Cities all over the globe are on the frontline of climate change. GrowingFor more urban cooling. Many governments are spending big on new trees in public places – but which species are most likely to thrive in a warmer world?
Numerical targets such as “one million trees” dominate tree-planting programs in cities such as Los Angeles, New York, Shanghai, Melbourne Sydney. But whacking a million trees into the ground won’t necessarily mean greener suburbs in decades to come.
It is not often enough to ensure that the trees are selected and watered properly so that they can withstand a hotter, dryer climate in the future.
Our website Recent researchWe evaluated the effects of drought and extreme heat on urban tree species. Some beloved tree species, which are often planted in large numbers throughout our cities, were unable to adapt to the changing conditions. This shows how crucial it is to make decisions today in order for urban greening programs that will succeed in a warmer world.
Experiment in a hothouse
In January 2020, following several years of drought, Penrith in Western Sydney hit 48.9℃ – the hottest temperatureGreater Sydney has ever seen such a record. Later, researchers assessed around 5,500 street trees. FoundMore than 10% of the canopy damaged were found to be severe. The worst was experienced by exotic deciduous species.
This event demonstrated how intense heat and drought can simultaneously damage urban trees.
Trees cool down in high temperatures by losing water through tiny openings in their leaves called Stomata. Trees that have been adequately watered are more likely to tolerate extreme heat than trees that aren’t.
Our researchStress testing of 20 broadleaf, evergreen tree species in habitats ranging between tropical rainforests and semiarid woodlands was involved.
The seedlings were grown in a coordinated greenhouse experiment. After the plants were established and acclimatised, half of them – five plants per species – were exposed to a gradual, five-week drought.
All plants were exposed during the final week of water deficiency to conditions that simulated a six day heatwave.
What we found
These conditions were different for each of the 20 plant species.
Two species of plants were subject to heat and drought. The crowns of four other species suffered severe crown damage.
Although most plants recovered from the heatwave, a few plants died.Banksia robur) and one crimson bottlebrush (Callistemon citrinus).
Species that have dense wood and small, thick and dense leaves are more water efficient and drought-tolerant. Our study found that orange jasmine was the most successful species.Murraya paniculata), inland rosewood (Alectryon oleifolius) and Australian teak (Flindersia australis).
Even though plants have water access, the tolerance of heat stress is different for different species. Swamp banksia (Banksia robur) and powderpuff lilly pilly (Syzygium wilsonii) suffered extensive crown dieback even with access to water. This shows that both wet- and dry-year heatwaves could be dangerous for urban trees.
Some species may be able to withstand heat and drought, but they may not be the best option for cooling cities. Many species, such as leopardwood, are drought-tolerant.Flindersia maculosa) grow slowly and have sparse foliage that provides little shade or cooling. These species could be planted in sunny and dry areas to increase biodiversity and habitat.
So what about trees such as the weeping fig?Ficus microcarpa) and London plane tree (Platanusx Acerifolia), which are Widely plantedSydney, Melbourne, or other Australian cities
These trees are more vulnerable to heat and drought due to their soft, low-density timber and large, thin leaves that are susceptible to heat. But they are fast growing and can create large canopies to cool urban areas.
These trees should be planted wherever water is available, whether it comes from rain or active management like irrigation.
Looking forward to a hot future
Our research reveals how water access is critical for urban trees to survive during hotter, drier summers.
That means urban greening programs must also incorporate elements of so-called “blue” infrastructure – retaining water in urban landscapes via engineered solutions and making it available for plant uptake. Such infrastructure comes together under the umbrella of “water sensitive urban design”.
Examples include passive irrigation, which is where trees draw water from. Storage pits(containing stormwater) raingardens – garden beds that filter stormwater runoff. The best way to increase your chances of survival is to plant young trees in areas where this design is used.
These methods have multiple benefits: they increase the health of trees, prevent flooding during storms, and reduce the need to add irrigation from local water sources.
Extreme heat in cities around the globe will have a negative impact on infrastructure, natural environments, and citizens. Planning for urban trees in cities is crucial to ensure that the right balance is struck between trees that cool down our cities and those which can withstand more severe conditions.