Source: American Forests

Are trees one of the most effective tools cities have for keeping cool in a warming world? Or “more worrisome than advantageous”?

Morningside resident Donyelle Pressley has a complicated relationship with her neighborhood’s trees. Many streets in this east side Detroit neighborhood have large oaks and maples that form a solid tunnel of greenery in the summer. But the trees are often old, and they frequently drop large branches or even whole trees onto roadways and rooftops, creating headaches for residents.

“I love them, they make the street look beautiful,” Pressley told Planet Detroit. “But I’d like to see less of them, and some that can be better cared for.”

Pressley worries that the pollen from flowering trees may be aggravating her asthma, and she said her neighborhood experiences more blackouts because storms frequently take down trees and branches, knocking out power lines. 

“They create dangerous situations,” she said, recounting how a large tree fell in front of her house, blocking the street. For a week, cars drove through her front yard rather than backing up and turning around. 

Another Morningside resident, Ellen Davis, has similar concerns. She said that the city-managed trees on the median next to the street are cracking the sidewalk, and that her home insurance might not be renewed because of the threat from overhanging branches. She acknowledges the aesthetic and environmental benefits of the urban canopy, but said, “It saddens me to say that right now I am not a tree lover anymore.”

And yet, there may have never been a time when Detroit has needed trees more. A healthy urban canopy represents one of the most effective strategies for dealing with periods of extreme heat and flooding, which — according to the National Climate Assessment — are likely to be the two most significant impacts of climate change in the Midwest. Planting trees could be the difference between life and death when heat waves hit the city, as they are expected to do more often as the climate warms.

Treed areas of cities are as much as 20 degrees cooler than those lacking a solid tree canopy and they could buffer against the extreme heat waves that some estimate could kill 760 Detroiters every summer by 2046. That scale of loss is roughly equivalent to the notoriously deadly Chicago Heatwave of 1995, except that it would be occuring in a smaller city and it could happen every single summer. 

Eric Candela, manager of the Community ReLeaf program for the non-profit American Forests argues that Detroit needs an ambitious tree planting program akin to its $185 million investment in streetlights several years ago. 

“We should think about trees probably in a very similar way,” Candela said. “The net effect is that we would be creating a much healthier, better functioning cooling system for the city of Detroit.” But the city will likely need to do a lot of planting in the next few years to head off some of the worst impacts of climate change while also getting buy-in from residents who have been frustrated by years of poor maintenance, power outages and other issues that have made trees seem like more of a liability than a vital piece of city infrastructure.

Davis’ and Pressley’ experiences are what one might expect in a city where Candella noted the plantings are “older, less diverse, and generally not as well-suited to an urban environment as they should be.”  

Detroit’s canopy was hit hard by Dutch elm disease in the 1950s and then, more recently, emerald ash borer, which still routinely fells large trees. The trees that remain are often old, with a limited number of species representing the majority of plantings. Maples, for example, make up about 41% of street trees in Detroit, according to a 2016 study from the Davey Resource Group, rendering them vulnerable to new pests and diseases like the Asian longhorned beetle, which has emerged as a problem in several eastern states.

The Davey study also found that the “stocking level”– or percentage of city land on streets that is suitable for trees and currently occupied by them – is 48%, where the report recommends levels of at least 90%. 

Yet, most Detroit trees are on private property. The overall canopy cover for the city stands at 24%, with wide variations between different areas. For example, the coverage in southwest Detroit stands at 15.6%, while the city’s northwest corner has canopy cover over 37.5% of its land. Candela said Detroit’s overall level of tree cover is, “not horrific, but less than what would be desirable”. 

Fai Foen, director of green infrastructure for The Greening of Detroit, says the nonprofit currently plants between 2,000 and 2,500 trees a year. In 2017 the city introduced its “10,000 Up” program to put in 10,000 over several years. So far, the city has planted 3,600 street trees through that program and an additional 900 in public parks since 2016, according to Jeremy Thomas, communication and marketing manager for Detroit’s Parks and Recreation division. 

However, these efforts may not be keeping up with the current level of die-off on city-owned rights of way.  Davey estimates a 1% to 3% annual mortality rate, meaning that of the roughly 170,789 trees on city streets,  somewhere between 1,707 and 5,123 die every year. Many more trees need to be planted to make a meaningful difference, but residents are likely to want better maintenance and perhaps more information about the role trees can play in protecting them from the dire impacts of a changing climate.

Surviving heat waves

“Survivability” is the word that Brian Stone uses when he talks about heat waves in cities like Detroit, which he is studying as part of his work with the Georgia Institute of Technology’s Urban Climate Lab. “We are seeing temperatures that cannot be survived,” he said. 

The threshold Stone has in mind is a“wet bulb” temperature of 95 or above. (This scale–similar to the heat index often used during the summer­–incorporates temperature, humidity and factors like wind speed and the angle of the sun to provide a clearer picture of how heat impacts the human body.) At these temperatures, said Stone, “A perfectly healthy person outside for three or four or five hours will…suffer organ failure and die.”

And Detroit –despite its northern latitude — is especially vulnerable to the effects of warming, second only to Miami in the number of additional deaths that could be produced by a three degrees Celsius of warming, according to a study in the publication “Science Advances”. The urban heat island effect — in which buildings and pavement trap heat and increase temperatures — adds to the danger. Many Detroiters also lack access to air conditioning, making it difficult to cool down during periods of extreme heat.

“About a third of Detroit residents during a heatwave are experiencing conditions that are conducive to heat illness,” Stone said. “Tree planting is one of the few things we can do that mitigates that risk.”  

