6 Climatic modification
Considerable research is being undertaken into the climatic modification benefits of Green Infrastructure in urban areas. Urban trees, other forms of greening, as well as Water Sensitive Urban Design practices have been found to contribute to:
- Urban heat island mitigation
- Reduced energy use and emission
- Air pollution interception and mitigation
Issues of global climate change have further highlighted the need to address a range of environmental issues, including making better use of Green Infrastructure in the public realm.Figure 38 summarizes the climate modification roles of Green Infrastructure.
Figure 38: Summary of climatic benefits of Green Infrastructure. By author.
6.2 Temperature reduction
Urban microclimates are characterized by significantly higher temperatures, higher wind speeds and lower net rainfall inputs than rural or natural landscapes (Miller, 1980). The most significant environmental benefit of Green Infrastructure, and trees in particular, is probably their ameliorating effect on urban climate and microclimate (McPherson and Rowntree, 1993; McPherson, 1994). According to (O’Brien, 1993).
‘Trees improve cities climatically, indeed this is probably the greatest benefit of tree planting in a built up area’. p.14.
A large body of research over the past twenty or so years has shown that trees and vegetation can improve local microclimate and help reduce the ‘urban heat island effect’. The climatic benefits provided by trees and vegetation include:
- Improving human comfort for street users
- Modifying local microclimates
- Reducing the urban heat island effect
- Providing health benefits especially for the aged
- Reducing energy use and carbon emissions
- Assisting in climate change mitigation and adaptation
6.2.2 The Urban Heat Island Effect
The ‘urban heat island effect’ (UHI) refers to the phenomenon where the air and surface temperatures of cities are typically much higher than surrounding rural or forest areas, especially at night (Bornstein, 1968; McPherson, 1994; Rosenfeld et al., 1998). Temperatures in cities have been found to be as much as 120C warmer than surrounding rural areas, on cloudless days (Oke, 1987). In Melbourne researchers have reported an urban heat island effect of a mean of around 2 to 4°C and as high as 7°C depending onthe location, time of the year and day (Morris and Simmonds, 2000; Coutts et al., 2010). (Akbari, 2001) modelled the effects of the urban heat island on elevated summertime temperatures and corresponding increased energy use in ten large US cities, including trends over the last 100 years. The analysis concluded that temperatures in urban areas had increased by about 0.5-3.10C since 1940. The researchers also estimated increases in cooling energy demand to compensate for temperature increases. It was also noted that urban trees and high-albedo (light-reflecting) surfaces can help ameliorate the urban heat island effect.
The urban heat island has also been described as an ‘unintended consequence of urbanization (GHD, 2011b). The urban heat island effect results from the storage and re-radiation of heat by building materials and paved surfaces, and from urban heat sources such as the burning of fuel for heating and transportation. The predominance of artificial surfaces is known to substantially modify surface energy budgets (Quattrochi and Ridd, 1994). Lack of vegetation in cities also contributes to the urban heat island effect. Reduced tree cover leads to a reduction in both shading of surfaces and in transpiration cooling by tree canopies, compared with rural areas (Federer, 1976). In cities natural ground surfaces are usually replaced with asphalt and concrete surfaces which create higher surface temperatures and reduce soil evaporation that may normally cool the surface (Miller, 1980). Humidity tends to be low in cities due to increased heat loads and the removal of rainfall as stormwater. Vegetative evapotranspiration increases humidity levels and levels of human physical comfort.
While the general urban heat island effect is well documented, micro-energy exchanges between trees and adjacent built surfaces are less easily categorized (Miller, 1980). At a local level heat is reflected and re-radiated from adjacent surfaces, with daily variations. Metallic structures such as cars heat and cool quickly, while concrete and masonry surfaces and structures continue to re-radiate heat into the evening.
184.108.40.206 Health implications of the urban heat island
Increases in average and peak temperatures due to urban heat island effects have a number of implications for cities including detrimental effects on human comfort in the public realm, increased costs of energy use in buildings (with associated greenhouse gas emissions) and increased health stress and related mortality rates (GHD, 2011b).The relationship between heat and mortality has long been recognised (Haines et al., 2006). Several researchers have attempted to quantify this relationship for the city of Melbourne. The urban heat island is recognized as contributing to health risks in large cities such as Melbourne (Loughnan, 2009). Urban heat island effects can contribute to increased morbidity/mortality rates in ‘heat wave’ events, especially among the aged (Loughnan et al., 2008; Loughnan et al., 2010; Tapper, 2010). In Melbourne on days over 30 degrees C the risk of heat related morbidity and mortality of people over 64 years of age increases significantly. Evidence suggests that buildings with little or no surrounding vegetation are at a higher risk of heat related morbidity (Loughnan et al., 2010). (Chen and Wang, 2012) also observed a triggering mean daily temperature of around 30°C for Melbourne based on analysis of historical mortality data from 1988 to 2009 for people over 75. Increasingly, the focus has been on health prevention rather than alerts, trying to mitigate heat stress in the first place, for example through improvements to urban vegetation coverage and the use of cool roofs. Shade provided by large canopied trees during hot summer days can help reduce localized day time temperatures up to two degrees C. Reports from the USA and Australia indicate that, for the last century, excessive heat exposure has contributed to more deaths than that from natural disasters such as hurricanes, lightning, tornadoes, floods, and earthquakes combined (McKeon, 2011; U.S. EPA, 2011).Victoria’s Chief Health Officer found that the heatwave preceding the Black Saturday bushfires in 2009 contributed to an increase of 374 excess deaths over what would normally be expected for that period for inner Melbourne, more than double the number who died in the fires. This also represented a 62% increase in total all-cause mortality and an eight fold increase in direct heat-related presentations in the emergency departments (DHS, 2009).The situation of urban summer heat accumulation is likely to be further exacerbated with global warming. Climate change projections for Australia suggest an increase in the number of warm nights and heat waves which can pose significant threats to human health (Alexander and Arbalster, 2008).
