration to tailor greening strategies to meet specific needs and preferences.

Set targets

A key policy driver that can significantly boost urban greening initiatives is the establishment of targets, such as tree canopy percentage targets. This provides councils a clear and measurable goal to increase the amount of tree cover in urban areas and helps foster accountability and proactivity in reducing the urban heat island effect.

In Australia, there are several instances of urban greening target-setting, including:

  • City of Adelaide, South Australia: The City of Adelaide is making urban greening a central part of its climate change strategy, recently committing $5.7 million to a public realm greening program. This initiative aims to reduce temperatures of road and pavement surfaces by over 9 degrees, achieve a net increase of 485 street trees each year, and reach a 40% canopy cover by 2035, up from the current 33%. The city will also install microclimate sensors to accurately assess the heat island effect. A consultation document is currently out for public comment.
  • City of Melbourne, Victoria: The City of Melbourne has set an ambitious target to increase urban green spaces and tree canopy cover to 40% by 2040. This commitment aims to create a more sustainable and resilient urban environment, enhancing the liveability of the city.
  • NSW Government’s Greening Our City program: Focused on Greater Sydney, this program aspires to boost urban tree canopy and green cover. The goal is to plant 5 million trees by 2030, aligning with the Greater Sydney Region Plan’s overarching target of achieving a 40% canopy cover for Greater Sydney.
  • City of Brisbane, Queensland: Brisbane’s Urban Forest Strategy outlines a target to increase the city’s tree canopy cover to 40% by 2031. This initiative underscores the city’s dedication to fostering a resilient and green urban landscape.
  • Town of Bassendean, Western Australia: Council developed a Tree Canopy Action Plan 2023-2027, which sets a target of 20% tree canopy cover by 2027.
  • Mackay Regional Council, Queensland: Council’s draft Urban Greening Strategy aims to achieve an average of 40% tree canopy cover over pathways by 2045.
  • Wagga Wagga City Council, New South Wales: Council has set an ambitious goal to enhance the urban environment by increasing the tree canopy cover to 20% by 2030.

Select the right species

Careful selection of tree species ensures that the plants provide maximum benefits and remain resilient to climate change, heatwaves, and other ecological threats. Trees are ‘long-term assets’ with a potential lifespan of over 100 years. However, some species may struggle to endure higher temperatures expected in the coming decades. A review of hundreds of eucalyptus species published in Nature estimated that an increase of two degrees would “see 40 per cent of species stranded in climate conditions to which they are not adapted”. Similarly, research also shown 35% of tree species are at high risk in the business-as-usual emissions scenario by 2070.

Research generally suggests that trees with smaller leaves, higher wood density, and higher water use efficiency are more likely to cope with high temperatures and drought. Conversely, trees with larger leaves, lower wood density, and lower water use efficiency are more likely to suffer from heat stress and water loss. However, a recent study published in Global Change Biology, reveals a paradoxical response in some native plants like banksia and callistemon to high heat and water scarcity, compromising their survival in forecasted hotter and drier conditions. For example, instead of storing water, the banksia and callistemon squandered it – opening the stomata (the pores on their leaves used to cool the plant and take in carbon). Conversely, plants you might not expect to thrive in extreme conditions, species like the Orange Jasmine Murraya paniculate, did thrive.

Figure 3. Orange Jasmine or Murraya Paniculata (Image source: victorsgardeningservices.com.au/gallery)

The ACT Government, a leading proponent of best practices in urban greening, undertook research to determine which tree species are suitable (will survive and thrive) in Canberra’s climate change future (2030, 2070 and 2090). The process they followed included:

  • Reviewing the current list of tree species used on public land;
  • Ranking species by suitability and capacity as street trees;
  • Identifying and recommending species for different scenarios and future climate; and
  • Considering other factors in tree species selection such as allergens, weeds, longevity, irrigation, and asset protection

The City of Whittlesea’s Street Tree Management Plan follows the vision of planting ‘the right tree, in the right place, in the right way, and at the right time’ where tree species are selected for their suitability to the site, biological diversity and minimal management effort required. Based on these factors, Council annually updates their Street Tree Species List to include a variety of small, medium and large trees from native, exotic, evergreen and deciduous species that are suitable for planting within the city.

Urban greenery faces unique challenges during heatwaves, as highlighted by a study on Platanus acerifolia (London Plane) trees in Melbourne. While these trees are popular for shade, research indicates that during >43 °C heatwaves, they undergo substantial canopy leaf loss, affecting essential ecosystem services. Despite this, areas under the tree canopy maintain cooler conditions than open areas, emphasising the importance of having at least some cover.

Interestingly, the ACT Government has recently discouraged planting London Plane trees, not out of concern for heatwaves, but rather for health reasons. The trees produce large amounts of pollen and fine hairs that can trigger allergic reactions and asthma attacks in some people. Selected species should also exhibit resistance to infestations by bugs and other ecological threats, which may change over time with changes in climate. Insects and pests can pose significant challenges to the health and vitality of urban greenery, potentially compromising the functionality and aesthetics of planted areas.

