Cities around the world are planting more trees to cope with rising urban heat. But our research shows trees alone are often not enough. In some cases, the wrong kind of greening can even make streets feel less comfortable on a hot day.
We compared field measurements from Melbourne, Munich and Hong Kong to test how different kinds of urban planting changed the heat people experience outdoors.
The results showed layered vegetation – where trees are combined with shrubs and ground cover – often cooled cities more effectively than trees alone. We also found local climate and street design strongly shaped whether greening worked well.
These findings matter because urban greening is no longer just about aesthetics. As cities spend billions adapting to extreme heat, planting design may matter as much as planting quantity.
Cities are getting hotter
Cities trap heat. Roads, buildings and asphalt absorb solar energy during the day and slowly release it back into the air, especially at night.
This “urban heat island” effect, combined with climate change, is making heatwaves more intense and more dangerous in our cities.
Trees are one of the most popular responses because they provide shade and reduce the amount of heat absorbed by surrounding surfaces. But outdoor comfort depends on more than air temperature alone.
People experience heat through sunlight, reflected heat, humidity and airflow. A shaded street can still feel uncomfortable if humidity is high or if wind cannot move through the space.
That is why a “one-size fits all” greening strategy can fail. A planting design that works well in Melbourne may behave very differently in Hong Kong or Munich.
What we found
To better understand how urban vegetation affects heat stress, we did field measurements in three cities with different climates: temperate Melbourne, cooler Munich and humid subtropical Hong Kong.
Rather than relying only on computer models, we measured real conditions in streets and green spaces during summer.
We compared open urban spaces (with no plantings), sites with trees only, and layered planting (which means trees, shrubs and ground cover together).
Importantly, we did not just measure air temperature. We also measured “mean radiant temperature”, which captures the heat radiating from roads, walls and other surfaces onto the human body.
In Melbourne, street trees reduced radiant heat absorbed by pedestrians by more than 18°C, compared with open streets. Even where air temperatures changed only slightly, shaded streets felt substantially cooler.
Munich showed the strongest benefits from layered planting. There, streets and green spaces containing trees, shrubs and ground cover reduced afternoon heat stress by almost 8°C compared with more open spaces.
Hong Kong also benefited from vegetation, especially through shade created by overlapping tree canopies. But the results there were more mixed because the humid climate changed how cooling worked (more on that later).
Across all three cities, one finding stood out: vegetation structure matters.
Combining trees with shrubs and ground cover often performed better than trees alone, but the benefits depended on how the planting interacted with the local environment.
Why some greening can fail
The study showed that more vegetation is not automatically better.
In Hong Kong, dense vegetation sometimes increased humidity enough to reduce some of the cooling benefit. Plants release water vapour into the air through transpiration, which can help to cool dry climates. But in already humid cities, extra moisture can make outdoor spaces feel sticky and uncomfortable because sweat evaporates less efficiently.
In some Munich streets, dense vegetation reduced airflow through narrow urban corridors, trapping warm air and slowing the movement of vehicle pollution away from pedestrians.
These findings highlight why cities cannot rely on generic canopy targets copied from elsewhere. Climate, street width and airflow all shape whether vegetation improves comfort or creates unintended side effects.
Designing cooler cities
The solution is not to stop planting trees. It is to design urban greening more carefully.
Cities need planting strategies tailored to local conditions rather than universal greening formulas. In parks and open green spaces, layered vegetation can provide strong cooling while also supporting biodiversity. In dense streets, planners may need to balance shade with ventilation.
The findings also suggest cities should move beyond measuring success through tree numbers alone. The arrangement, density and type of vegetation matter just as much as canopy cover.
Designing for local conditions
Our research shows urban vegetation can reduce heat stress, but the benefits depend on how and where cities plant it.
Melbourne demonstrated the strong cooling effect of street trees on radiant heat, Munich showed the added value of layered vegetation, and Hong Kong revealed how dense planting can sometimes backfire in humid conditions.
Cities need climate-smart green spaces designed for local conditions, airflow and human comfort to remain liveable as temperatures rise.
Mohammad A Rahman receives funding from the German Research Foundation (DFG), TREE Fund, Humboldt Foundation, Bavarian State Ministry of the Environment and Consumer Protection, German Academic Exchange Service (DAAD), Sustainable Consumption Institute (SCI), University of Manchester and the European Union.
Last year, we noted early signs of recovery in Australia’s high country, following the reduction of feral horse numbers.
These had dropped from 17,000 in 2023 to around 3,000 in 2024 across Kosciuszko National Park, thanks to the management efforts of NSW National Parks staff and contractors.
But horse numbers are already bouncing back. The latest survey data estimate between 6,476 and 16,411 horses now roam the national park.
So, what happened?
A mild summer
The answer is simple. If feral horse eradication is impossible — or politically and legally off the table — then continuous management of horse numbers is essential.
