TREE EQUITY: how are some of America’s hottest cities using IT to protect vulnerable communities?

Editor’s note: This article is part of a collaboration between APM Research Lab and the Ten Across initiative, housed at Arizona State University.

by RITHWIK KALALE | Sept. 6, 2023

Earlier this year, the Biden-Harris administration announced $1 billion in grants to increase access to trees and green spaces in urban communities as part of the president’s Inflation Reduction Act. The initiative was created in order to combat record high temperatures in vulnerable areas across the country.

Since then, multiple cities have developed urban tree plans and strategies for allocating the federal dollars they will be receiving. Nonprofits, research centers and state governments are partnering to understand how trees can be planted strategically to curb heat in high-risk areas.

Funding couldn't come soon enough for places like Maricopa County, home to the city of Phoenix, Arizona. In 2022 alone, Maricopa County experienced 425 heat-associated deaths, more than half of which occurred in the month of July. This was a 25% increase from 2021. Two hundred forty-five of those deaths occurred in Phoenix.

This past July, the daily high temperature in Phoenix exceeded 110 degrees Fahrenheit for 31 straight days, shattering the previous record span of 18 days there set back in 1974. Continued high temperatures like these will likely result in even more heat-related deaths. In 2023 so far, 194 heat-associated deaths have been reported by Maricopa County Department of Public Health.

“Tree equity” explained

“If you look at a map of tree cover in American cities, it's very oftentimes a direct correlation to income and race,” said Jake Simon, senior manager for the Urban Forestry Southwest division of nonprofit organization American Forests.

In cities that have experienced redlining and other discriminatory policies, according to Simon, trees are often sparse in neighborhoods with low-income residents and people of color.

“Having those fewer trees, especially in the Phoenix area, means it’s hotter,” he said.

He added the hottest parts of Maricopa County directly correlate to the places with the highest concentration of people of color, people in poverty and unemployment.

“There's a concept known as tree equity,” said Dr. Ryan Bare, a research scientist at the Houston Advanced Research Center. “It's about identifying some of the hottest areas, some areas most at risk, where tree canopy can have the greatest benefits.”

Quantifying tree equity

The Tree Equity Score website quantifies this concept by ranking U.S. cities and neighborhoods on a scale from 0 to 100, depending on how complete the area’s canopy coverage is, combined with the demographic characteristics of the area's population. The initiative is a collaboration between American Forests, University of Vermont and Google.

“It's a tool for looking for where to plant trees and prioritize replanting through an equity lens, in any population center in the U.S., and that includes Alaska and Hawaii,” said Julia Twichell, director of data design and user experience for American Forests.

Twichell says the data team uses a combination of satellite imagery, Google Tree Canopy and demographic census data to score the urban areas. The online score is used by many cities and nonprofits across the country to determine how to implement their tree planning strategies.

“If you look at the neighborhood level in any of these urban centers, [the score] gives you a sense of priority on where trees are needed, but also where the greatest human need is,” said Twichell.

Twichell said the data team also uses a composite index of demographics and environmental climate factors when determining how to score an area.

“[We want] to try to get data into everyone's hands and make it freely available because not everyone has a [Geographic Information Systems] team to crunch that kind of data and figure out their own prioritization system,” she said.

Within the Maricopa County cities, the areas with the highest tree equity scores, meaning they have adequate canopy coverage in relation to their population, are Carefree (100), Paradise Valley (100), Scottsdale (92) and Anthem (94) -- all in the northern area. The lowest scores, signaling highest priority for more tree planting, are in the western metro area, including Tolleson (63), Buckeye (74) and Goodyear (76).

The Arizona Department of Forestry and Fire Management is a government agency that uses the Tree Equity score, in conjunction with other research, to prioritize spending on tree planting efforts.

“A lot of our funding comes from the state of Arizona, but the Urban and Community Forestry Program is pretty much fully funded by the federal U.S. Forest Service,” said LoriAnne Warren, urban forestry program manager. “That federal urban community forestry program is one of the only ones in any of the national agencies that deals specifically with the ecosystems in the urban habitats, and the relationship between humans and the trees that we see in the communities that we live in.”

The cost of tree equity in Phoenix, Houston and L.A.

According to the U.S. Department of Agriculture’s Forest Service website, Arizona is set to receive $6 million in the 2023 fiscal year for tree planting efforts alone.

Warren says there is more to tree planting than just giving people seedlings and calling it a day. It is resource-intensive and requires constant maintenance, especially in desert landscapes like Arizona.

