Expert Commentary

Tree equity and trees’ impact on surface temperatures, human health: A research roundup

The absence of trees is not just an aesthetic discrepancy — it can impact human health and well-being, a growing body of research shows. We highlight several studies that examine this association and highlight residential tree inequities.

A tree canopy
Photo by Nick Page on Unsplash

In both cities and suburbs, affluent neighborhoods are more likely to enjoy the cool shade of trees than lower-income neighborhoods, some of which bear the scars of historic redlining and housing segregation.

The absence of trees is not just an aesthetic discrepancy — it can impact human health and well-being, a growing body of research shows. Neighborhoods devoid of trees often grapple with the urban heat island effect, where concrete and asphalt absorb and magnify summer’s heat and elevate the risk of heat-related illnesses. Heat islands are often linked to factors such as income and race.

Some studies have linked long-term exposure to green surroundings to health benefits such as living longer. In contrast, unsheltered sidewalks can deter residents from walking, exercising or socializing outdoors.

More than half of the world’s population now lives in urban areas. In the U.S. and Canada, nearly 80% of people live in urban areas, according to estimates from academic research. Meanwhile, in the U.S., urban tree canopy cover is declining at an estimated rate of four million trees per year due to urbanization and tree diseases.

In addition, climate change is making summer days hotter and heat waves more frequent and longer. Researchers warn this can further exacerbate the divide, calling on policymakers to invest in planting more trees and creating more green spaces in barren neighborhoods. Some cities are beginning to address these inequities by investing in new green spaces or testing creative solutions, such as green roofs.

American Forests, a nonprofit organization in Washington, D.C., has created Tree Equity Scores for nearly 200,000 urban neighborhoods in the U.S. to measures how well the benefits of urban tree canopy are reaching those who need them most, including low incomes, communities of color and those disproportionately affected by extreme heat, pollution and other environmental hazards.

Journalists can assess tree equity in their coverage area and find out whether local officials are implementing plans to reduce inequities in green space. To bolster journalists’ knowledge and reporting, we’ve gathered and summarized several recent studies that assess tree and green space inequities in cities and neighborhoods and examine their association with health. The studies are organized by publication date.

Research roundup

The Tree Cover and Temperature Disparity in US Urbanized Areas: Quantifying the Association with Income Across 5,723 Communities
Robert I. McDonald, et al. PLoS ONE, April 2021.

The study: Researchers use digital images from the U.S. Department of Agriculture’s National Agriculture Imagery Program to survey tree cover inequity in 100 large urban areas in the U.S., including 5,723 municipalities and home to 167 million people. They compare tree cover with summer land-surface temperature, using NASA’s Landsat imagery. The study focuses on one benefit of tree canopies: reducing temperature. Tree canopies primarily cool the air by shading surfaces such as concrete and asphalt, preventing heat storage and reducing the urban heat island effect. Tree cover can reduce land surface temperature by 10 to 20 degrees Celsius (18 to 36 degrees Fahrenheit) on a summer day, the authors write.

The findings: In 92% of the areas studied, low-income blocks had less tree cover than high-income blocks. More specifically, low-income blocks on average had 15.2% less tree cover and were 1.5 degrees Celsius (2.7 F) hotter than high-income blocks. In the Northeast U.S, low-income blocks had 30% less tree cover and were 4 C hotter (7.2 F) than high-income areas, showing some of the greatest differences in the U.S. At 54%, Connecticut had the greatest difference between high- and low-income neighborhoods. Researchers also find a positive association between more tree cover and populations that are white and have higher income. But, in 22% of the urban areas there was not a statistically significant relationship between income and tree cover.

Quote from the study: “A targeted investment in tree planting of $15.8 billion would close the urban tree cover disparity for 34 million people in low-income blocks of moderate or greater population density, although it would likely take at least 5–10 years for planted trees to be large enough to deliver significant ecosystem service benefits. Some of the needed tree planting would occur through public sector investment in tree planting and maintenance on the public right of way and publicly owned land… But some of the needed tree planting would have to occur on private land, which would require incentives or regulations that motivate the private sector to conduct this tree planting.”