According to his research, trees are 1.5 to three times more effective than other cooling strategies such as painting rooftops white, and they offer other benefits as well like filtering air pollution and improving mental health.  By shading heat trapping surfaces and transpiring water–which can lower temperatures through evaporative cooling–trees can help counteract the effects of the urban heat island — but whiter and wealthier neighborhoods in the U.S. often enjoy these benefits the most.   

Power outages only multiply the threat from extreme heat. If one were to hit at the same time as a heatwave, it would take away residents’ ability to keep cool with fans and air-conditioners. 

“A concurrent blackout and heatwave in Detroit, Atlanta, or Phoenix is perhaps the deadliest thing we can imagine from climate change,” Stone said. 

In Detroit this poses something of a dilemma. The canopy helps keep things cool, but poor tree maintenance also contributes to the region’s frequent blackouts. Yet, Stone said that cutting down trees simply to maintain power would be “compounding the problem” especially for the many Detroiters that lack air conditioning. 

Waste heat produced by air conditioners could also contribute to an increasingly unequal situation where some crank their units to deal with the high temperatures, making the outside air even hotter and causing those without cooling to swelter.  

Long-term, Stone said cities should receive help from the federal government to bury power lines while also working for a more resilient grid with rooftop solar and other forms of decentralized generation, he adds. In the short term, Detroit may need better tree care — especially around power lines — which DTE said it’s doing.

Stone emphasizes that despite its drawbacks, the urban canopy should be regarded as a vitally important tool  for fighting climate change. “Trees sound like a neighborhood amenity and a nice thing to have,” he said. “I would say it is just absolutely critical infrastructure for public health.”

Weathering storms

For many Detroiters, flooded streets and basements may seem like a more immediate problem than heat. Trees may help prevent floods like those produced by the historic storms of 2014 and more localized events like the recent high water in the Jefferson Chalmers neighborhood.  Asia Dowtin, professor of urban forestry at Michigan State University said that, depending on the species, the canopy can absorb up to 25% of rainfall, allowing it to evaporate before it hits the ground and contributes to flooding.

As for the other 75%, Dowtin said trees can significantly increase the amount of water the ground can absorb. The soil soaks up some rainfall as it either passes through a tree’s leaves or drips off the edges of the canopy, but much of it is funneled down branches and the trunk in a process called “stemflow”, moving directly into the soil where it follows the channels created by large roots until it can get taken up by finer roots or root hairs. Dowtin describes this underground network as “roadways below the soil” that offer multiple pathways for water to infiltrate the ground, keeping it away from pavement, storm drains and basements.

Like heat waves, it’s likely that the flooding Detroit has seen over the last decade is about to get even worse. For every degree Celsius of warming, the air will hold 7% more water vapor, producing additional precipitation. A study from the University of Notre Dame predicts that so-called 100-year floods will increase by as much as 30% in the Midwest and Great Lakes region by the end of the century. 

In other words, in addition to “green infrastructure” projects like rain gardens and porous pavements, Detroit is likely to need more trees to help mitigate the effects of storms. And it’s likely to need them soon, as larger trees provide exponentially more benefits than smaller ones.

Building tree equity

Getting trees in the ground will require massively scaling up planting efforts and targeting areas most vulnerable to climate change. But perhaps most importantly, any such effort must win over Detroiters who have suffered the consequences of a poorly managed urban canopy for years. 

American Forests has started a nursery in the Virginia Park neighborhood, and in the next year or so they plan to work with the US Forest Service, the Greening of Detroit and others on a pilot project to plant “significantly more than 5000 trees a year.” Planting at the rate of 20,000 a year, Candela said they could put in the 200,000 thousand or so trees needed to get the city to 30% canopy coverage within ten years

The concept of tree equity is critical in marshalling resident support for planting programs like what American Forests is proposing. The approach underscores the ways the urban canopy helps low-income and minority neighborhoods, which often suffer more from air pollution and are likely to be hit hardest by climate change. 

Tree equity  also means addressing disparities in who gets employed by urban forestry initiatives.  “The tree care industry…has not employed a lot of people who live in dense urban cities,” Candela said, something that American Forests and Greening of Detroit are trying to address with their job training programs. 

Dowtin, the MSU professor, has met with a number of community groups and schools to listen to resident concerns and offer education around tree planting. She said it’s important to think beyond just the numbers when carrying out a city forestry program. 

“You can put 10,000 trees up, and within two years 8,000 of those can die because there were no stewardship initiatives to help people take care of them,” she said.

Part of the process said is building “environmental literacy” or helping people think critically about the services trees provide, while also being honest about the drawbacks. 

“When you think about stewardship, it’s all about protecting a resource that’s of value,” Dowtin said. “But if you don’t know that that resource is of value, it’s likely that you don’t even think about it as a resource.” 

As a general rule of urban forestry, 90% of city trees are on private property. For planting efforts to succeed in cities, residents need to be willing to commit time and money for their upkeep.

Stone feels that the data he’s compiling on how trees might save lives with tree planting could help influence public opinion. 

But on the east side of Detroit, Pressley continues to feel that trees are “more worrisome than advantageous”. As for the threat of heatwaves, she said she could see tree planting as a solution, “someplace where you have heat for more than two months of the year,” but not Detroit. 

As summers get hotter and rainstorms more intense, that sentiment may change. But for now, it appears that tree planting advocates have some work to do to convince people like Pressley that the benefits of an urban canopy outweigh the risks.