«Proceedings of the th 14 National Street Tree Symposium 2013 ISBN: 978-0-9806814-1-3 TREENET Proceedings of the 14th National Street Tree Symposium ...»
Throughout the world and particularly in industrialised countries, Green Infrastructure is being embraced as an important component in the development and redevelopment of urban environments. Without Green Infrastructure cities and towns risk becoming urban deserts in the sense of being hostile and barren places where people are disconnected from nature and from each other. A rapidly growing body of evidence supports the key role of Green Infrastructure in providing critical life support for human habitats.
Approach Three main perspectives of Green Infrastructure have been identified in literature and in practice. They include an Ecosystem services approach in which GI delivers services and benefits similar to those delivered by natural processes (Daily 1997) a Linked green spaces approach whereby GI provides a healthy and sustainable alternative to the traditional ‘grey’ or engineering services-based infrastructure (Benedict and McMahon 2002) and a Green engineering approach in which GI is seen as a specialised form of engineering infrastructure that replaces conventional elements with ‘green’ elements that perform ecosystem service functions such as storm water harvesting, waste management and energy efficiency (Margolis and Robinson 2007). For example the City of Sydney labels energy trigeneration and a decentralised water networks as ‘green infrastructure’ responses to climate change (City of Sydney 2012).
The way of thinking most relevant to the work that needs to be done to create healthy and sustainable places to live and work integrates all three with emphasis on the Linked green spaces approach. Networks of plants and water systems deliver services and functions that urban environments require and provide a ‘green’ framework for sustainable living and development. Evidence of the importance of Green Infrastructure in urban environments has been gathered from studies and reports around the world. In order to consider the values and benefits we have grouped them: human health and well-being; water, air, soil and climate; climate change; biodiversity; food; and economics.) The 14th National Street Tree Symposium 2013 Human Health and Well-being Research over the last 20 years has investigated connections between contact with nature and human health and well-being. Health and well-being are defined in the broadest sense to mean not only the absence of disease, but a state of physical, mental and social well-being. Abraham et al. (2010) reviewed the health promoting aspects of GI, which include physical well-being, mental well-being and social well-being. There is a clear link between the built environment (including landscapes) physical activity and health (Kent, Thompson et al. 2011). Physical activity is promoted by access to nearby green spaces and well designed ‘walkable streets' (Giles-Corti, Broomhall et al. 2005).
A large body of research supports the psychological benefits of contact with nature. Early work undertaken in the United States by Rachel and Stephen Kaplan (Kaplan and Kaplan 1989) included research into the ‘restorative effects of nature’ and found that the natural environment can foster people’s wellbeing and their ability to function effectively. Steven Kaplan ‘s Attention Restoration Theory proposes that contact with nature engages our ‘involuntary attention’ giving our ‘directed attention’ (voluntary attention) the opportunity to rest, thus helping overcome the mental fatigue associated with continual directed attention (Kaplan 1995).
One of the best known studies of the restorative powers of nature was conducted by Roger Ulrich who showed that abdominal surgical patients had shorter post-operative hospital stays when accommodated in a room looking out on a stand of trees (Ulrich 1984). Positive connections with nature have been found in a range of studies including reduction of ADHD symptoms (Faber Taylor and Kuo 2009), reduced crime in inner city neighbourhoods (Kuo and Sullivan 2001) and strengthening of a sense of community (Kuo, Sulivan et al. 1998).
At the University of Illinois Landscape and Human Health Laboratory, Kuo, Sullivan and others are researching inner-city residents’ responses to trees and other vegetation and the ways in which the physical and psychological health of individuals and communities can be improved with enhanced access to nearby nature.
Importantly, more attractive urban green spaces can enhance opportunities for social interaction, fostering community ties and a sense of identity that has been found to be fundamental to human health and well-being (Maas, van Dillen et al. 2009a). According to Armstrong and Leyden, urban parks and other public places can enhance social integration if they facilitate social contacts, exchange, collective work, community building, empowerment, social networks and mutual trust (Armstrong 2000; Leyden 2003). Trees and greenery increase the attractiveness of places for people, in turn promoting community socialisation and passive surveillance, which can reduce crime and increase personal safety (Coley, Kuo et al. 1997; Kuo and Sullivan 2001; Kuo 2003).
A particular focus is on the role of Green Infrastructure in dealing with the emerging physical and mental health epidemic in Australian children (Malkin 2011). Writer Richard Louv in his book Last child in the Woods has coined the term Nature-Deﬁcit Disorder to describe the effects on children of enforced alienation from nature. These effects include a number of behavioural issues including diminished use of the senses, attention difficulties and higher rates of physical and emotional illnesses. Louv argues that sensationalist media coverage and paranoid parents have literally ‘scared children straight out of the woods and fields’ while promoting a litigious culture of fear that favours ‘safe’ regimented sports over imaginative play (Louv 2005). Two recent studies by Planet Ark have investigated the changing role of children’s play, and the health and well-being benefits to children of contact with nature. Rises in childhood obesity and mental health issues have been linked to dramatic lifestyle changes in the last 20 years. The 2011 study, Climbing Trees, found a dramatic shift in childhood play activity in the space of one generation (Planet Ark 2011). For example 64% of parents said that they climbed trees when young, compared with only 19% of their children. A 2012 study, Planting Trees, investigated the intellectual, psychological, physical and mental health benefits of contact with nature for children (Planet Ark 2012). This study reviewed local and international research in this field and revealed an emerging body of evidence that ‘contact with nature during childhood could have a significant role to play in both the prevention and management of certain physical and mental health problems, and in forming environmentally responsible attitudes in future adulthood’.
