How Urban Trees Help Dayton Cool Hotter Cities

Cities are becoming hotter in ways that people notice every day. On a summer evening, stepping outdoors no longer brings much relief, as roads and buildings continue to radiate heat long after the sun has set.

Some neighbourhoods remain cooler because their streets are lined with trees. Others are left exposed, with minimal shade and increasing discomfort. The difference has consequences for health, energy consumption and everyday life.

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

Keeping them alive in demanding urban environments is much harder. A recent study in Dayton, Ohio, illustrates what is needed to establish trees in the places where they are most necessary.

Cities will continue to get hotter

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

This is known as the urban heat island effect, which makes cities warmer than surrounding rural locations.

Trees can lessen this effect by providing shade and reducing surface temperatures.

Research indicates that tree cover can lower daily temperatures in cities by around 3 degrees Celsius. By reducing the demand for cooling, trees can also cut electricity use.

“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 legacy city: a place with a formerly robust industrial economy that has subsequently lost both residents and income. Since the 1970s, Dayton has lost roughly half of its population.

With reduced resources, green spaces are more difficult to maintain. Tree cover is extremely limited in many neighbourhoods, with canopy levels reaching only five to eight percent in some areas.

The shortage of shade increases exposure to heat and related health risks, particularly for older adults.

“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 Dayton

The researchers planted 640 saplings at 20 locations across Dayton, including parks and one vacant plot.

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

Eight native tree species were chosen, including red maple, northern catalpa and white oak. Every site received the same combination of species, enabling the researchers to compare survival in varying conditions.

Testing watering methods

Water is crucial to a tree’s survival during its early years. The researchers assessed four watering methods. One group received rainfall alone, while another was given 19 litres of water every two weeks.

A third group received the same volume once a month. The final group was fitted with gator bags, which hold around 75 litres and gradually release water over time.

These methods represent the practical decisions cities face. Regularly watering trees involves both labour and expense, whereas slow-release systems require an initial investment.

Half of the trees survived

At the end of the growing season, only around 48 percent of the trees were still alive. A number were also missing, probably because they had been removed or damaged.

Survival rates differed substantially between species. Red maple performed strongly, with 91 percent survival. Black gum fared poorly, surviving at a rate of just 10 percent. White oak also had a low survival rate.

Tree condition at the time of planting mattered as well. Several species were unhealthy from the outset, indicating that nursery quality may be a factor.

Gator bags delivered the best results

For most species, gator bags produced the strongest results. Their slow release of water enabled it to soak further into the soil, helping trees cope more effectively with dry conditions.

The bags are costly, however. Each costs about 30 dollars, so the expense rises quickly when hundreds of trees are involved. Some bags were also damaged by machinery or by people, creating an additional problem.

Heat affects tree species differently

Trees do not all react to heat in the same manner. Certain species struggled more in places with extensive paving, and black gum survival fell sharply at hotter sites.

Other species coped better with heat. Northern catalpa performed well at every site, while red maple and honey locust also demonstrated strong resilience. These species continued to grow healthily even in warmer conditions.

Human impact on trees

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

“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 spaces rely on care and protection from local people.

Diversity remains important

Planting only the most robust trees may appear to be the practical solution. Yet dependence on a small number of species creates its own risk, because pests or diseases can spread rapidly through uniform tree populations.

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

Key lessons for city trees

The study provides clear advice for city planners. First, tree species should be matched to local conditions, as not every tree is suitable for every location. Second, early care is worth investing in: watering and maintenance in the first few years raise survival rates.

Cities should also prepare for damage. Safeguarding young trees from mowing equipment and public activity helps them establish, while involving local residents can strengthen care and reduce 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.”

Planting trees without providing follow-up care wastes resources. A tree that dies young delivers no benefit, whereas one that survives can cool streets for decades.

This makes aftercare among the most important elements of urban forestry. Even modest maintenance investment can improve long-term results.

Resilient cities in a warming climate

Half of the trees in the study survived. Although this may appear low, it demonstrates what can be achieved even under difficult conditions. The surviving trees endured heat, poor soil and restricted water supplies.

Improved planning can raise survival rates. Cities can select more resilient species, use targeted watering and involve their 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 mature. Decisions made now will influence life in cities for many years ahead.