Figure 39: Graphical representation showing the relationship between increased mean temperatures and the effect on hot weather based on a normal temperature distribution. Source:(South West Climate Change Network, 2012) modified from IPPC 2007.
6.2.3 Reducing the urban heat island with trees and vegetation
One initiative to mitigate extreme summer temperatures in the urban areas is the ‘cool cities’ strategy (Luber, 2008). A ‘cool cities’ strategy aims to reduce the urban heat island effect by (a) promoting tree planting to shade buildings and to cool the ambient environment by evapotranspiration of vegetation and (b) using reflective roof and paving surfaces to reduce heat accumulation due to solar radiation.
Research shows that trees and other vegetation can modify urban microclimates and help reduce the UHI effect through two major natural mechanisms: (a) temperature reduction through shading of urban surfaces from solar radiation, and (b) evapo-transpiration which has a cooling and humidifying effect on the air (McPherson et al., 1988; McPherson, 1994; Akbari et al., 2001; Pokorny, 2001; Georgi and Zafiiridiadis, 2006). In fact it has been suggested that trees can operate as natural ‘air conditioners’ which require only natural energy inputs to operate (Pokorny, 2001).
220.127.116.11 Shading Effects
The surfaces of pavements and buildings can reach very high temperatures unless shaded (Kjelgren and Montague, 1998). By shading ground surfaces, trees can reduce the amount of radiation reaching, being absorbed by, and then being re-radiated from paved surfaces (Roberts et al., 2006). Trees can intercept the majority of the sun’s energy, and while some of it is reflected, most is absorbed and used in photosynthesis. Research shows that tree canopies can reduce the temperatures of the surfaces they shade by as much as 10-250C (Akbari et al., 1997; Livesley, 2010). Shading effects of different tree species vary according to their Leaf Area Index (LAI) a ratio of leaf area per unit of ground surface area. It is also known that shading by trees is more effective than shading by non-natural materials (Georgi and Dimitriou, 2010). Research in the USA showed that increasing the amount of leaf area in urban or suburban areas can have a substantial effect on surface temperatures (Hardin and Jensen, 2007). Recent research in Melbourne supports these findings, with inner city areas and the western suburbs experiencing higher temperatures than the more leafy eastern or southern suburbs (Loughnan et al., 2010). It appears that leafy suburbs can be 2-3 degrees cooler than new tree-less suburbs. A study by (Taha et al., 1996) found that the addition of a large number of trees to the public realm should result in an air temperature reduction of 1-30C in the hottest areas.
18.104.22.168 Evapotranspiration effects
Transpiration is the process by which water is taken up from the soil by tree roots, and lost to the atmosphere through the stomata in its leaves. Most of the water taken up by a tree is used for transpiration, which has the important function of cooling the leaf during evaporation (Kopinga, 1998).
The conversion of water from a liquid to a gas during evaporation requires energy, which removes heat from the leaf, cooling the leaf and the surrounding air. (Evapo-transpiration is a term that refers to the combined effects of transpiration from plant leaves, and evaporation from exposed soil surfaces).
Plants, however, need water for transpiration, highlighting the need to maintain soil moisture in cities to maximize the temperature reduction effects of evapotranspiration (Tapper, 2010). However, in cities rain falls on extensive impermeable surfaces, resulting in increased runoff and reduced soil infiltration and soil water recharge. Urban runoff is also directed away from the areas where soil moisture recharge is most needed, below the tree canopy, via engineered systems of gutters and stormwater drains (Bernatzky, 1983). Drought and prolonged water restrictions can also impact on soil water levels resulting in premature tree deaths and loss of a valuable community resource (Connellan, 2008). Mature trees however should be viewed as a valuable resource which can deliver significant benefits including amelioration of the UHI effect both through shading of urban surfaces and atmospheric cooling through evapotranspiration (McPherson, 1994).
In 2012 the Kestrel Design Group developed a formula for the Minnesota Pollution Control Agency to estimate evapotranspiration benefits from trees to include in their stormwater crediting system. The formula, included in the Minnesota Stormwater Manual is possibly the first in the US to formally quantify this benefit (Minnesota Pollution Control Authority 2014). To maximize the incentive to plant trees for stormwater, the Kestrel Design Group recommended giving credits based on the projected mature canopy size for trees that are planted and maintained correctly and provided with adequate rootable soil volume.
22.214.171.124 Australian urban heat island research
Studies have been conducted in a number of Australian capital citiesusing thermal imaging to evaluate urban heat island effects and potential mitigation strategies.UHI effects in Melbourne City were investigated using aerial and ground thermal imaging, in the development of the Melbourne Urban Forest Strategy (City of Melbourne, 2011; GHD, 2011b; GHD, 2011a). Thermal mapping was related to site conditions such as pervious area, vegetation area or leaf area index, bitumen area, urban canyon dimensions, anthropogenic heat sources, thermal mass and retention of heat and albedo levels. A set of UHI mitigation strategies was then developed which includes:
- Increasing vegetation cover to shade ground surfaces and increase evapotranspiration.Research shows that ‘trees and planting in the urban canyon layer provides, perhaps, the greatest benefits in terms of mitigating the UHI effect’ due to increased evapotranspiration rates, and shading thermally massive ground surfaces from solar radiation (GHD, 2011b).
