Interviewer: Listening to Pacific Mornings, we're wondering you how do trees keep our cities cool and do plants and trees in cities make you feel good?
Well, we're talking about that next here on Pacific Mornings.
Well, there's an urban rainforest building outside my workplace that's won many awards overseas for its positive environmental impact and I'm told that a Pacific Island relative of mine was one of the engineers who worked with the architects and helped to bring these new type of building a green rainforest Skyrise building into the middle of the city.
Now, I'm wondering, do you have any plants where you live perhaps in your backyard or in parks or on the side of the road? Do you enjoy keeping pot plants in your home? How important are green spaces in the cities, and does exposure to plants make us feel better with our planet keeping warmer and getting warmer each year? Will plants survive in hotter climates?
Joining me now is Dr. Renee Marchin Prokopavicius, a Postdoctoral Research Fellow at Western Sydney University. So, Renee, why are green spaces important in cities?
Renee: Well, cities heat more than the surrounding countryside and that's what's called the urban heat island effect. And cities can be several degrees warmer and that's due to the concrete, the buildings, the asphalt roads and all of the dark flat surfaces in cities that absorb the sun's energy.
The temperature difference is usually highest at night and at night it can be as much as seven or even 12 degrees Celsius warmer than the surrounding areas. And such high temperatures have negative effects on human health. They lead to incidents of higher numbers of heat stroke victims and deaths on heatwave days.
So having green spaces in cities is important to help prevent these high temperatures and counteract that urban heat island effect.
Interviewer: Right. So how did trees do it? How do they cool our cities?
Renee: Trees provide two important functions that help to cool cities. So the first is shading so kind of much as an umbrella, tree leaves reflect about one-third of the sun's energy rather than absorbing it. And that's in fact why plants appear green because they reflect the green wavelengths of visible light. And they only absorb the red and the blue wavelengths for photosynthesis and plants only need about five to 10 percent of the energy and sunlight to drive photosynthesis.
So the second function that trees provide is that they transpire a lot of water out of their leaves and this has a cooling effect. So there is a recent study that was done at the Hawkesbury Institute for the Environment at Western Sydney University and that's where I work and the study found that Eucalyptus trees increase rates of transpiration during these hot heatwave days and that's in order to cool their leaves.
So that's a lot like how humans sweat, how having water evaporate off your skin cools your skin. And so that's how trees regulate a cool temperature and, all in all, trees can cause cities by seven to 15 or even more degrees Celsius on the really hot sunny days.
The temperature difference is usually highest at night and at night it can be as much as seven or even 12 degrees Celsius warmer than the surrounding areas.
Interviewer: Wow, that's incredible! So in what ways are green environments more pleasant to be in?
Renee: Well, besides the shading and the cooling factors, trees have a number of other benefits. They can help reduce home energy consumption.
So, for instance, my parents live in the hot deserts of Arizona in the southwestern United States and they recently planted trees in their front yard to help shade the house and reduce their air conditioning costs.
Trees in urban streets also remove air pollutants from the atmosphere and they take up carbon dioxide from the air. Vegetation along creeks or rivers can help reduce runoff and flooding during major storm events.
And then, of course, it's kind of obvious but large green spaces such as parks provide space for people to interact on the weekends for social events with family and friends for sports activities and the green spaces are just nice to look at. It's not just a concrete jungle anymore. And then there are wildlife green spaces provide food sources and shelter for wildlife.
And here in Sydney, we had some recent really hot days in January where in Penrith where I live it got up to 47 degrees Celsius and then other suburbs such as Campbelltown and south and it got to 44 degrees and this led to the death of hundreds of flying foxes. But in other Sydney suburbs besides Campbelltown that had more vegetation understorey, there is more shading and more cooling and the death toll wasn't quite as high.
Dr Prokopavicius is training our research assistants how to measure thermal and drought tolerances of horticultural plant species for the WPW project. @MQBiology @westsyduhie @Macquarie_Uni @Hort_Au @NGI_NEWS #climatechange #WomenInSTEMM #urbangreening pic.twitter.com/ZW7bSRtZIH— WhichPlantWhere (@WhichPlant) April 9, 2018
Interviewer: Are these the reasons why green spaces are important in cities?
Renee: Yes yes. I think that there is are so many reasons why green spaces are important and in Australian cities we're seeing that there is less green space over the last few years and so we need to work to try to improve them because of all these important benefits.
Interviewer: So in improving them, is plant diversity important too to the wildlife that exists in an urban space?
Renee: Yeah. So often in streets, you'll see that one species lines the entire street. Yes, but throughout the city, it's important to vary the different tree and shrub species and that increases the diversity of the wildlife.
