Risks to Australia’s urban forest from climate change and urban heat

Australian cities contain millions of trees that provide amenity, important ecosystem services such as cooling and slowing stormwater, and provide habitat for birds and animals. There is growing recognition that increasing temperatures due to urban heat and climate change are a threat to some tree species in our cities. This study analyses the risk of temperature increases to 1.9 million trees in 29 LGAs across Australia, from Launceston to Darwin, and Brisbane to Perth. Every tree was analysed to see how close it was to known temperature limits in current climates, an emissions limited climate change scenario in 2040 assuming emissions stabilisation, and a business as usual emissions scenario in 2070. We find that 14% of all public trees (22% of species) in Australia’s cities are at high risk (red flagged) from increased temperatures in the emissions limited climate change scenario, and 24% of all public trees (35% of species) in the business as usual emissions scenario by 2070. A further 33% of trees are at some risk (yellow or orange flagged) in the emissions limited scenario and 29% in the business as usual scenario. There is great variation in the risk to urban trees of temperature increases from city to city, and across areas within each city. This risk from increasing temperatures will present a major challenge to land managers across Australia. There are likely to be unequal impacts on the different benefits provided by the forest (e.g. cultural heritage, biodiversity), and these impacts vary from place to place. Change in management of natural areas and natural resources can lead to conflict. Urban trees are important to people for different reasons, and a wide range of concerns must be addressed when planning our urban forests of the future. Particular care must be taken so the inequality in the distribution of urban trees is not reinforced when planning and managing this change, and that ‘maladptation’ leading to feedback loops with increasing temperatures (e.g. through reduced shade) does not occur. There are several strategies that are available for urban forest managers to adapt to increasing temperatures. For important trees, a strategy of resistance can be used to improve the environmental conditions and prolong useful life e.g. by providing irrigation or improved pest and disease management. More generally, a strategy of promoting resilience can be used e.g. through careful site selection for vulnerable species, and improved tree maintenance. Lastly, managers can respond to change by selecting trees that are better adapted to future climates. While this report has focussed on the risks of increasing temperatures, there are also many opportunities that will arise from this. New tree species will need to be introduced to our cities to maintain resilience and provide a wide range of benefits. We have an opportunity to improve the sustainability of our cities through this renewal process. Meaningful engagement with the community and industry will help create successful urban forests of the future that provide a wide range of benefits for people and wildlife in cities.