Flood prone councils should take care to select flood resilient species. In south-eastern Australia, a prolonged drought (1998-2010) stressed trees, followed by La Niña events causing floods. Some trees recovered, but others faced a second dose of stress, succumbing to fungal diseases and pest attacks. This emphasises the importance for councils to consider tree species’ resilience to both drought and excess water, understanding how waterlogged conditions, by displacing soil oxygen, can lead to hypoxia and anoxia, adversely affecting root health and overall tree survival.

In selecting tree species, projection tools can be used to simulate possible future climate conditions. This can provide valuable insights into the most appropriate trees for a specific locale.

Don’t forget about ground cover

It’s not just all about the canopy; ground level cover is also effective in providing cooling and aesthetic benefits. Ground cover can sometimes be more effective than canopies at stopping the sun from hitting hard surfaces as it can be quite extensive and provides cover at all times of the day and year with the sun at various angles.

Figure 4. Ground level planter boxes with ferns and shade-loving plants used as ground cover (Image source: Snap from caportal.com.au/act/city-walk)
Figure 5. Ground level plantings with jasmine and other plants used as ground cover in the City of Sydney (Image source: Screen cap from Cooling Our Streets – Gardening Australia (abc.net.au)

Reclaim space

At first glance, it might seem like there is limited area for planting new trees, and this can be true in some cases. But even in towns with narrow sidewalks and paved areas, there are often great opportunities for expanding greenery. For example, installing road islands that separate traffic lanes can offer enough room to plant trees, providing shade across the whole street width. Another option is to reclaim green space from parking areas, by converting a percentage into areas for planting trees.

Figure 6. Trees on an island in the suburb of Braddon Canberra
Figure 6. Trees on an island in the suburb of Braddon Canberra
Figure 7. Carpark space turned into a plant box, Dickson Canberra
Figure 7. Carpark space turned into a plant box, Dickson Canberra

Integrate stormwater management

Research has shown that street trees with well-integrated stormwater management can help reduce the risk of floods. Tree canopies act as a natural barrier by intercepting and absorbing rainfall. This reduces stormwater volume and lowers flash flood risk. The concentrated root networks of trees in upper soil layers enhance soil permeability, while shallow roots prevent soil compaction and promote effective water infiltration to reduce surface runoff.

Research has aimed to quantify the impacts of tree-based stormwater control and runoff harvesting measures on tree growth. Vaughn Grey et al (2018), for example, found that effective designs doubled the growth of standard street tree plantings, although tree performance was reduced if waterlogging occurred.

Figure 8. Street trees can reduce stormwater runoff volumes by interception, infiltration, slowing and capturing runoff (Image source: citygreen.com/stormwater-management/strataflow/)

Urban greening initiatives can therefore achieve their full potential only by integration with civil works programs. This integration allows for more holistic design, effectively capturing and using rainwater runoff from hard surfaces, such as roads and pavements, to water green infrastructure including trees and raingardens. Innovative solutions like underground rainwater storage systems can further enable councils to save excess runoff during rainy periods and use it during dry spells, providing a more dependable and sustainable water supply for urban green spaces such as parks.

Build green roofs and walls

Green roofs are roofs with a layer of vegetation on top that acts as natural insulation and shade, reducing the need for heating and cooling. While green roofs can improve the environmental performance of buildings and towns, they are not always easy or affordable to implement, especially as they often compete with solar PV in terms of “bang for buck” use of available space.

However, green walls can be relatively easy and cheap to establish while providing significant cooling benefits. In the southern hemisphere, especially during late summer afternoons, the low trajectory of the western sun creates substantial heat load on walls and windows, presenting challenges for building design. Standard eaves are less effective during this period, so alternative shading options like green walls, pergolas, or vertical elements such as trellises with climbing vines are a good idea. This approach can also improve and expand local amenity and increase economic activity, for example around cafés and food outlets.

Encourage community involvement

Engaging communities through initiatives like community tree planting, workshops, and community gardens can foster a sense of ownership, pride, and inclusivity in urban greening efforts, contributing to sustainable outcomes. To encourage community participation in urban greening initiatives, Logan Council has been giving away native plants to residents and community groups who want to green their private land. This initiative distributed 12,910 free trees in a single year through various conservation programs.

Capture carbon with trees

Some councils with adequate land reserves close to town could consider incorporating carbon offsetting or insetting through tree planting into their urban greening strategy. For example, using CSIRO’s FullCAM software, 100% Renewables recently analysed the carbon sequestration impact of a NSW council’s urban revegetation projects, indicating that achieving useful amounts of carbon abatement is possible through tree planting initiatives within urban areas.

Conclusion

As the impacts of climate change increase, it is important to recognise the interconnectedness of infrastructure, urban greenery, and public health. Urban greening can play an important role in mitigating rising temperatures and heatwaves in Australia. By better understanding your baseline conditions, identifying opportunities to implement greening projects, setting targets, and planting the right species in the right locations, you will be much better positioned to adapt to whatever the future climate may bring. As they say, ‘the best time to plant a tree was 20 years ago, and the second best time is now.’

If you need any advice regarding the role of urban greening in your climate change strategy, we’re more than happy to assist. Please contact Ian or Theresa for more information.

 

If you need help developing a climate transition strategy and action plan, or a climate risk assessment, contact us today to learn how we can support your transition. Reach out to  Barbara or Patrick for more information.

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