Impacts of green infrastructure on urban heat often focus on trees and their canopies. But seeing the wood for the trees is important as it is the detail that counts.
Urban heat research has found that it's not as simple as planting a few trees and considering how the top layers reduce heat. Local climate comes into the mix. Ground covers and shrubs contribute to comfort and biodiversity. And grey infrastructure such as reflective rooves is important too.
Cooling and greening is complex and multiple solutions are important to mitigate against the impacts of climate change in cities and suburbs.
Canopy cover is the horticultural method mostly utilised to combat the urban heat island effect. Research has shown that trees can reduce air temperatures as they block sunlight through shade, use energy from the sun to grow and cool the air, reduce the amount of heat absorbed by surrounding surfaces, and provide transpiration released water vapours.
Findings by university academics looking at nearly 9,000 cities around the world found that trees almost halve how much heat is trapped by the urban heat island effect. But, as the researchers say, climate change and the resulting rises in extreme heat mean that tree canopies will help – but will come under increasing pressure to do the job of cooling our cities and suburbs.
Part of the limitations arise from the process of transpiration where trees release water vapour through their leaves which assists in cooling. In extreme heat, plants retain more water and this effect is significantly reduced. Researchers at the University of NSW have found that trees' ability to regulate urban temperatures have been significantly overestimated because of this effect. If the same transpiration rates are to be seen in the summer months, irrigation is an important factor – meaning water management is an issue as well.
A focus on multi-level plantings
Research out of the University of Melbourne, in collaboration with The Education University of Hong Kong and The Technical University of Munich, looked at increased vegetation complexity (trees, shrubs and grasses) across a variety of urban landscapes and their impact on the urban heat island effect. They found that local climate conditions and the city layout are important factors to consider when designing new green infrastructure. Hong Kong’s climate is humid, and a dense/complex planting strategy would lead to increased water vapour which only adds to the sticky humid conditions. Munich's heat issues arose as a lot of vegetation in narrow streets trapped air and reduced wind speed. This is ideal in winter months as it reduces body heat loss, but has the opposite effect in the summer, as dry hot air is stagnant, decreasing comfort.
However, the research found that complex plantings are highly effective in large open spaces which don't have airflow issues. Multi-layered planting techniques in Munich of trees, shrubs and grasses showed significant cooling benefits with reductions from 7.8 to 18 degrees Celsius. Additionally, the planting of indigenous plants improved biodiversity and habitat complexity for native species.
It's clear a ‘one size fits all’ approach is not applicable for greenery when it comes to combating the urban heat island effect. Solutions should be tailored for each city with the natural climate and urban landscape playing important roles. Hong Kong would likely benefit from reduced plant complexity focusing instead on shade and canopy cover from large trees, and Munich could take advantage from cool roofs which are designed to have high reflectance.
The right type of plant for the right environment
Planting the right type of plant for the right environment will help future-proof cities. This is where programs such as the ‘Which Plant Where’ project at Western Sydney University and Macquarie University are important for helping to improve planting practices.
Which Plant Where is an online database backed by years of scientific research. The research provides recommendations about what could be planted around Australia, based on the specific climate conditions for that area. It also shows how future-proof the species is if average temperatures continue to rise up to 2070.
These programs are highly important for future-proofing our cities and improving green infrastructure. While large-scale urban planning can benefit from resources such as these, it is also beneficial to members of the public.
Public involvement in combating the urban heat island effect is vital, based on a Macquarie University studyconducted in Western Adelaide. The researchers found that while backyards only make up 20% of urban land, they make up 40% of urban tree cover, and these small green spaces reduced localised temperatures by approximately 5-6 ˚C. Making sure these green spaces last with strategic planting practices will ensure temperatures remain cool in the suburbs.
And greenery is part of a broader strategy to reduce urban heat and the effects of climate change. Wholesale causal approaches focusing on renewable energies are an obvious part of the solution. And other mitigation strategies such as using reflective materials and green rooves, and increasing air flow between buildings will be important too.
Article by Finnegan O'Hara
Photo by Muhammad Nabeel