How trees in urban areas are key to cooling down a warmer world

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How trees in urban areas are key to cooling down a warmer world

Forest-building a sustainable solution to future heat waves, study finds

Planting new forests may be a low-cost way to combat warming temperatures in urban areas, suggests a new study.

In a large-scale field experiment, researchers planted 640 tree saplings across 20 parks in Dayton, Ohio, and implemented varying irrigation methods. After monitoring sapling survival, growth and health in response to their irrigation methods and nearby temperatures, the team found that the effects of both water treatment and surrounding heat varied among tree species.

An analysis of saplings at season's end revealed an overall survival rate of about 48%, indicating that irrigation approaches impacted species' health differently: Certain species, such as red maple, northern catalpa and honey locust, consistently thrived over others, particularly the white oak, black gum and sassafras saplings.

Understanding the causes behind these environmental disparities could make urban forests both a lucrative and sustainable option for preventive climate action, said Erika Wright, first author of the study and a graduate student in entomology at The Ohio State University.

"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.

Urban forests - essentially functioning as citywide ecosystems because they include all the green spaces in a community - also provide valuable benefits to the health and well-being of local residents. Besides beautifying the environment and supporting biodiversity and wildlife, increased shade offers natural cooling that can reduce consumers' summer electricity usage.

Yet obstacles to expanding urban forests often include limited financial and labor resources to care for and maintain them, said Wright. To better consider these challenges, researchers sought to partner on a planting project with a legacy city, an urban area that once had a booming economy but whose shrinking population now lacks the means to manage long-term reforestation projects.

"It's going to keep getting hotter in the future," said Wright. "So we need to know if we should be trying to create environments that are more resilient to those changes."

The study was recently published in the journal Urban Forestry & Urban Greening.

The study goal was to test the efficacy of different irrigation strategies on tree development, but with the saplings scattered throughout the city, researchers did not have access to public water sources and had to rely on water transported from nearby fire hydrants.

Nonetheless, the team predicted that irrigation investments with higher input costs would lead to higher sapling survival, increased growth and improved health compared to less costly irrigation investments, outcomes expected to be partly related to lower water stress in regularly watered saplings.

Actual sapling survival rates turned out to be slightly lower than the five-year post planting survival rates recorded in previous studies, but the researchers did find that the best watering tactic involved using gator bags - plastic bags that are typically applied for slow-release watering.

While filling them once per month was not labor-intensive, researchers did note that the up-front and replacement cost of gator bags was high, illustrating the challenging nature of even securing the right supplies to successfully establish and expand urban forests, said Mary Gardiner, senior author of the study and a professor of entomology at Ohio State.

"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 Gardiner. "But even small differences in management can mean huge variations in survivorship."

According to the study, of the 640 trees seeded, a large portion either went missing or were destroyed by outside factors, likely due to environmental or human interference. "Even after the trees were pretty well established, we saw losses, which was surprising," said Wright.

For legacy cities, such threats make gator bags a low- to mid-cost irrigation option as long as precautions like protective fencing are taken to ensure that the bags are not damaged. These investments, combined with having volunteers ensure sapling stock is of high quality before planting, could lead to impressive strides in future forest growth, said Gardiner.

Overall, while the study's results indicate that certain species performed better under heat and water stress, researchers don't recommend limiting planting to only those few species, as urban forests with a mix of trees tend to be more resilient to pest and disease outbreaks.

Instead, researchers recommend a tailored approach that takes elements like tree species, irrigation resources and surrounding infrastructure into consideration. It may also be worth exploring whether urban forest resilience could be bolstered by non-native tree species.

"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."

Other co-authors include Ellen Danford and Samuel Ward from Ohio State and Christopher Riley from Casey Trees Farm.

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