- Trees should have a Leaf Area Index of 5.3 (the same as a deciduous temperate forest) to achieve greater than 30% canopy cover over ground level (Hardin and Jensen, 2007). Deciduous trees are favoured as they provide summer shade while allowing winter sun penetration.
- Water management systemsneed to be matched to planted vegetation to maximize evapotranspiration effects. Research has shown that, without an adequately matched water supply, the evapotranspiration benefits of vegetation will be minimal. The most desirable means of achieving this is through natural water infiltration and retention, rather than irrigation with potable water.
- Increasing permeable surfaces to encourage water infiltration and subsequent evapotranspiration. Pervious ground areas should be as large as possible but no less than 30% of available site area.
- Use of stormwater harvesting (‘low flow stormwater management’) for the passive irrigation of street trees.
- Encouraging appropriate micro level wind movement.
- Utilising active adiabatic cooling systems like water features and pools.
- Replacing dark asphalt surfaces with surfaces of albedo greater than 0.4, with pavements having albedo values greater than 0.5.
In Sydney in 2010 a pilot research program examined the degree to which different components of the urban environment, and associated landscapes, contribute waste heat to the urban climate and to the UHI effect (City Futures Research Centre, 2010). A study was made of the thermal emissivity of different elements in the designed urban environmentat the Victoria Park medium density housing development, and their transience to the surrounding urban air. Thermal imagery was used to measure radiant emissivity at micro-urban-climatic scale. The findings were used to develop a Thermal Performance Index representing the transience contribution of elements, ranked from hottest to coolest (radiators to coolers).The thermal imagery indicated that: albedo reflects heat and cool coloured elements apparently contribute less heat; and water stores heat as expected. However, unless the thermal energy is transformedby living vegetation, the problem of excess heat in the urban environment still persists. Trees providing shade in a grassed park had a 3-4Ctransient contribution over the diurnal cycle and a 7-8C transience shading light coloured paving. Unshaded grass in a park had a 12C transience, and vegetated swales had a 6-9C transience. Swales located along dark asphalt roads were less effective coolers (9C transience) than along a light coloured/non-porous concrete path at a park edge (6C transience).
In South Australia the Adelaide Urban Heat Island Project, is currently being undertaken by Flinders University and the City of Adelaide, measuring spatial differences in temperatures across the city. Preliminary findings indicate a 3-5 degree difference between the CBD and surrounding Adelaide Park Lands (Vinodkumar et al., 2011; Ewenz, 2012).
In a study for Nursery and Garden Industry Australia (NGIA), Dr. Dong Chen of the CSIRO modelled the potential benefits of vegetation in reducing extreme summer temperatures in the Melbourne CBD under different climate scenarios (NGIA, 2012). The potential benefit of urban vegetation in mitigating extreme summer temperatures in Melbourne for 2009 (referred to as the present day) and for future climate in 2047, 2050 and 2090 were investigated using an urban climate model TAPM UCM developed by CSIRO (Thatcher and Hurley, 2012). Vegetation and building coverage ratios of the generic urban types were based on measurements by Coutts et al (Coutts et al., 2007), while the vegetation and building coverage ratios of Melbourne CBD areas were estimated from Google Earth images. Considering the average summer daily mean (ASDM) temperature, the following predictions were made:
- Suburban areas are predicted to be around 0.5°C cooler than the CBD.
- A relatively leafy suburban area may be around 0.7°C cooler than the CBD.
- A parkland (such as grassland, shrub-land and sparse forest) or rural area may be around 1.5 to 2°C cooler than the CBD.
- Doubling the CBD vegetation coverage may reduce 0.3°C ASDM temperature.
- 50% green roof coverage of the CBD area may result in 0.4°C ASDM temperature reduction.
- ASDM temperature reduction of around 0.7°C may be achievable by doubling the CBD vegetation coverage and having 50% green roof coverage in the CBD area.
(Morris et al., 2001) reported UHI observations of around1.3°C for Melbourne summers between 1972 and 1991. Simulationstudies by (Coutts et al., 2008) also showed that day time UHI is in the range between 1 and 2°C. By reviewing a number of observation studies, (Bowler et al., 2010) concluded that, onaverage, an urban park would be around 1°C cooler than asurrounding non-green site, while 2.3°C cooler was reported whencompared with a town or city further away. Chen concludes that the predicted 0.5 to2°C temperature differences in the ASDM temperatures betweenMelbourne CBD, suburbs, rural areas are reasonable. In summary, the results showed that the cooling benefit of various urban forms and vegetation schemes may be in the range of 0.3°C to 2°C and that although Melbourne is projected to be warmer in 2050 and 2090, the relative benefit of urban vegetation will not change significantly.
In 2013 Chen was again commissioned by the NGIA to investigate the impacts of urban vegetation on heat related mortality (Chen, 2013). This research represents one of the first attempts to develop quantitative estimates of the potential benefit of urban vegetation in reducing heat related mortality. The study involved modelling of vegetation and mortality relationships for the summer of 2009 and projected future climates for the city of Melbourne. Simulations of indoor thermal environment were carried out using the AccuRate software to quantify the potential benefit of urban vegetation in reducing heat related mortality. This was done for the 2009 summer and also for projected 2030 and 2050 future climates in Melbourne. The results show that urban vegetation can potentially reduce excess heat related mortality. Different urban vegetation scenarios were tested, with the forest scheme predicted to achieve 60-100% reduction in excess mortality rate in comparison with the CBD vegetation scheme. From these results it was recommended that urban vegetation be a key component in heat wave mitigation and for preventative health. The model established as part of the study is currently undergoing further testing, verification and development.
Figure 40: The potential impact on excess mortality rate with different urban vegetation schemes relatively to the CBD vegetation scheme. Source:(Chen, 2013).