Water, Air, Soil and Climate Green Infrastructure provides a wide range of natural functions, often called ‘ecosystem services’, including cleaner water and air, healthier soils and more amenable urban climate and microclimates (Millennium Ecosystem Assessment 2003). Issues of global climate change and local extended drought have further highlighted the need to address a range of environmental issues, including making better use of Green Infrastructure in the public realm.
The 14th National Street Tree Symposium 2013 Vegetation cover plays an important role in the natural water cycle, modifying rainfall inflows, soil infiltration and groundwater recharge, and patterns of surface runoff (Xiao, McPherson et al. 2006). Urbanisation, however, has seen the natural water cycle replaced by the ‘urban water cycle’, with extensive impervious surface and highly efficient drainage systems dramatically increasing the quantity but reducing the quality of urban storm water runoff. This has negative impacts on the ‘receiving’ aquatic ecosystems, while removing a valuable water resource from the city (Wong 2006). Water Sensitive Urban Design (WSUD) emerged during the 1990s as a new paradigm for the more sustainable management of the water cycle in the urban landscape (Argue 2004). Green Infrastructure can play a vital role in helping to restore or better replicate the natural water cycle in urban areas. In particular, vegetated WSUD systems contribute in a number of ways: canopy interception (Xiao et al. 2006); soil infiltration and storage (Bartens, Day et al. 2008); improved storm water runoff quality through biofiltration processes (Denman, May et al. 2011); and flood damage control (Lull and Sopper 1969; Craul 1992).
More recently the concept of Water Sensitive Urban Design has been expanded to embrace the many ways in which water and more appropriate water management can enhance the ‘liveability’ and ‘resilience’ of our cities (Living Victoria Ministerial Advisory Council 2011). Along with the provision of safe, secure, affordable water supplies, WSUD supports green landscapes that significantly enhance urban amenity, help to combat the impacts of the urban heat island effect, improve the health of urban waterways and provide opportunities for active and passive recreation. The emerging concept of the Water Sensitive City has three main ‘pillars’: Cities as Water Supply Catchments; Cities Providing Ecosystem Services; and Cities Comprising Water Sensitive Communities (Wong 2011).
An ecosystem service provided by urban trees and other vegetation is that of improving air quality in cities.
Plants have several natural functions: they remove atmospheric pollutants; oxygenate the air; and absorb carbon dioxide through photosynthesis (Brack 2002; Nowak, Crane et al. 2006). Studies show that gaseous pollutants are absorbed by leaves and either metabolized or transferred to the soil by decay of leaf litter, which may be particularly important in streets with high traffic volumes (Nowak 1994; Scott, McPherson et al.
1998). The leaves of trees also collect and trap airborne particles on their surfaces. The most significant impacts on air quality, however, are through reductions in carbon dioxide and atmospheric pollutants (Nowak, Stevens et al. 2002).
A valuable benefit of Green Infrastructure to cities is that of climate modification, especially temperature reduction. 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 vegetated areas, especially at night (Bornstein 1968; Rosenfeld, Akbari et al. 1998). Temperatures in cities on cloudless days have been found to be as much as 120C warmer than surrounding rural areas (Oke 1987). In Melbourne, researchers have reported a mean UHI of around 2 to 4°C and as high as 7°C depending on the location, time of the year and time of day (Morris and Simmonds 2000; Coutts, Beringer et al. 2010).
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. Lack of vegetation in cities also contributes to the urban heat island effect. Reduced tree cover leads to both a reduction in shading of surfaces and a reduction in transpiration cooling by tree canopies in comparison with rural areas (Federer 1976). Cities are also drier than surrounding areas, as the natural ground surfaces are frequently replaced with asphalt and concrete surfaces which create higher surface temperatures and reduce evaporation from the soil that may otherwise cool the surface (Miller 1980).
The urban heat island is now recognised as contributing to health risks in large cities such as Melbourne (Loughnan 2009). Urban heat island effects contribute to increased morbidity/mortality rates in ‘heat wave’ events, especially among the aged (Loughnan, Nicholls et al. 2008; Loughnan, Nicholls 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, Nicholls et al. 2008; Loughnan, Nicholls et al. 2010;
One method of mitigating extreme summer temperatures in the urban areas is to adopt the ‘cool cities’ strategy (Luber 2008). Trees and other vegetation modify urban microclimates and help reduce the urban heat island effect through two major natural mechanisms: temperature reduction through shading of urban surfaces from solar radiation, and evapotranspiration which has a cooling and humidifying effect on the air The 14th National Street Tree Symposium 2013 (McPherson, Herrington et al. 1988; McPherson 1994; Akbari, Pomerantz et al. 2001; Pokorny 2001; Georgi and Zafiiridiadis 2006).
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, Cane et al. 2009). Green Infrastructure, especially urban trees, can play an important role in the two responses to climate change: climate change mitigation, and climate change adaptation.
While climate change mitigation strategies often include reduction of CO2 emissions through increased use of public transport and energy efficiency (ClimateWorks 2010), urban trees can contribute to net reductions in atmospheric CO2 through carbon sequestration and storage and also through avoided CO2 emissions due to building energy savings. Moore (2006) estimated that the 100,000 public trees in Melbourne would sequester about one million tonnes of carbon. In 2000, Brisbane’s residential tree cover was estimated to absorb the equivalent amount of CO2 emitted by 30,000 cars per year and to cool surface temperatures in the relatively mild month of October 1999 by up to 5 degrees Celsius (Plant 2006). Such performance can result in reduced demand for air-conditioning energy, leading to a reduction in carbon emissions from power stations (McPherson and Simpson 2001). It is important to balance these reductions against CO2 released by the decomposition of dead trees and vegetable matter, and emissions produced in the management of urban trees (McPherson, Simpson et al. 2009). Similarly, 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).