With no aerial culling within the national park in 2025, two factors likely contributed to this rapid rebound.
First, horses move. Control efforts have largely focused on remote parts of Kosciuszko National Park, away from people, trails and roads. Once resident herds in these areas have been culled, horses from surrounding regions – particularly adjacent state forests – likely moved in.
Second, horses breed. After a mild summer with significant rainfall across the high country, most mares will have bred. During Autumn fieldwork, we observed large numbers of foals accompanying herds throughout the region.
If feral horse numbers aren’t rapidly reduced again, things will get worse for the alpine environment and the horses themselves.crbellette/Getty
A numbers game
If numbers aren’t rapidly reduced again, things will only get worse, both for the fragile alpine environment and the horses themselves. With winter conditions imminent, many horses will struggle to maintain condition as snow covers grazing areas and energy reserves are depleted.
Ironically, some of the strongest opposition to culling overlooks these very real animal welfare consequences. Leaving horse populations unmanaged may ultimately result in prolonged suffering from starvation and exposure, compared with humane control conducted by trained professionals.
Forecast El Niño conditions may further compound these pressures, with drought likely to persist through spring and summer. As water and food become scarce, horses will likely concentrate around creeks, wetlands, alpine bogs, fens and meadows. These are precisely the alpine ecosystems most vulnerable to trampling, grazing and erosion.
And this is where hard-fought gains will be rapidly lost. Banks will become eroded, clear waters fouled and our fabled high plains replaced by overgrazed paddocks.
A long-term effort
We don’t need to look far to see what happens when a population of feral animals goes unchecked. Great Keppel Island, for example, is overrun with a thousand or more feral goats, denuding dune and forcing increasingly exasperated locals to erect fences around their properties
As with horses in Kosciuszko, political hesitancy and delayed action on Great Keppel have allowed ecological damage to escalate while management becomes increasingly difficult and expensive.
New South Wales Environment Minister, Penny Sharpe, recently said the latest Kosciuszko feral horse numbers confirmed the need for “continued management”, required to meet the target of reducing feral horse numbers to 3,000 by mid-2027.
But where did that target come from? It’s a holdover from the repealed Kosciuszko Wild Horse Heritage Act and, when even basic population growth models are applied, the implications become clear. Maintaining a population of 3,000 horses would still require the removal of well over 1,000 animals every two years — indefinitely.
In other words, there is no “set and forget” solution. If horse populations are to remain capped, ongoing culling will be necessary in perpetuity.
Alternative solutions?
Some have suggested that instead of culling, rehoming and fertility control should be used. While many Australians might like the idea of a “brumby” or two grazing in the back paddock, the number of landholders willing and able to care for these animals is far smaller.
Even retired racehorses struggle to find suitable long-term homes once their racing careers end, highlighting the practical limitations of large-scale rehoming programs.
Likewise, although various fertility control options have been suggested, vaccines, intra-uterine devices or surgical sterilisation are all invasive procedures for which horses need to be caught and sedated. These may be effective to maintain a small herd in an easily accessible area. But previous assessments have warned such an approach must be carried out in concert with large scale culling efforts.
Population dynamics vs politics
We don’t have to look far to find other examples of how invasive species management could be improved. In 2016, then New Zealand Prime Minister John Key introduced a bold plan to rid Aotearoa of all introduced predators in the next 30 years.
Predator Free 2050 is the first national-scale initiative to reduce the impacts of introduced predators, capitalising on the invention of new technologies including real-time automated species identification to trap targeted species and mobilising neighbourhoods across the country to join the effort.
Australia faces a different set of challenges — larger landscapes, divided jurisdictions and deeply entrenched cultural and political debates around invasive species management.
But the broader lesson remains the same: meaningful conservation outcomes require long-term commitment, clear targets and the willingness to act before ecological problems become too difficult to reverse.
David M Watson receives funding from the Australian Government (DAFF and DCCEEW).
Patrick Finnerty does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
Fireworks set off without permission by a police officer during a rehearsal for Italy’s Republic Day parade startled dozens of horses, sending them bolting through the capital and injuring several riders.
Australia is pioneering a revolution in home renewables and battery use, proving what is possible with the right policies
The timing was rich with symbolism. As intense heatwaves pummelled Europe and Asia, and oil markets around the world leapt and sputtered, the two big chimneys of one of Australia’s largest power stations were being demolished. Meanwhile, the Australian energy minister was holding a media conference to hail a fall of up to 10% in the benchmark electricity price in parts of the country.
Quietly, and with surprisingly little fanfare from the rest of the world, Australia is pioneering a revolution in home renewables and battery use, proving what is possible with the right policies. The country was already one of the global leaders in domestic solar power, with panels on one in three homes. It also remains, however, a major contributor to the climate crisis through its vast fossil fuel exports.But it is batteries that are giving Australia a new burst of speed.
