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How Vegetation can Beat the Summer Heat

By Dr Anthony Kachenko, National Environmental and Technical Policy Manager, Nursery & Garden Industry Australia.

The mecury has risen high this summer. Many welcome a rise in temperature; however, extreme heat in our urban environments can prove fatal for some. In the heat wave event in Melbourne during the summer in 2009 the Department of Human Services noted that the extreme heat may have resulted in 374 excess deaths over what would normally be expected for the period. That equated to a 62 per cent increase in total all-cause mortality and an eight-fold increase in direct heat-related presentations in the emergency departments. These figures should ring alarm bells for our decision makers across the country, particularly as an increase in extreme heat events is expected under a changing climate. Indeed, Arbalster and Alexander (2008) indicate that climate change projections for Australia suggest an increase in the number of warm nights and heat waves could pose significant threats to human health. The question to ask is, how can decision makers minimise the risk of extreme heat events to the wider Australian community? The answer is quite simple – more vegetation, in particular by growing the urban forest being all the trees and vegetation in built-up areas in urban, peri-urban and regional locations. Throughout Australia, rapid urban population growth and development has transformed native environments to engineered infrastructure, accompanied with increased heat generation from our human activities and summer heat accumulation due to massive heat absorbing surfaces. These all result in higher temperatures in urban areas in comparison with rural areas, and consequently the phenomenon known as the urban heat island (UHI) effect. For example, in Melbourne, research has shown that an UHI of around 2 to 4 ºC and as high as 7 ºC can occur, depending on the location and time of the year and day (Morris and Simmonds, 2000; Morris et al, 2001; Coutts et al 2010). Vegetation provides many benefits to humans, the environment and biodiversity, from carbon capture through to reductions in crime rates. However, one of the most important benefits from vegetation in our cities is the improved microclimate conditions and the associated reduction in the urban heat island (UHI) effect. Nursery and Garden Industry Australia (NGIA) is currently funding research with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to examine the degree of cooling offered by urban vegetation in Melbourne using current and future climatic predictions. The following results have been reported:

  • Suburban areas are predicted to be around 0.5 ºC cooler than the CBD;

  • A relatively leafy suburban area may be around 0.7 ºC cooler than the CBD area;

  • A parkland (such as grassland, shrub-land and sparse forest) or rural area may be around 1.5 to 2 ºC cooler than the CBD;

  • Doubling the CBD vegetation coverage may reduce the average summer daily maximum temperature by 0.3 ºC;

  • 50% green roof coverage of the CBD area may reduce the average summer daily maximum temperature by 0.4 ºC;

  • An average summer daily maximum temperature reduction of around 0.7 ºC may be achievable by doubling the CBD vegetation coverage and having 50% green roof coverage in the CBD area.

The data from this research presents clear evidence that more vegetation is required not only in Melbourne but in built up urban environments right across Australia. To ensure this vision turns into a reality, owners of private residential land can introduce, increase and manage vegetation canopy cover within their own gardens, whilst local government authorities can do the same within residential streetscapes and landscapers can encourage greater use of greenlife in their projects. Furthering this necessary research has been the recent development of the National Urban Forest Alliance of which NGIA has become a Platinum Partner.  The Alliance has been formed to promote the growth and recognition of the urban forests across Australia. The formation of the Alliance brings together stakeholders with an interest in urban forest policy, planning, measurement and promotion. At present the National Urban Forest Alliance has 12 major stakeholders including the city councils of Melbourne, Sydney, Brisbane, and Darwin as well as the Australian Landscape Industry and Arboriculture Australia. The National Urban Forest Alliance arose from the partnerships and collaborations during the development of i-TreeEco for application in Australia (refer to Landscape Contactor July/August 2012 page 14). The vision of the Alliance is to promote a thriving, sustainable and diverse Australian urban forest that creates a contiguous and healthy ecosystem that is valued and cared for by all Australians as an essential environmental, economic, and community asset. The National Urban Forest Alliance values the Urban Forest for:

  • reducing the urban heat island effect in a time of climate change to improve liveability and comfort

  • improving the health of urban residents and workers though the provision of more attractive public spaces which encourages increased activity and walking

  • reduced energy (air-conditioning) costs and associated emissions for buildings through transpirational cooling

  • improving air quality for our atmosphere and water quality for our waterways and bays

  • sequestering carbon from the atmosphere to help mitigate climate change

  • providing food such as fruits, nuts, spices and olives

  • increasing habitat to support biodiversity

  • increasing real estate value of properties with tree-lined streets

  • increasing the life of infrastructure through weather protection

  • improving the visual amenity of streetscapes and neighbourhoods

It is hoped that all decision makers and urban planners will value research supporting the urban forest as a necessary environmental, economic, and community asset and the activities of the National Urban Forest Alliance.
In the meantime, I would encourage any landscaper contractors, designers and managers to help build awareness of, and importantly, grow the urban forest – our futures depend upon it.


For more information on this research or activities relating to the National Urban Forest Alliance visit www.plantlifebalance.com.au or contact Dr Kachenko on (02) 8861 5100.

Built up and Urban area's with little vegetation.

Research shows clear evidence that more vegetation is required not only in Melbourne but in built up urban environments right across Australia.