How Urban Trees Can Cool Cities in a Warming Climate

Cities are becoming hotter in ways residents notice every day. On a summer evening, stepping outdoors brings little relief as the heat lingers. Streets and buildings retain warmth well beyond sunset.

Some neighbourhoods remain cooler thanks to tree-lined roads. Others are left exposed, with minimal shade and increasing discomfort. That difference influences health, energy consumption and everyday living.

Urban trees provide a practical means of cooling cities. They shade surfaces, release moisture and enhance air quality. However, putting trees in the ground is only the beginning.

Helping them survive demanding urban environments is considerably harder. A recent study in Dayton, Ohio, demonstrates what is required to establish trees in the areas that need them most.

Cities will become hotter

Concrete, asphalt and brick cause urban areas to retain heat. During the day, these materials take in sunlight, then gradually give off that stored heat overnight.

This is known as the urban heat island effect, and it increases temperatures relative to nearby rural locations.

Trees can limit this effect by creating shade and reducing surface temperatures.

Research indicates that tree cover can lower daily urban temperatures by roughly 3 degrees Celsius (5.4 degrees Fahrenheit). By reducing the need for cooling, trees can also cut electricity demand.

“It's going to keep getting hotter in the future,” said Erika Wright, first author of the study and a student at The Ohio State University.

“So we need to know if we should be trying to create environments that are more resilient to those changes.”

Legacy cities face risks

Dayton is an example of a city type referred to as a legacy city. These places previously had thriving industrial economies but have since experienced losses in population and income. Since the 1970s, Dayton has lost around half of its population.

With reduced resources available, green-space maintenance becomes more challenging. Tree cover is extremely limited in many neighbourhoods, with canopy coverage reaching only five to eight percent in certain areas.

The shortage of shade raises heat exposure and associated health risks, particularly for older people.

“People don’t necessarily think of how their yard connects to the surrounding environment, but we all contribute to the urban forest by planting in our own space,” said Wright.

Planting trees in cities

Across 20 locations in Dayton, researchers planted 640 saplings, including in parks and on one vacant plot.

The sites differed in the proportion of nearby land covered by hard surfaces, ranging from 7 percent to 65 percent.

The researchers chose eight native tree species, among them red maple, northern catalpa and white oak. Every site received an identical species mix, allowing survival to be compared across varying conditions.

Testing watering methods

Water is crucial to the survival of young trees. The research team examined four watering methods. One set of trees received rainfall alone, while another was given 19 litres (5 gallons) of water every fortnight.

A third set received the same volume once per month. The final group was supplied with gator bags, which hold around 75 litres (20 gallons) and slowly release water over time.

These approaches represent the practical decisions cities face. Regular tree watering involves both labour and expense, whereas slow-release systems require an initial financial outlay.

Half of the trees survived

At the end of the growing season, just about 48 percent of the trees were still alive. A number of trees were also missing, probably having been removed or damaged.

Survival rates differed between species. Red maple performed particularly well, achieving 91 percent survival. Black gum fared poorly, with survival of only 10 percent. White oak likewise had a low survival rate.

The health of trees when planted was also significant. Certain species were in poor condition from the outset, indicating that nursery quality may be a factor.

Gator bags performed best

For most species, gator bags produced the strongest outcomes. By supplying water gradually, they enabled deeper absorption into the soil. This allowed trees to cope more effectively with dry conditions.

The bags are not without cost, however. Each costs approximately 30 dollars, making them costly when used for hundreds of trees. Some were also damaged by machinery or by people, creating an additional difficulty.

Heat affects species differently

Trees do not all react to heat in the same manner. Some species had greater difficulty in locations with extensive paving. Black gum survival fell sharply at hotter sites.

Other species coped with heat more successfully. Northern catalpa performed strongly at every site. Red maple and honey locust also demonstrated considerable resilience, maintaining healthy growth even under warmer conditions.

Human impact on trees

Environmental pressures are not the sole danger to young trees. Human activity can also reduce survival. At one park, 30 of 32 saplings vanished after being snapped or completely removed by people using the area.

“Even after the trees were pretty well established, we saw losses, which was surprising,” said Wright.

This underlines the importance of community awareness. Trees in public areas rely on care and protection from local people.

Diversity remains important

Planting only the hardiest trees may appear to be the sensible option. Yet depending on just a small number of species introduces risk, as diseases or pests can spread rapidly through uniform tree populations.

A more balanced strategy is preferable. Robust species can be planted initially to create shade, followed later by more sensitive varieties. This approach produces a more resilient and stable urban forest.

Key lessons for city trees

The study provides straightforward advice for city planners. First, tree species should be matched to the conditions at each location, since no single tree is suitable for every site. Second, early care deserves investment, because watering and maintenance in the first years increase survival.

Cities should also anticipate damage. Safeguarding young trees from mowing equipment and activity by the public supports their growth. Involving local residents may improve care and limit losses.

“When a city invests in reforestation of a park, you have to plant more trees than you want to last long term because there will be some percentage that dies off,” said co-author Mary Gardiner.

“But even small differences in management can mean huge variations in survivorship.”

Resources are wasted when trees are planted without continuing care. A tree that dies young delivers no benefit, while one that survives can cool streets for decades.

This means aftercare is among the most important elements of urban forestry. Even modest spending on maintenance can lead to better long-term results.

Resilient cities in a warming climate

In the study, half of the trees survived. Although that figure may seem low, it illustrates what can be achieved under difficult conditions. The surviving trees withstood heat, poor soils and restricted water supplies.

Survival rates can rise with improved planning. Cities can select more resilient species, use targeted watering and engage local communities.

“Looking for cost-effective strategies for cities to be resilient in the face of a warming climate is an important tactic,” said Gardiner. “Conservation like this leads to a great payoff in the long term.”

Urban forests need time to develop. Decisions taken now will influence life in cities for many years ahead.