She added that trees planted as part of “million tree initiatives” end up dying due to being exposed to the reflective heat of urban structures, such as buildings and pavements. This is especially prevalent in cities like Mesa and Tucson.

“There's a lot of risk in planting a tree that needs to be taken into consideration and thinking about how you're going to nurture that tree, so that it can grow up to 10, 15, 20 feet tall and provide the shade and the resources…in our urban environments, especially in areas where it's most needed,” she said.

Phoenix is not the only city that is trying to use greenery to mitigate heat equitably.

“Within Houston we do see a disproportionate amount of tree canopy coverage in more affluent areas compared to underserved areas or neighborhoods,” said Dr. Bare.

Bare mentioned that HARC has partnered with the Mitsubishi Corporation to identify at-risk areas in Houston, with the first phase involving scientists going out into the city using thermal instrumentation to collect heat data.

Nonprofit organizations like Climate Resolve in Los Angeles are taking a different approach–recruiting people from at-risk communities, paying them and providing tree care courses to construct their own area-specific tree planting projects.

“In the past, a lot of trees have been planted in places that weren't the right fit. And so it's led to the legacy of a lot of trees that are definitely necessary in the community, but messing up some of the infrastructure,” said Gabriel Varela, outreach and community liaison for Climate Resolve.

“Communities ended up having these bad experiences with trees and so now, the saying is ‘right tree, right place, right reason,’ so that we can dispel some of that previous experience that folks have gone through.”

Varela said the organization’s Tree Ambassador program emphasizes the importance of trees as a source for heat relief.

“A big piece of the workshops that we present is couching all this tree planting within the context of climate change, and extreme heat within the city of LA,” he said.

“We're definitely deeply concerned about building resilience in areas that are impacted by extreme heat conditions that can actually cause so much damage, especially within those working class communities of color that are on the frontlines of extreme heat and other climate impacts.”

Neither HARC nor Climate Resolve receive federal funding for these initiatives.

Ensuring tree health

If temperatures are high even at night, many plants, including trees, are tricked into thinking it is still daytime. Because of this, they are not able to perform essential functions in order to grow.

“Nighttime is very important for trees, and the increasingly high temperatures at night negatively affect their biological processes,” said Simon. “If we can't protect those trees and protect the canopy in these low tree equity score areas, it's just an exacerbating effect,” he said.

Warren says there are other barriers to proper tree care that most private residents and cities are just not able to afford.

“There are a lot of those medians and things which also have trees along the road spaces and that's something that the municipality is responsible for monitoring,” said Warren. “But a lot of municipalities do not have that because they don't have that as a staff person. They don't have the funding to do it. And that makes it more challenging.”

She added that commercial properties like shopping centers which have trees within their area should think of a plan for its long-term maintenance.

“Whether you're a homeowner or business owner or whatever, hiring somebody that is a certified arborist is really important,” she said. “There is a proper way to take care of trees and prune them and maintain them, and an improper way.”

Although tree planting initiatives have been around for decades, the concept of “tree equity” is a relatively new concept that nonprofits, researchers and governments alike are using to inform both their greenery projects and heat mitigation strategies. The objective is to view and maintain trees as accessible infrastructure that can transcend generations.

“You don't just run plumbing or electrical lines and then that's it,” said Simon. “Those require maintenance, right? Same as trees. If you don't have a maintenance plan in place, or clearly defined roles and responsibilities, you probably shouldn't plant the tree because it's just going to die.”

Factors like invasive plant species, city versus private resident responsibility and fluctuating temperatures make it difficult to not just maintain the greenery, but also have it be spread equally across urban areas.

“It's a dedicated, lifelong journey. Trees have a much different concept of time than we do. So, [the goal is] trans-generationally, we’ll get to experience these trees,” said Simon.

Correction: When originally published, the article quoted Jake Simon as saying, “Nighttime is when trees release their stomata. They do all this evaporation and transpiration stuff.” On Sept. 11, 2023 he indicated that this comment was meant to apply to problems currently observable with saguaro cacti. He provided an alternate quotation (“Nighttime is very important for trees, and the increasingly high temperatures at night negatively affect their biological processes,”) which we have incorporated into the article as of 1:30 p.m. Central Time on Sept. 11, 2023.

Thoughts? Questions?

We want to hear from you! Leave us a message below.

…or email us at info@apmresearchlab.org