Brisbane’s urban forest strategic planning, targets, policies and programs are focused on optimising the multiple benefits of the extensive and diverse public and private tree cover whilst balancing the risks, costs and other priorities of a growing city (Brisbane City, 2013a). Targets are therefore relative to recognised values of the urban forest and not just tree canopy cover, or tree planting numbers and include:
- Restoring 40% of Brisbane to natural habitat by 2026
- Reconnecting ecological corridors
- Providing 50% tree shade cover for residential footpaths and off-road bikeways by 2026
In 2012, Brisbane’s natural habitat cover was assessed at 34.9%, greatly assisted by the Two Million Trees project which restored almost 500 hectares of available waterway and ecological corridors, including bushland acquisition program sites. This intensive four year project was completed in 2012. In 2010, there was an estimated 575,000 street trees of over 200 species, mostly of maturing age (72% estimated less than 15 years of age). This provided an average of 35% tree shade cover to footpaths supporting walking and cycling in residential suburbs. Ongoing progress is focusing on partnerships with community and other Council programs and policies.
Brisbane’s shade cover target recognises the importance of shade in a subtropical city and increasingly so with climate change studies predicting an increasing number of extreme hot summer days. A direct relationship between areas of Brisbane with high levels of tree cover, and cooler surface temperatures was confirmed more than ten years ago in a University of Queensland research project. Currentmeasures of tree cover use a combination of high resolution satellite imagery and aircraft acquired LiDar (www.lidar.com.au) to help identify the city’s most shade hungry hot spots. This information is then used to target Council’s tree planting policies.
Some of the suburbs with the most shade hungry footpaths are those already targeted for increased dwelling density because of their proximity to public transport and employment centres. Within these suburbs and many others, Neighbourhood Shadeways projects target the most shade-hungry pathways used to walk or cycle to shops, school, bus and train stations for footpath and bikeway shade tree planting. Neighbourhood Shadeways projects have added 50,000 new shade trees since the program commenced in 2006. Almost half of those trees have been planted with local residents at community planting events on Saturday mornings. These partnerships are encouraging stewardship of the newly planted trees and engaging residents on the many values of street trees and urban greening.
6.2.4 Reducing the Urban Heat Island with Water Sensitive Urban Design
As mentioned earlier, maximizing the temperature reduction effects of vegetation with shading, and especially evapo-transpiration, requires the maintenance of adequate soil moisture levels (Tapper, 2010). It has been demonstrated that Water Sensitive Urban Design practices can also be used to maintain soil moisture levels and help in reducing the UHI effect. According to (Wong, 2011):
‘Green Infrastructure supported by stormwater can provide microclimate benefits by reducing excess urban heating (through shading and cooling by evapotranspiration). Use of harvested stormwater and vegetated stormwater management systems has the potential to limit exposure to extreme heat’.
Mitigation of the UHI effect is one of the research focuses of the Water Sensitive Cities project at Monash University (Wong, 2011). Researchers consider that UHI mitigating responses should place particular emphasis on the implementation of WSUD technologies and Green Infrastructure including (in order of priority):
- Trees, which have a particularly important place in the urban landscape, providing shade and supporting evapotranspiration (cooling).
- Irrigated landscapes using harvested stormwater to encourage cooling through evapotranspiration, and support healthy and productive vegetation. Irrigation of trees provides greater cooling efficiency than irrigation of grassed areas.
- Vegetated stormwater treatment measures that retain water in the urban landscape and return moisture to urban soils. Trees should be incorporated into these systems to provide additional benefits from shading.
- Green roofs and green walls, with green walls providing more outdoor cooling benefit at street level than green roofs. The outdoor street level benefit of green roofs reduces with building height.
- Parks and water bodies.
Importantly, thedifferent types of Green Infrastructureto be implemented in the urban landscape should be linked to stormwater harvesting practices, to use the large volumes of unutilized stormwater runoff. They also recommend that microclimatic improvement should be included under WSUD and stormwater harvesting policy objectives. A good example of this is the City of Melbourne which has recently developed an Urban Forest Strategy aimed at ‘building a resilient, healthy urban forest that can thrive in the future’ (City of Melbourne, 2011).Council anticipates that with ‘increases in urban temperatures and density we can expect that Melbourne’s Urban Heat Island (UHI) effect will intensify’.A key principle of the Strategy, therefore, is to reduce the urban heat island effect.
‘Established research and ongoing studies by the City of Melbourne confirm that the addition of trees and vegetation in the built environment provides the greatest benefit in terms of mitigating the Urban Heat Island effect. Through the natural process of transpiration trees help reduce day and night-time temperatures in cities, especially during summer. Trees provide shade for streets and footpaths and their leaves reflect and absorb sunlight, minimising the heat absorbed by the built environment during the day’(City of Melbourne, 2011) p.7.
Council also recognizes that maintaining healthy vegetation that delivers both shading and evapotranspiration benefits, requires maintaining adequate soil moisture levels.
‘Mitigating the urban heat island effect may mean increased water usage during periods of low rainfall ‘to maintain the health of urban forests’(City of Melbourne, 2011) p. 27.
'The higher the level of moisture in the soil, the more trees are able to transpire at maximum efficiency, allowing for cooling of the urban environment and combating the urban heat island effect’(City of Melbourne, 2011) p.40.
Council also has a related strategy to ‘improve soil moisture and water quality’, with a target that ‘soil moisture levels will be maintained at levels to provide healthy growth of vegetation’.
