Trees, vegetation, and green infrastructure in urban areas can significantly reduce stormwater runoff, especially during heavy rainfall events, as they can absorb vast amounts of water, thereby reducing flooding and its damaging impacts.
A report by Prof. Margaret Stanley, School of Biological Sciences, University of Auckland, for the New Zealand Ecological Society Hot Topics series explains the role of vegetation and actions like decreasing impervious surfaces and protecting streams and wetlands in alleviating the pressures off the stormwater infrastructure that could mitigate floods.
According to the report, the amount of rainfall at the end of January 2023 exceeded the city’s stormwater system, resulting in a devastating flood that resulted in the loss of lives and massive property and infrastructure damage. “In Aotearoa, we have lost 90% of our wetlands and replaced them with impervious surfaces and highly compacted soils, leaving little to intercept high rainfall and fast-flowing water”.
The report highlights the importance of trees, vegetation cover, nature-based solutions, preservation of wetlands and streams, and application of water-sensitive design to decrease the impacts of extreme weather events. A mindset change is also essential to increase the uptake of nature-based solutions to support the grey infrastructure.
“We can no longer rely on pipes and concrete alone. We should be investing in nature-based solutions throughout our cities, on private land throughout existing suburbs as well as public land. At the very least, instead of removing large trees to make way for new developments, these should be built in a way that works with existing vegetation.”
In New Zealand, while there is a clear need to build more houses in the country to address high housing demands and affordability, we should do it in a manner that protects trees and the natural environment.
An article in The Guardian written by Prof Stanley emphasises the importance of protecting mature trees and considering their valuable environmental services before cutting them off in favour of developing an area for housing in New Zealand. However, when mature trees are cut down, it also removes their benefits, such as improving water quality, biodiversity, and health and well-being of residents, and reduction in flood risk, energy, noise, and air pollution. Hence, the government should uphold the laws that protect and preserve the environment and not loosen them. For instance, amendments to the RMA allowed the removal of trees on private lands unless it was a protected tree, which means that only 15% of trees in Auckland are covered.
Prof Stanley also cites the proposed international guidelines, the 3:30:300 rule, on preserving trees and green spaces in communities. This means three trees for every home, 30% forest canopy cover for every neighbourhood, and a minimum 300m distance of each house to green spaces.
A study by Nanjing and Yale University scientists found that urban greening – planting street trees, constructing rainwater gardens, and de-paving can reduce urban heating caused by climate change. The study shows the difference in the rate of warming between urban and rural areas and between small and big cities over ten years. It shows that urban areas are warming 29% faster than rural areas, and megacities are warming more than smaller cities.
Continents with the most megacities, particularly in Asia, are warming more than their European and Oceania counterparts. While about 90% of warming in the cities surveyed is attributed to climate change, urban sprawl, like in parts of China and India, also influences warming.
However, urban greening strategies like tree planting and green spaces generate a cooling effect and combat urban warming.
Read the article here: Urban greening can reduce impact of global heating in cities, finds study
The negative consequences of increased urbanisation, paving over porous surfaces, cutting down trees, and building over wetlands and streams have been well documented. While these developments created many economic opportunities and benefits, it has contributed to environmental damage and climate change and exacerbated its impacts.
As scientists have pointed out, the solution to mitigate climate change and its effects is to scale back on concrete pavements or impermeable surfaces, increase the application of water-sensitive urban design via well-designed green infrastructure, and urban greening by planting more and protecting existing ones. Policymakers and urban planners should use these urban greening strategies more and invest in them more seriously to address climate change effects.
Can urban greening help make cities more resilient to the ongoing impacts of extreme events? (2023 February). New Zealand Ecological Society. Retrieved from https://newzealandecology.org/can-urban-greening-help-make-cities-more-resilient-ongoing-impacts-extreme-events
Stanley, M. (2021, December 14). Mature trees are key to liveable cities – housing intensification plans must ensure they survive. The Guardian. Retrieved from https://www.theguardian.com/world/commentisfree/2021/dec/14/mature-trees-are-key-to-liveable-cities-housing-intensification-plans-must-ensure-they-survive
Mohamed, W. (2022 September 29). Urban greening can reduce the impact of global heating in cities, finds study. The Guardian. Retrieved from https://www.theguardian.com/environment/2022/sep/29/urban-greening-reduce-impact-global-heating-cities-study