How equitable is tree cover in Michigan’s cities?

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By Nina Ignaczak

Cities are made of pavement and rooftops. And those hard surfaces that make up the physical structure of our cities contribute to a crucial ecological challenge that will only worsen as the world warms world: the urban heat island effect.

Hard structures absorb sunlight and radiate it back at ground level, increasing daytime temperatures in urban areas about 1–7°F higher than in surrounding suburban and rural areas. At night, temperatures rise about 2–5°F higher in the city than in outlying areas.

Those same surfaces also contribute high volumes of stormwater runoff into local sewer systems and waterways. And according to the sixth Intergovernmental Panel on Climate Change IPCC report, heatwaves and intense precipitation events are expected to increase in frequency and intensity.

The situation is often worse in poor areas with high concentrations of poverty and people of color. One study found that surface air temperature in formerly redlined areas across the United States is approximately 2.6 °C warmer than non-redlined areas.

Maps created by the federal government's Home Owners' Loan Corporation between 1935 and 1940 assigned risk categories to residential neighborhoods primarily based on race and ethnicity. Areas with the lowest grade of "D" were considered "hazardous” and colored red. Local real estate investors and financial institutions used these maps to make decisions on mortgage applications. The result: investment flowed to desirable — primarily white — areas and away from redlined areas.

1940 Home Owners' Loan Corporation Map of Flint


The implications for climate vulnerability today are apparent. Residents of formerly redlined areas are less likely to have an adequate tree canopy to help mitigate against the urban heat island effect. A tree-lined street may be one of the best defenses against a warming climate for urban dwellers, substantially reducing the urban heat island effect. Maintaining an urban tree canopy is a challenge. Once a tree canopy in an urban area drops below 40%, temperatures rise substantially.

The combination of severe storms, power outages, and the urban heat island effect is a potent threat. Hurricane Ida’s death toll was driven mainly by heat experienced by elderly residents who had lost power and had no way to stay cool in the scorching days following the storms.

In Detroit, researchers predict that 450,000 people — more than two-thirds of the city’s population — are at risk of heatstroke and heat exhaustion in a combined power outage and heat event. This week, President Biden announced a federal interagency initiative to address the effects of extreme heat on vulnerable populations.

"My Administration will not leave Americans to face this threat alone," Biden said in a statement. "Today, I am mobilizing an all-of-government effort to protect workers, children, seniors, and at-risk communities from extreme heat."

The urban heat island effect is less pronounced in the Midwest than in the Southeast and the West, but dangerously hot days are projected to increase here nonetheless. As it seems with so many aspects of climate change, Michigan is a bit buffered from extremes, but we still suffer consequences.

In July, the nonprofit American Forests released a Tree Equity Score data set that describes the equity of tree cover across urban areas within the United States. The data assigns a “tree equity score” to each census block group within a city based on various factors.

The analysis starts with a tree canopy gap, defined as the potential amount of tree canopy in a given area minus the existing tree canopy. Based on that gap, a score is calculated using local physical and socioeconomic factors like income, employment, age, race, and health, and the severity of the existing urban heat island.

The result is Tree Equity Score on a scale of 0-100. A lower Tree Equity Score means a greater priority for closing the tree canopy gap (more trees needed!). The website also includes filters for the raw data, so you can see how block groups, cities, and urbanized areas stack up in terms of socioeconomic variables. American Forests’ goal is to get every block group to a score of 75.

So how do Michigan cities shape up when it comes to tree equity? Let’s look at some examples.

Bloomfield Hills: Tree Equity Score = 100

Bloomfield Hills has a perfect Tree Equity Score, meaning it has a low priority for closing the tree canopy gap –not surprising for a leafy and wealthy suburb that ranks above the top five most affluent cities in the country.

Tree Equity Map of Bloomfield Hills

Pontiac: Tree Equity Score = 79

While Pontiac has a whole exceeds American Forests’ Tree Equity Score goal of 75, blocks within the city range from 34 to 100. Only 21 of the 65 block groups within the city have a score of 75 or better.

Tree Equity Map of Bloomfield Hills

Flint: Tree Equity Score = 80

Flint meets the American Forests’ Tree Equity Score goal of 75 citywide, but within the city it ranges from 40 to 100. Only 42 of140 block groups in the city have a score of 75 or better.

Tree Equity Map of Flint

Grand Rapids: Tree Equity Score = 88

Grand Rapids scores 88 overall for tree equity, and only 21 of 52 block groups score below 75. Block groups range from 35 to 100 within the city.

Tree Equity Map of Grand Rapids

Detroit: Tree Equity Score = 80

Detroit scores 80 overall for tree equity, but 282 of 875 block groups have a tree equity score below 75. Block groups range from 36 to 100 citywide.

Tree Equity Map of Grand Rapids

Bay City: Tree Equity Score = 65

Bay City scores 65 overall for tree equity, with 22 of 89 block groups scoring below 75. Block groups range from 36 to 100 citywide.

Tree Equity Map of Bay City

Learn more about your city’s tree equity here.

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