6.3 Wind speed modification
Vegetation modifies wind patterns by obstructing, guiding, deflecting or filtering (Miller 2007). Trees decrease wind speed by either deflecting it or allowing a portion of it to pass through them. Studies on the effect of trees on wind speed in residential areas found that trees in a dense arrangement may reduce the mean wind speed by 90% compared with bare land (Heisler and DeWalle 1988, Heisler 1990). McGinn (1983) found that trees in residential areas to reduce wind speed by 65 and 70% in winter and summer respectively. A more recent study in Bahrain found that wind speed under trees was always below wind speed in bare land, and the mean reduction in wind speed under trees compared to bare land ranged from 60 to 90% at all sites (Tahir and Tawhida 2013).
Tree planting to reduce wind speeds has long been practised around the world, especially the planting of semi-porous windbreaks in rural settings. A barrier of approximately 35 percent transparent material can create a long calm zone that can extend up to 30 times the windbreak height (Caborn 1965). In cities tall buildings create pathways of high wind velocity (wind tunnels) and vegetated buffers can help disrupt these straight pathways. Reduced wind speeds can improve human comfort by reducing wind chill factors and improving human mobility, including walking or cycling in places subject to ‘wind tunnel’ effects (Trowbridge and Mundrak 1988). Reduced wind speeds lead to microclimate modification within the protected zone by reducing air temperature and evaporation, and increasing humidity levels. Air flow modification by trees depends on the area, surface roughness and type of vegetation (Wilmers 1991).
Evergreen tree species are generally preferred as windbreaks as deciduous species are only about 60 percent as effective in winter compared with summer when they are in leaf (Heisler 1991). Movement of air along trees depends on tree spacing, crown spread and vertical distribution of leaf area within height. Therefore, extensive tree cover in urban areas can trap air below their crowns (Grant 1991). The more compact is the foliage on the tree or a group of trees, the greater is the influence of these trees on wind speed. Therefore, many city designers use trees for wind reduction in many urban situations. However, for sites to be used during both warm and cold weather, trees should not form an extremely tight enclosure as a thicket that would essentially reduce wind speeds to zero (Herrington, Bertolin et al. 1972) .
Studies on the effect of trees on wind speed in residential areas found that trees in a dense arrangement may reduce the mean wind speed by 90% compared with bare land (Heisler and DeWalle 1988, Heisler 1990). McGinn (1983) found that trees in residential areas to reduce wind speed by 65 and 70% in winter and summer respectively. A more recent study in Bahrain found that wind speed under trees was always below wind speed in bare land, and the mean reduction in wind speed under trees compared to bare land ranged from 60 to 90% at all sites.
Heisler (1990) describes wind reductions due to buildings and trees in residential neighbourhoods. Modification of wind speed and direction can also affect cooling and heating costs in buildings. Decreasing wind speeds can reduce heating costs in winter, but reduce cooling effects in summer (Akbari and Taha 1992). One study investigated the combined effects of increased shade and reduced wind speeds on residential air conditioning costs, giving an annual savings of 2-23% (Heisler 1989).
6.4 Other climatic benefits
6.4.1 Energy use reduction
Air conditioning is a major consumer of electricity in ‘urban heat islands’ (with associated increases in carbon emissions by power stations) (Rosenfeld et al., 1998). Research has shown that appropriately planted trees can play a role in reducing energy consumption especially in ‘urban heat islands’. Trees can help reduce energy consumption, by reducing air temperatures, and by the direct shading of buildings (Heisler, 1986; Simpson and McPherson, 1996; Akbari et al., 2001; Coutts et al., 2007; Donovan and Butry, 2009; Laband and Sophocleus, 2009). (Wilraith, 2002) found that by applying the effects of tree shade on the eastern and western sides of a Brisbane single storey three star energy rating home to the Building Energy Rating System model,energy savings of up to 50% per year could be achieved.
Trees can also redirect winds or reduce wind speeds, reducing winter heat loss (Heisler, 1986). However such energy savings can only occur with appropriately located trees, and inappropriate placement may actually increase energy use (Dwyer et al., 1992). Researchers have quantified these energy saving costs of the urban forest in a number of US cities (McPherson et al., 1988; McPherson and Rowntree, 1993; McPherson and Simpson, 2003).
Recent research at Ryerson University in Toronto assessed the energy conservation merits of a residential Toronto tree planting program (Sawka et al., 2013). Researchers adapted the Sacramento Municipal Utility District's (SMUD) Tree Benefits Estimator for application in Toronto, Canada and used it to model the air conditioning energy conservation savings delivered by 577 trees planted in Toronto backyards between 1997 and 2000. The researchers found that the study trees contributed 77,140kWh (167kWh/tree) of electricity savings as of 2009, 54.4% of which was due to shading of neighbouring houses. The researchers also found an average tree conserves 435-483 kWh of electricity over 25 years post planting. The research findings also indicate that densely settled urban neighbourhoods should prioritize tree survival over shading potential, as the energy conservation benefits of a mature tree often outweigh the benefits of a strategically planted one.
6.4.2 Pedestrian and human comfort
(Heisler, 1974) reviewed literature on the role of trees in regulating urban microclimate. Shading was found to only minimally reduce air temperature, but provided significant solar radiation shielding, the component of the sun which humans actually feel. Depending on crown density, various trees allow only 2-40% solar penetration. Shade-tolerant species (such as maples) have denser canopies and provide more shade than shade-intolerant species (such as honeylocusts). He concluded that streets are an ideal place for trees due to evaporative cooling and shading benefits, as well as shielding of people from long-wave radiation from nearby buildings. These findings reinforce the importance of providing shade in streets for two main reasons:
- The need to shade pedestrians from the direct sun.
- Urban surfaces (buildings and pavements) consistently absorb and re-radiate heat, and shading by trees can reduce the amount of absorption and protect pedestrians from re-radiation.
