It is almost 10 years since I last wrote on the economic value of urban trees for Sustainable Gardening Australia, and much has changed. As climate change takes greater effect on the continent, the economic value of trees has in some contexts soared, bolstered by a massive increase in tree related research. But urban trees continue to be under-valued. State governments in particular, and some more than others, place almost no value on urban trees in the face of development and a housing crisis that sees urban tree clearing on both public and private land at an unprecedented scale. This threatens the environmental and economic sustainability of the places where people will live under a warmer, dryer climate.
As research continues into the importance of urban trees for urban ecology, environmental sustainability and people’s health and well-being, the number and the diversity of published papers can be overwhelming. It is no longer possible for a single researcher to keep up to date with all aspects of the research undertaken, so it can be convenient to think of their contributions from two perspectives. One set of contributions comes from the environmental services trees provide and their economic value, and other contributions come from the perspective of human health and well-being, and the impacts that trees and treed greenspace have on State and Federal health expenditures.
Urban Trees Help to Cool our Cities
At present tree canopy cover is an important metric for those studying urban trees. In the face of climate change, the shade provided by trees is known to reduce ambient temperatures. However, many people are unaware of the cooling effect that evapotranspiration by trees has in cooling the environment around them, provided there is sufficient soil moisture. This cooling and shade can have a dramatic effect in reducing temperatures, particularly in heatwaves. This cooling may reduce heat related deaths. To be effective, using trees to cool our environment might require the use of irrigation at specific times of extreme temperatures, however this is not part of current government policies. The shade and cooling effects of large trees will diminish as uncontrolled urban tree clearing continues.
The urban heat island effect (UHIE) is of increasing concern as our climate changes and temperatures warm. With the prospect of many more days above 40OC and days approaching 50 OC, there are concerns about the economic impacts this could have on cities and towns. Air conditioners are not as effective when temperatures exceed 45OC, heat related hospitalizations and deaths increase as do their costs, school and workplace activities may be suspended for health reasons. All of these have an economic impact.
The simplest and cheapest way to reduce the influence of the UHIE is through treed greenspace and tree canopy cover. Concerns about the UHIE and heatwaves precipitated a review of tree canopy cover in Adelaide by the State Government. The city had lost over 11% of its tree canopy in a decade and was losing 75,000 urban trees per annum. Attempts are being made to preserve both trees and livability in Adelaide, but in other cities, such as Melbourne, where the losses have been greater, reductions in tree canopy cover continue unabated. It is possible to envisage a future heatwave claiming 500 to 1,000 lives at an enormous social and economic cost.
Other Economic Benefits
Other benefits from the presence of urban trees include removing common particulate air pollutants, which are still responsible for illnesses and deaths in Australia; the role of trees in minimizing local flooding and their use with porous paving and water sensitive urban design (WSUD) have been further documented. Some aspects of both WSUD and the use of permeable paving have now entered mainstream design, engineering and urban landscaping. However, other aspects such as the role of shade trees in prolonging the life of urban bitumen surfaces, stabilizing steep slopes and sequestering carbon are still under-appreciated. In States such as Victoria a powerline clearing regime that unnecessarily and harshly prunes trees around low voltage (LV) powerlines in low bush fire risk areas (LBRA) has been maintained by government despite a willingness by electricity distribution companies and other stakeholders to support change. Changing regulations that are clearly unfit for purpose would bring an immediate benefit of about $1billion per annum to Victorians and the increased shade would reduce fuel bills as electricity prices soar.
Health Benefits of Urban Trees
The disruption of Covid-19 had a significant impact on how treed urban greenspace is viewed by society. These spaces can be potent reminders of how important trees and the natural environment are to the health and well-being of humans. During the periods of lockdown, especially in Melbourne, visitation rates and the use of local parks, along waterways and in other greenspaces sky-rocketed. It was well understood by many that access to these spaces was vital to peoples’ health, well-being and resilience during stressful times. Even the State government recognized the importance of the connection during the period of lockdown. But a few years later, without an immediate crisis, greenspace is once again under threat and tree canopy cover is reducing at an increasing rate in suburbs, regional centres and country towns. It is curious and concerning that with threats of climate change and future pandemics, we will have fewer trees and less greenspace to allow people to cope with stressful change where they live.
Unsurprisingly, over the past decade the research into the human health benefits of urban trees has literally exploded. We now know, following a meta data study of the effects of living in a green and treed space on women’s health, that they recover more quickly when hospitalised, give birth to babies with higher birth weights, use fewer prescription drugs, have better general health and live longer. There are other benefits too in that people are more resilient under stress, have lower rates of type-2 diabetes and are less prone to blood pressure and cardiac related disease. Some of these health benefits derive from the exercise and lower rates of obesity that a treed environment and summer shade encourage. They are nonetheless benefits that are worth many billions of dollars to the nation’s health economy each year.
Recent studies here and overseas have emphasized the benefits or treed greenspace on the wellbeing, health and development of children. The effects can be subtle, ranging from the multi-tasking that children undertake as they move through a complex biological space using all of their senses and developing skills in interpreting complex spaces, to the health of their gut flora. Exposing primary school-aged children to a natural forest/woodland environment where there was leaf litter, humus and soil improved their gut microbiomes and for those with dermal conditions there was a significant improvement. Brief exposures to treed greenspaces can also assist students in focusing and concentrating on classroom activities and can reduce disruptive behaviours, and in some cases the need for medications. Once gain these benefits can be valued at millions of dollars per annum to the Australian economy.
Urban trees continue to provide high value services and functions in the places where Australians live. However, the full value of these services and benefits continues to be undervalued by most State governments. “You don’t know what you’ve lost till it’s gone” is a famous line from the Joni Mitchell song, but it could also be the epitaph for the loss of urban trees in many Australian States.
Selected References:
- Croeser, T, Ordóñezb, C, Threlfall, C, Kendal, D, van der Ree, R, Callow, D and Livesley, S J. (2020) Patterns of tree removal and canopy change on public and private land in the City of Melbourne. Sustainable Cities and Society, 56, 102096.
- James, P., Hart, J E., Rachel F. Banay,R F. and Laden F (2016) Exposure to greenness and mortality in a nationwide prospective cohort study of women. Environmental Health Perspectives, 124 (9) 1344-52.
- Mills J G. 2023) Urban Microbiome Restoration. Johnson T Editor, Proceedings of the Twenty Fourth National Street Tree Symposium, 17-23, University of Adelaide/Waite Arboretum, Adelaide.
- Moore, G M. (2022) Connected treed greenspace: a salve for climate change. TREENET.com