Philip Hewett explains why our most famous shade dweller was well ahead of his time.

“Once a jolly swagman camped by a billabong; under the shade of a Coolibah tree,” wrote ‘Banjo’ Patterson in 1895. A puddle of cool water and the shade of a gum tree in a parched land. A hundred years later you’d think shade would still appeal but it seems not. Australia has the world’s highest skin cancer rate with $294 million spent on skin cancers in 2001 and over 1,500 deaths in 2003 alone. We’d be better off in the shade of trees than in the sun yet all that planners, developers and politicians give us are treeless environs. Our oldest urban trees need replacing, and drought, diseases and pests are emerging threats as climate change alters nature’s dynamic balance.

Despite an appearance of plenty, shade tree loss is accelerating in every Australian city. Planting space is contracting and imperviousness increasing. Queensland’s Griffith University studied the implications of losing suburban backyards to McMansion style housing. The study concluded that the loss of backyards not only affects resident’s physical and mental health but also increases the risk of local flooding and increases air pollution, urban heat and stormwater management costs. This is the price of ill-considered tree loss and a poorly planned urban consolidation model.

We know that North American city tree cover has declined 30% in 20 years as the urban footprint has increased by 20%, forcing increases in the cost of managing stormwater and air quality, and contributing to declining community health. So that’s America – could it be the same here? In fact it’s very likely the same here, but the lack of Australian research means a lack of understanding and thus investment in integrated planning for city growth.

Planting more trees is not enough because the space they need is now given to vehicles, utilities, housing and ‘lifestyle’. The miniscule space available is good for shrubs or small trees at best, yet research proves that large mature trees provide at least eight times the benefits of small trees.

a large mature tree can intercept 10,000 litre of rainfall and filter two-kilograms of fine particulate air pollution every year

So, what benefits would accrue if we successfully retained trees and got the necessary space for new trees as urban densities increase?

Urban forest benefits accrue in three areas – human services (part one in last issue) economic development services, and climate and environmental services. Numerous North American studies conclude that quality urban forest has a positive ripple effect on real estate. Rental rates for commercial office properties may be seven percent higher on sites with a quality landscape incorporating trees, and consumers will pay from nine to 12 percent more in shopping areas with trees compared to treeless sites.

Melbourne University’s Dr Greg Moore believes the climate and environmental services of mature trees, whether native or exotic are of such value that trees should be managed as significant assets. He views mature trees in cities as significant carbon sinks, sequestering atmospheric carbon dioxide for very long time periods. Using rounded data Dr Moore estimates that Melbourne’s 100,000 inner city trees have sequestered one million tonnes of carbon.

North American researchers have developed the Urban Forest Effects (UFORE) computer model to quantify urban forest structure and effects for cities. UFORE has been applied to 22 US cities and nine other international cities. The model calculates carbon sequestration, pollution removal, energy conservation and microclimate effects.

Non-profit group, American Forests developed a GIS software system called CITYGREEN that quantifies urban ecosystem benefits for stormwater management, clean air and energy consumption. UFORE and CITYGREEN studies have led to significant increases in investments in urban tree programs and research. In other words, when tree benefits are scientifically proven and quantified, trees effectively compete for political support.

It might come as a surprise to find that a large mature tree can intercept 10,000 litre of rainfall and filter two-kilograms of fine particulate air pollution every year. Scale up these benefits to a city with 100,000 or more trees and the value of trees is breathtaking.

Tree shade is a city asset – for example, shaded asphalt has a 30% longer service life than unshaded asphalt. An Adelaide University and TREENET study found UV screening effects of mature street trees equaled UV protection creams and clothing – worth considering when packing the kids off to the school bus stop or if you like to walk without SPF cream or a hat.

The heat island effect is described by the US Environment Protection Agency as a phenomenon whereby urban temperatures rise one- to six-degrees celcius above nearby rural areas. Elevated temperatures increase peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality. Take a Google Earth view of any Australian city and note the concentrated heat islands from expanding areas of commercial and industrial development – these are treeless heat sinks where thousands of people spend much of their working life. Any wonder that respiratory diseases are plaguing urban populations?

A 2006 NASA study of the urban heat island in NY found vegetation to be the most effective tool to reduce surface temperatures in the city. Columbia University NY scientist Stuart Gaffin, co-author of the NASA study, says ever-increasing urban populations around the world means the heat-island effect will become more significant in the future – in other words, cities need a lot more shade.

So there you have it – the benefits of urban forest far outweigh management costs. At least that’s the conclusion from North America after 30 years and nearly a billion dollars of research. Banjo Paterson’s ‘jolly swagman’ knew the value of tree shade but could never have imagined the multifaceted role trees would play in this, the world’s most urbanised nation.

Philip Hewett has managed municipal trees and taught arboriculture for over 25 years. He is City Arborist at Newcastle Council and a member of the NSW Local Government Association Urban Forest Policy Working Group. This is Part Two of a two-part Read & React series by Phil on the values and benefits of urban forests and green-space in our urbanised society.

Source: Outdoor Australia