6.4.3 UV Radiation
Trees can provide shade from UV radiation and its associated health problems such as skin cancer (Heisler et al., 1995; Parisi et al., 2000; Grant et al., 2002). It has been shown that shade alone can reduce overall exposure to UV radiation by up to 77% (Parsons et al., 1998). Shading by urban trees reduces ultraviolet irradiance when they obscure both the sun and the sky, (i.e. when there is dense shade). When only the sun is obscured, leaving much of the sky in view, UV irradiance is greater than suggested by the visible shade. A recent study developed a methodology to estimate the amount of protection tree canopies can provide (Grant et al., 2002). The paper recommended a number of improvements to the urban environment including increased tree canopy coverage.
6.4.4 Shading of cars and car parking spaces
(Scott et al., 1999) studied the effects of tree cover on parking lot microclimate and vehicle emissions in Davis, California. An analysis was made of the temperatures of pavements and parked vehicles in different parts of the car park with different levels of shading (1-25 %, 25-75%, or 75-100% shaded). The findings indicate a clear connection between shade and the temperatures of both:
- The parking lot pavement and above-ground air, and,
- The interior cabin temperatures of the automobile.
Although the shaded area of the car park was warmer than the air temperature of a control site (a nearby area of irrigated turf) the unshaded area was warmer for nearly the entire duration of the experiment. The air temperature of the shaded site was approximately 1.30C cooler then the unshaded site during the hottest period of the warmer days, while the pavement temperatures were nearly 200C cooler during the same period. Emissions modelling also showed the shaded areas released 3% less emissions than unshaded areas. Although these findings relate to parking lots, they would also be applicable to car parking spaces in urban streets.
6.4.5 Extended Materials Life
(McPherson and Muchnick, 2005) examined the effects of street tree shade on asphalt pavement performance in Modesto, California. The researchers studied 48 street segments divided into 24 low shade/high shade matching pairs. The streets were controlled for age, use, tree history and maintenance history and traffic volumes. A Pavement Condition Index (PCI) and Tree Shade Index (TSI) were calculated for each street segment. Three hypothetical scenarios were developed to demonstrate how the amount of tree shade can influence preventative maintenance expenditure over a 30-year period: an unshaded segment; a segment with small trees (crepe myrtle) with close spacings; and large trees (hackberry) at wider spacings. Analysis indicated a positive association between better pavement condition and higher levels of tree shade. Although further research is required into different road types, the results indicate that tree shading can contribute to extended pavement life.
6.5 Climate change
It is now widely accepted that human activities are contributing to global climate change due to increased levels of greenhouse gases in the atmosphere (Thom et al., 2009). Figure 41 summarizes Green Infrastructure roles in climate change mitigation and adaptation.
Figure 41: Climate change roles of Green Infrastructure. Source: M. Ely.
6.5.2 Climate change impacts
The impacts of climate change are difficult to predict and vary from region to region, however the likely impacts in Australia include: increased temperatures; reduced rainfall and extended periods of drought; increased bushfire risks; and more extreme weather events such as storms and flooding (Suppiah et al., 2006). Table 19 summarizes potential climate change impacts on the ‘built environment’ in Australia.
Table 19: Climate change impacts on the built environment. Source: Adapted from NCCARP: Settlement and Infrastructure. Sept 2009.
Climate projections prepared by CSIRO and the Bureau of Meteorology in 2007 suggest that the future climate of South Australia will generally be characterised by:
- Lower average rainfall
- More intense extreme rainfall events
- Higher storm surge events
- Higher average sea levels
- Higher average temperatures
- More frequent occurrence of extreme temperatures
- More frequent very high and extreme fire danger days
Small changes in average annual and seasonal temperatures and precipitation can reflect large changes in individual extreme weather events, such as heat waves, storms, strong winds and higher intensity rainfall. According to (AECOM, 2009) changes in extreme weather events projected for South Australia include:
- An increase in the frequency of hot days (days over 35°C)
- An increase in both peak precipitation intensity (measured in millimetres per hour) and the number of dry days (days with less than 1mm of precipitation) leading to longer dry spells interrupted by heavier rainfall events
- An increase in storm surge events characterised by high–intensity storms intersecting with a high tide and amplified by increases in mean sea level can be expected in coastal areas.
These changes in average climatic conditions and extreme weather events will increase the risks of damage to the state’s natural and man–made assets (caused by floods, bushfires and storms)if appropriate adaptive measures are not implemented.
Changing climatic regimes mayalso impact detrimentally upon the health, structure and management of the urban forest (Moore, 2006). Urban trees, however, can play an important role in the two main responses to climate change: mitigation and adaptation
6.5.3 Climate change mitigation
Climate change mitigation strategies include reduction of CO2emissions through increased use of public transport and energy efficiency (ClimateWorks, 2010). Urban trees can help mitigate climate change bycontributing to net reductions in atmospheric CO2 through:
- Carbon sequestration and storage (sequestering atmospheric carbon from carbon dioxide, and storing it tree tissues.
- Avoided CO2 emissions due to reductions in building energy use, consequently reducing carbon dioxide emissions from fossil-fuel based power plants (Abdollahi et al., 2000).
Figure 42: Role of trees in climate change mitigation. Source: (McPherson, 2010).
6.5.4 Carbon sequestration and storage
Urban trees can help reduce the amount of carbon in the atmosphere by sequestering carbon in new growth every year, as part of the carbon cycle. The amount of carbon annually sequestered increases with the size and health of the trees. Since about 50% of wood by dry weight is comprised of carbon, tree stems and roots act to store up carbon for decades or even centuries (Kuhns, 2008). Over the lifetime of a tree, several tons of atmospheric carbon dioxide can be absorbed (McPherson and Sundquist, 2009).
