Trees are vital for cooler cities, but they need help too

Walking beneath the trees this historically hot summer, sometimes felt like autumn had already arrived: dry, dead leaves crunching underfoot, and crowns tinged yellow or completely bare. 

That's a problem because large leafy trees help make our cities more livable. They block sunlight, reduce pollution, store carbon, provide habitats for biodiversity to thrive, soak up otherwise harmful stormwater runoff and even boost the mental and physical health of residents. 

"When trees go away, those things go away as well," Nina Bassuk, who runs the Urban Horticulture Institute at the US-based Cornell University, told DW.  

That's why cities and scientists are racing to figure out how to save trees and choose smarter ones for the future. 

Why cities need trees 

As humans have burned fossil fuels to power industry and heat homes, the planet has become hotter, making the role of urban trees ever more vital.  

Why? Because they essentially act like giant air conditioners, which makes a big difference in hot cities. Inside a tree's canopy, temperatures can be up to three degrees cooler than the surrounding area. This is down to evapotranspiration, the process of trees absorbing water and releasing water vapor through their leaves. 

In addition, trees cast shade, which keeps their surroundings from heating up in the first place. 

"When we stand under a tree, the temperature we perceive can be 10 to 12 degrees Celsius (18 to 22 degrees Fahrenheit) lower than right next to it," said Matthias Beyer, a hydrologist at the Institute of Geoecology at TU Braunschweig.  

And it's not just cooling. There have been numerous
studies correlating all sorts of other positive effects of robust trees.

One showed that cities with more tree canopy had lower rates of cardiovascular disease. Another concluded that greener school surroundings were associated with better grades in high schools. And further research suggested that more greenspace exposure during pregnancy can lead to healthier outcomes for newborns."

How much water do trees need to survive drought? 

Though a warmer climate makes trees more necessary, it also makes it harder for them to thrive. 

Healthy trees need a minimum of 10 to 15 liters of water a day, but mature ones with full foliage require up to four times that. 

While there may be a temptation for cities to focus on planting new, less water-hungry trees, experts agree that the priority should be protecting older, struggling ones. 

Research shows it would take around 400 saplings to replace the benefits of an old tree with a crown circumference of 20 meters. Once trees reach maturation, their benefits can be exponential, but this takes time and lots of maintenance.  

"A lot of trees can't deliver the full function until 40, 50 years from the day they were planted," said Henrik Sjoman, a researcher at the Swedish University of Agricultural Sciences. 

The best way to water trees 

It's better to give trees a large amount of water over a short period so it can soak down deep, rather than letting it stay in the upper soil layers, where much of it evaporates. 

For newly planted trees, a ring of metal or sturdy plastic is often fixed into the ground around the trunk. Where that's not available, a shallow depression around the tree can be created, allowing the water to pool. What matters is not damaging any surface roots in the process. 

When the soil is extremely hardened, so-called irrigation bags or cushions can help. Water drips slowly but steadily into the soil through small holes on their underside. It's  important that the water doesn't sit on the surface for long periods, since that can cause rot, which damages the tree.  

But sometimes the soil has been heavily compacted to support sidewalks above, meaning water can't get through at all. Thirty years ago, Bassuk helped pioneer a method that mixes specially-sized gravel in with the soil. It allows the stones help bear the load of the sidewalk, while leaving more space for roots to stretch across dense urban ground. 

"As long as you get the soil and the stone proportions right, you can compact it and yet trees can access that and grow for long distances," Bassuk said. 

How to choose the right trees for cities

When it comes to climate adaptation, there's also the question of which trees to plant in the first place.  

Historically, Sjoman says urban tree selection was often driven by aesthetics and design, which led to the over-planting of single species that were vulnerable to threats like pests or disease. Horror stories of Dutch elm disease wiping out entire populations in the 20th century helped to transform urban forestry planning, placing the focus far more on diversity and climate adaptation. 

The 10-20-30 rule of thumb was coined in 1990 by the late
Frank Santamour, who worked as a research geneticist at the US National Arboretum. It states that urban forest populations should include no more than 10% of any one species, 20% of a single genus or 30% of any family. 

In Germany, there are several tools and databases that offer planting guidance, and a number of cities are taking part in an official street tree test. The idea is that 45 species are tested at more than 220 locations to gather knowledge of
how they thrive. 

Another resource is the CITREE database, which categorizes almost 400 species of woody plants, sortable by more than 50 different features, including leaf density, fruit color, allergy potential, limb breakage and more.  

The key feature cities need to select for is droughttolerance — considering the immense biological strain urban
trees are facing in an ever-warming world. But knowing the benefits trees offer a city, means that is not the only factor.

"The demands of trees today — it's so much bigger, so much deeper than it was just a couple of decades ago," Sjoman said.

This article was adapted from German, with additional reporting.

Edited by: Tamsin Walker

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