And so studies have found that having shrubs that have different growth forms and having trees that have different flower colours they help to increase the diversity the diversity of insect communities and those insects then form the diet of birds bats and mammals that are in city wildlife.
And then there are also issues such as native trees that provide more benefits for native wildlife over exotic tree species.
Interviewer: You're listening to Pacific mornings. Right now I'm speaking with Dr. Renee Marchin Prokopavicius and she's a Postdoctoral Research Fellow at Western Sydney University and at the moment we are discussing trees and green spaces in urban spaces. So how will climate change affect the plants and cities?
Renee: Well, most people today are aware that climate change involves a rise in temperature and that's due to the increase in carbon dioxide in Earth's atmosphere.
We're also starting to experience more frequent heatwaves and more extreme temperatures at the same time that we're experiencing more frequent and more severe drought and heatwaves during drought.
That's a double whammy for our plants and it can lead to scorch leaves and canopy dieback and even death of individuals and in cities, it's even worse than in surrounding countrysides because of the urban heat island effect.
So the heat is compounded and when plants become stressed, then they are weakened. And that leaves them more susceptible to insect pest and pathogens attack.
Interviewer: Right, so with rising temperatures, do we need to plant more plants or different kinds of plants in the future to cope with the changing environment?
Renee: That's a good question. Ideally, we would do both. I already mentioned that there has been a decline [in greenspace area] in many cities in Australia due to urban development of our green spaces and so we definitely need to plant more plants. This is something that's already been recognised so in Sydney there is a movement for the government to plant 5 million trees by 2030 and that's to boost the tree canopy from 16 percent which is currently up to 40 percent.
But to be successful efforts like this that are going on and planting a lot of trees they need to plant the right types of plants and trees in cities typically lasts for 10 to 50 years but global temperature has already increased by 1 degree Celsius and within the next few decades we could see further temperature rises of 2 or even up to 5 degrees Celsius.
And so some of the plant species that are popular to plant in cities right now such as Elms and Poplars, these trees that are really from the northern hemisphere from cooler environments they're not going to be able to handle the extreme temperatures in the future. And so instead we need to find different horticultural varieties or species that can tolerate the warmer future climates.
Studies have found that having shrubs that have different growth forms and having trees that have different flower colours they help to increase the diversity the diversity of insect communities and those insects then form the diet of birds bats and mammals that are in city wildlife.
Interviewer: Right. So how are you helping to improve how green cities are?
Renee: I work on the Which Plant Where project which is a collaborative project between Western Sydney University and Macquarie University and it's funded by Hort Innovation Australia and the Office of Environment and Heritage.
So our project aims to select the right plants for the right urban space with an eye to the future. I'm a plant ecologist and a plant physiologist and this past year I've been running a series of glasshouse experiments.
We've already tested and grown 50 plant species under the same conditions and the glasshouse in order to measure their heat and drought tolerance. And we're planning to do another series of experiments next summer. Then some of my colleagues are working to develop maps that show the suitability of different plant species to current and future climates throughout Australia.
We're also starting to establish test plantings of different tree species in Melbourne and Sydney and so, at the end of this project, all of the work is going to be compiled into an interactive online tool that will help users to select plant species that would be ideal for any location throughout Australia based on a range of filters and those filters do include climate tolerances.
This is a big project so we're not due to finish until 2021.
Interviewer: Right. Are there different considerations for greening cities in the Pacific Islands?
Renee: Yes the recommendations of plant species selection are really always tied to the climate of a city and, for example, plants that are native to human cities here in southeastern Australia they don't always perform well in cities such as Perth that have a more hot and dry climate.
That also means that cities with similar climate profiles then can usually support similar plant species and it's best to use local knowledge when choosing species for urban forests and you want to avoid characteristics such as tree species that tend to have large limbs that drop out of the canopy or that have aggressive root development or they're known for being weedy or potentially invasive.
And it is also important to consider the benefits of native vs. exotic species for particular studies since native plants may be more important for native wildlife.
I recommend that individual cities hold meetings for all relevant stakeholders to discuss and choose what they want for their natural and cultural heritage in their own city and then also as cities go and develop. It's important to prioritize parks and to connect parks into networks so that they can be used for walking and cycling.
Interviewer: Yes. Thank you so much for joining us on Pacific Mornings.
Renee: Yeah thank you.
Interviewer: That was Postdoctoral Research Fellow at Western Sydney University, Dr. Renee Marchin Prokopavicius. And you're listening to Pacific mornings on ABC Radio Australia.