Carbon sequestration is the annual removal of CO2 through photosynthesis by plants. Photosynthesis is the process where plants use sunlight to convert CO2 to plant tissues.
Carbon storage refers to the carbon bound in above and below ground plant tissues, including roots, stems, and branches. Once plants die and begin decomposing, carbon is slowly released back to the system. For example, stored carbon can be released into the atmosphere as CO2 or stored as organic matter in the soil.
Source: (Green Cities Research Alliance, 2012) p.15.
In Australia (Moore, 2006) has estimated that the 100,000 public trees in Melbourne would sequester about one million tonnes of carbon. (Plant, 2006) estimated that in 2000, Brisbane’s residential tree cover was estimated to be absorbing the equivalent amount of CO2 emitted by 30,000 cars per year, and cooling surface temperatures in the relatively mild month of October 1999 by up to 5 degrees Celsius. These reductions must be balanced against CO2 released by the decomposition of dead trees and vegetable matter, and emissions produced in the management of the urban forest (McPherson and Sundquist, 2009). Unfortunately, however, urban vegetationis not currently included in calculations of greenhouse gas emissions for the purposes of creating carbon sinks to store carbon and reduce atmospheric CO2 levels (Moore, 2006).
126.96.36.199 Quantifying carbon sequestration
The i-Tree tool has been used to calculate carbon sequestration by an urban forest. For example the gross sequestration of Washington trees is estimated to be about 19,000 tons of carbon per year with an associated value of US$393,000. Net carbon sequestration in the Washington urban forest is estimated to be about 13,600 tons (i-Tree, 2010).
Figure 43: Carbon sequestration and value for species with greatest overall carbon sequestration in Washington. Source: (i-Tree, 2010) p.8.
6.5.5 Quantifying carbon storage
Carbon storage by trees, including urban trees, is another way that Green Infrastructure can influence global climate change. As trees grow they store more carbon as wood, and as trees die and decompose they release this carbon back into the atmosphere. Therefore the carbon storage of the urban forest is an indication of the amount of carbon that could be released if trees are allowed to die and decompose.Maintaining a healthy tree population will ensure that more carbon is stored than released. Utilizing the wood in long term wood products or to help heat buildings or produce energy will also help to reduce carbon emissions from other sources, such as power plants.
Trees in Washington have been estimated to store 596,000 tons of carbon ($12.3 million). Of all the species sampledthe Tulip tree was found to store and sequester the most carbon, approximately 15.4% of the total carbon stored and 11.2% of all sequestered carbon (i-Tree, 2010). Seattle’s urban forest stores approximately 36 metric tons of CO2 equivalent (or 9.9 metric tons of carbon) per acre and sequesters approximately 2.6 metric tons of CO2 equivalent (or 0.7 metric tons of carbon) per acre. Across Seattle, carbon storage in urban forest biomass amounts to almost 2 million metric tons of CO2 equivalent, with an additional 141,000 metric tons of CO2 equivalent sequestered in 2011. This equates to a city-wide savings of $10.9 million from carbon storage and an annual savings of $768,000 from carbon sequestration. The urban forest CO2 removal rate per year is 2% (or 7 days) of the city’s total annual emissions (Green Cities Research Alliance, 2012). These reductions must be balanced against CO2 released by the decomposition of dead trees and vegetable matter, and emissions produced in the management of the urban forest (McPherson et al., 2009).
It must be recognized however that the potential of urban trees for carbon storage should not be overstated, as street trees are often short lived and small in stature (Nowak and Crane, 2002; McPherson, 2008). The direct impacts of urban trees on carbon reduction therefore may seem to be negligible at first glance. However, the potential for the urban forest to reduce CO2 emissions through energy reduction, and its role in climate adaptation, lowering urban temperatures through evaporative cooling and protecting soil carbon should not be overlooked. For example it has been shown that increasing green cover by 10% within the urban area of Manchester could reduce surface temperatures by 2.2 - 2.5 % (Gill et al., 2007). Trees also play an important role in protecting soils, which is one of the largest terrestrial sinks of carbon. Soils are an extremely important reservoir in the carbon cycle because they contain more carbon than the atmosphere and plants combined.
6.5.6 Avoided emissions
Trees affect energy consumption by shading buildings, by evaporative cooling, and by blocking cold winter winds. Trees tend to reduce building energy consumption in the summer months and can either increase or decrease building energy use in the winter months, depending on the location of trees around the building. Estimates of tree effects on energy use can be made, based on field measurements of tree distance and direction to air conditioned residential buildings (McPherson and Simpson, 1999). Reduced energy demand from reduced air-conditioning can also lead to a reduction in carbon emissions from power stations (McPherson and Simpson, 2001).Based on 2002 prices, trees in Washington are estimated to reduce energy-related costs from residential buildings by US$3.45 million annually. Trees also provide an additional US$129,006 in value (Nowak and Crane, 2000) by reducing the amount of carbon released by fossil-fuel based power plants (a reduction of 6,240 tons of carbon emissions) (i-Tree, 2010).
Table 20:Annual energy savings in Washington due to trees near residential buildings. Note: negative numbers indicate an increased energy use or carbon emission. Source: (i-Tree, 2010) p.9.
Table 21: Annual savings (US $) in residential energy expenditure in Washington during heating and cooling seasons. Note: negative numbers indicate a cost due to increased energy use or carbon emission. Source: (i-Tree, 2010) p.9.