More, from the lead author: “Tree inequality is worse in the suburbs,” published in in May 2021.

Residential Housing Segregation and Urban Tree Canopy in 37 US Cities
Dexter H. Locke, et al. Urban Sustainability, March 2021.

The study: Researchers assess how the practice of redlining, a racially discriminatory housing policy established by the federal government’s Home Owners’ Loan Corporation during the 1930s, may relate to tree canopy coverage in city neighborhoods in 2020. “Tree canopy” typically refers to areas that are shaded by trees. The authors include 37 metropolitan areas, comparing predominantly white neighborhoods during the redlining era with areas where mostly racial and ethnic minorities lived.

The findings: Redlining influenced the location and allocation of trees and parks. The 37 metropolitan areas where mostly racial and ethnic minorities lived during the 1930s have, on average, 23% tree canopy cover today. Areas where U.S.-born white people lived in the 1930s have almost twice as much tree canopy, 43%.

Quote from the study: “Our investigation into 37 cities reveals a strong association between HOLC grades inscribed on maps roughly nine decades ago and present-day tree canopy. The study design cannot identify causal pathways, but the inequity invites careful scrutiny of the social, economic, and ecological processes that have created the demonstrably uneven and inequitable distribution of urban tree canopy in the United States.”

Green Spaces and Mortality: A Systematic Review and Meta-Analysis of Cohort Studies
David Rojas-Rueda, et al. The Lancet Planetary Health, November 2019.

The study: Researchers looked for English-language longitudinal studies that assessed the association between green spaces, or lack a thereof, and the risk of death from any cause. The meta-analysis of nine studies, published between 2012 and 2019, included more than 8 million adults from Australia, Canada, China, Italy, Spain, Switzerland and the U.S.

The findings: Increasing green spaces in neighborhoods is significantly associated with reducing risk of death. While researchers haven’t yet identified a causal relationship between green spaces and health, they have offered several theories. For instance, green spaces can foster physical activity, walking and cycling. They also lessen air pollution, noise and the heat island effect.

Quote from the study: “Although the benefits of green spaces and mortality that we found are robust, negative effects of increasing green spaces in the urban environment (such as gentrification) can occur, and these externalities should be considered when urban public policies are designed.”

Who Has Access to Urban Vegetation? A Spatial Analysis of Distributional Green Equity in 10 US Cities
Lorien Nesbitt, et al. Landscape and Urban Planning, January 2019.

The study: The study is an analysis of the relationship between urban vegetation and socioeconomic and demographic factors in 10 urban areas in the U.S.: Chicago, Houston, Indianapolis, Seattle, St. Louis, Los Angeles, New York, Phoenix, Portland, Ore. and Jacksonville, Fla. Researchers used high-resolution aerial imagery and Census data for their analysis. They define equitable access to urban vegetation as fair access, regardless of income, race or age.

The findings: Access to green spaces in urban areas is generally associated with higher income, higher education and higher percentage of white residents. Latino urban residents had the lowest level of access to urban greenery, followed by African American and Indigenous residents. Meanwhile, socioeconomic factors appear to be less often associated with access to park area, suggesting that parks are more equitably distributed.

Quote from the study: “The impact of urban vegetation exposure on the health and well-being of marginalized communities may become even more critical as climate change worsens. When health inequalities intersect with low access to urban vegetation, this intersection can create areas of high climate vulnerability.”

More on urban vegetation: The authors parse the effects of different types of green spaces. An area with a mix of vegetation, including shrubs, hedges, garden and crop plants and grassy areas, reduce stormwater runoff and offer green views that can reduce stress. Trees, or woody vegetation, can reduce the urban heat island effect by providing shade. Trees can also improve air quality, while parks offer space for physical activity and socialization.

More, from the lead author: “How cities can avoid ‘green gentrification’ and make urban forests accessible,” published in The Conversation in June 2021.

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