6.5.7 Carbon storage protocols
In the United Statesthe State of California has developed the Urban Forest Greenhouse Gas Reporting Protocol, an accounting and reporting tool that allows communities to obtain carbon credits for urban forest schemes (CCAR, 2008a). To be eligible, schemes must meet a number of criteria such as a stable tree population and long term ownership. The protocol includes a registry and annual online reporting of Net CRT (carbon stored minus carbon emitted). A Centre For Urban Forest Research (CUFR) Tree Carbon Calculator has also been developed as part of the reporting protocol. This comprises a standardized Excel based carbon calculation tool, optimized for California’s climatic zones (CCAR, 2008b). The tool calculates carbon sequestered, carbon stored in dry biomass, and carbon emissions avoided by energy conservation. However, urban forest projects have not yet been successful at registering under this protocol to serve as offset projects. Some of the main challenges are the protocol’s requirement of a 100-year, lifetime guarantee of project, the high costs of planting and maintaining urban trees and of monitoring/reporting costs, and the limited eligibility for applicants (e.g. non-governmental organizations and developers are not allowed to apply). Without being able to register under this protocol, urban forest projects cannot be credited as carbon offsets to be used in the California cap-and-trade market. Currently, the urban forestry project closest to registering under the Urban Forest Protocol for carbon offset credits is a Greenhouse Gas Tree-Planting Project in Santa Monica that is designed to plant 1,000 new trees in parkways along boulevards (Housholder, 2012).
6.5.8 Climate change adaptation
The other climate change role of Green Infrastructure is in adaptation to unavoidable climate change (Thom et al., 2009). Climate change adaptation strategies include cooling of buildings and houses and cooling of the outdoor surrounds (Nice, 2012). As discussed earlier the urban forest can assist in reducing temperatures in cities through shading, evapotranspiration and wind speed modification (Akbari et al., 2001). Urban trees can also play a role in relation to future climate change impacts such as shelter from extreme weather events, and flood reduction (McPherson et al., 2006). The recent City of Melbourne Urban Forest Strategy specifically addresses issues of climate change adaptation (City of Melbourne, 2011). According to the strategy:
‘As we anticipate increases in urban temperatures and density we can expect that Melbourne’s Urban Heat Island (UHI) effect will intensify. An increased canopy cover throughout the municipality will minimise the impact of the UHI effect’.
City of Melbourne Urban Forest Strategy
To achieve our vision of a healthy and resilient urban forest that contributes to the health and wellbeing of our communities and to a liveable city, we need to create better urban environments for everyone. Our guiding principles defined above highlight the importance of a well-designed city, and the following strategies list how we go about creating these ‘living spaces’.
Strategy 1: Increase canopy cover
Target: Increase public realm canopy cover from 22 per cent to 40 per cent by 2040.
Strategy 2: Increase urban forest diversity
Target: The urban forest will be composed of no more than 5 per cent of any tree species, no more than 10 per cent of any genus and no more than 20 per cent of any one family.
Strategy 3: Improve vegetation health
Target: 90 per cent of the City of Melbourne’s tree population will be healthy by 2040.
Strategy 4: Improve soil moisture and water quality
Target: Soil moisture levels will be maintained at levels to provide healthy growth of vegetation.
Strategy 5: Improve biodiversity
Target: Melbourne’s green spaces will protect and enhance a level of biodiversity which contributes to the delivery of ecosystem services.
Strategy 6: Inform and consult the community
Target: The community will have a broader understanding of the importance of our urban forest, increase their connection to it and engage with its process of evolution.Source: (City of Melbourne, 2011).
According to the US based (Sustainable Sites Initiative, 2009) climate change adaptation can be addressed through a range of landscape or Green Infrastructure strategies including:
- Increased temperatures:
- Use vegetation to reduce building cooling requirements
- Reduce urban heat island effects
- Extreme weather events:
- Protect and restore riparian, shoreline buffers
- Manage stormwater on site
- Reduce risk of catastrophic wildfire
- Reduced rainfall, drought:
- Reduce water use for landscape irrigation
- Design rainwater/ stormwater features to provide landscape amenity
- Maintain water features to conserve water
- Create a soil management plan
- Human health impacts:
- Provide views of vegetation and quiet outdoor spaces for mental restoration
- Provide outdoor spaces for social interaction
- Outdoor recreation demand:
- Provide opportunities for outdoor physical activity
- Sea level rise:
- Protect and restore shoreline buffers
6.5.9 Climate change and Water Sensitive Urban Design
It has been suggested that Water Sensitive Urban Design and Water Sensitive City principles and practices can be incorporated into the design of the urban landscape in response to climate change. For example vegetation and water can be incorporated into urban landscapes for their cooling effects (Shaw et al., 2007). Urban developments designed to encourage evapotranspiration such as trees and green-spaces, can also act as carbon sinks (Coutts et al., 2010). Cooling through increased albedo of urban surfaces and passive cooling techniques can decrease the amount of anthropogenic heat released through air conditioners, as well as savings in CO2 emissions from reduced mechanical cooling requirements (Coutts et al., 2010). The increased use of urban stormwater runoff also has a number of climate change mitigation benefits. For example urban areas can secure water supply without relying on centralized water systems that can be energy and emission intensive (Coutts et al., 2010).
- The literature review shows that climatic modification is one of the main ecosystem services provided by Green Infrastructure, especially in terms of mitigating ‘urban heat island’ effects in Australian cities.
- The urban heat island has also been linked to increased mortality of older people in extreme heat events in Australian cities.
- Research shows that vegetation, especially large canopied trees, can provide significant climatic benefits through shading and evapo-transpiration effects.
- Research also highlights the need to retain water in urban areas to maximize tree canopy cover and evapo-transpiration.
- Green Infrastructure can also play a role in climate change mitigation, and more importantly in adaptation to unavoidable climate change.
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