Not all tree canopy is created equally

This blog was written by Tuviere Onookome-Okome, a researcher at DDL and the Samuel Centre for Social Connectedness.

Trees are a vital part of any community. Benefits from trees include the ability to reduce the amount of greenhouse gases in the atmosphere through carbon storage,¹ decrease stormwater runoff during heavy rainfall,² and to mitigate the urban heat island effect by providing a cooling refuge during periods of increased air and surface temperatures.³ Additionally, recent studies have examined the effects of trees on mental health, anxiety and mood. One study found that depression decreased with forest immersion,⁴ and exposure to a forest environment has been shown to lower anxiety, anger and fatigue.⁵ Trees have immense benefits to humans and the environment alike, which is one reason why sustainable development goal (“SDG”) 11, which aims to “make cities and human settlements inclusive, safe and sustainable,” includes a target to provide “universal access to safe, inclusive and accessible, green and public spaces.”⁶

Trees, and their equitable access, are central to these SDG-11 targets. American Forests defines tree equity as having enough trees in an area that everyone can experience the health, climate and economic benefit of trees.⁷ Achieving tree equity means that all neighborhoods within a city reach a citywide standard of tree cover that is feasible and appropriate for each city’s unique climate and context. In short, tree equity is about ensuring that there is equitable distribution of trees throughout all neighborhoods and communities.⁸

But research demonstrates that U.S. cities do not have tree equity across the different neighborhoods.⁹ A recent meta-analysis confirmed that whiter and wealthier communities saw much higher levels of urban tree canopy and other studies have found that neighborhoods with a majority of people of color had one-third less tree coverage than white neighborhoods.¹⁰¹¹Additionally, neighborhoods in which 90% or more of residents lived below the poverty line had 65% less tree canopy than neighborhoods where 10% or less of the population was living below the poverty line.¹² A mapping of urban tree cover often correlates significantly with a map of race and income in the United States, in that more tree cover is found where areas tend to be whiter and wealthier.¹³

Recognizing the importance of trees, American Forests, a non-for-profit organization, created a way to evaluate the equitable distribution of trees in 150,000 neighborhoods across the U.S. with a “tree equity score.” This score ranges from 1 to 100 and more than 486 urbanized areas in the U.S. have already been assigned a score.¹⁴ Neighborhoods with a score of 100 signifies perfect tree equity where each individual benefits equally from the presence of trees.¹⁵ The score incorporates factors such as population density, income and employment, race and ethnicity, satellite data on tree cover, and surface temperature.¹⁶ At the core of the tree equity score is the message that all people should experience the benefits of trees.

To calculate which neighborhoods should be prioritized when increasing tree canopy, American Forests created the priority index within the tree equity score.¹⁷ Calculated at the neighborhood level, the priority index includes statistics such as the percentage of people with income below 200% of the poverty line, the unemployment rate, the percentage of people who are not white non-Hispanic, the ratio of seniors and children to working-age adults, the severity of urban heat island effects, and the prevalence of poor mental, physical, respiratory and cardiac health.¹⁸ These measures are normalized and combined to create a simple priority index from 0 to 1, where 1 indicates a greater amount of inequity.¹⁹ American Forests intends for policymakers and urban planners to utilize the priority index to identify neighborhoods that most urgently require investments and improvement in tree cover.

Tree equity within neighborhoods is especially a concern in the U.S. due to the historical practice of redlining. In the late 1930s, the federal government began insuring mortgages to lift Americans out of the Great Depression and help them become homeowners. However, these loans were not distributed equally.²⁰ The term “redlining” refers to the process in which neighborhoods with high concentrations of Black, immigrant, and working-class residents were color-coded as red due to the fact that these areas were deemed hazardous and excessively risky for investment.²¹ Residents in these “undesirable” neighborhoods, especially Black residents, did not have access to loans that would allow them to move into more desirable neighborhoods.²² This practice resulted in the systematic disinvestment by both public and private entities in areas of cities that were deemed less desirable or presented a greater “financial risk.”²³ As a result, many of these areas are still under-resourced today, and many studies have cited these policies as reasons for poor tree equity in the U.S.²⁴

Although redlining of minority neighborhoods was outlawed in the Fair Housing Act of 1968, many cities are still seeing its enduring effect, especially with respect to neighborhood green space and tree cover.²⁵ In response, many American cities have started campaigns to acknowledge and rectify this ugly history. New York City, Phoenix, Chicago, Raleigh, Baltimore, Philadelphia and Los Angeles all have active policies to address the history of redlining when it comes to tree cover.²⁶²⁷²⁸²⁹³⁰³¹³²³³³⁴ All seven of these cities have created strategies to plant new trees to increase overall tree canopy, and have promised to include an equity component to their tree planting initiatives, prioritizing marginalized and overlooked communities with less extensive tree canopies due to redlining.³⁵³⁶³⁷³⁸³⁹⁴⁰⁴¹⁴²⁴³

New York City, for example, developed a heat vulnerability index to map how vulnerable its citizens are to heat, particularly low-income residents and people of color.⁴⁴ As a result of the findings of this study, Mayor Eric Adams’ office has invested $100 million in targeted tree planting to reduce heat vulnerability in higher-risk neighborhoods, with plans to invest $130 million more within the next four years.20 These measures follow up on Mayor Michael Bloomberg’s largely successful “Million Trees NYC” campaign – in 2007, Mayor Bloomberg committed to planting one million new trees in New York City by 2017.⁴⁵ By 2015, two years earlier than the target year and under new mayor Bill de Blasio, the millionth tree was planted, with 250,000 of those trees planted in public housing developments, schoolyards, on New York State land, and near houses of worship, health centers, and campuses throughout the five boroughs.⁴⁶ The success of Mayor De Blasio’s campaign led to a political push from all five borough presidents for the subsequent administration under Mayor Eric Adams to restart the Million Trees NYC campaign 20 The goal of Mayor Adams’ plan, much like Mayor De Balsio’s, is to increase tree canopy in New York.⁴⁷

While New York City had success with increasing tree cover, cities like Los Angeles have not fared as well.⁴⁸ In 2019, Mayor Eric Garcetti appointed Rachel Malarich as Los Angeles’ first forestry officer. Her role was to help achieve Mayor Garcetti’s “Green New Deal” – an ambitious plan to supercharge the city’s climate resiliency in the next few decades, and under which Mayor Garcetti planned on doubling Los Angeles’ existing tree canopy over the next six years.⁴⁹ Their initial goal was to plant 90,000 new trees by the end of 2021, but the pandemic stalled progress and only 65,000 trees have been planted.⁵⁰ The main issue stems from the fact that the Los Angeles city government relies heavily on city residents to plant and care for trees. This expectation can be a problem, especially for residents of poorer neighborhoods who do not own land and can find it difficult to plant trees. Additionally, poorer residents can encounter difficulty maintaining new trees during the pivotal first three years of establishment for new saplings.⁵¹ Even more pressing in Los Angeles are historic droughts and corresponding rising cost of water use, which can make planting and maintaining trees costly for low-income residents.⁵²

A recent study by Natural Areas Conservancy, a New York-based non-profit dedicated to studying and helping to care for the city’s 20,000 acres of forests and wetlands, evaluated different types of tree cover in New York City, and found that the type of tree cover can be important when designing heat mitigation strategies. The study identified three different types of urban tree canopy: healthy natural areas, degraded natural areas, and tree canopy in streetscape. Natural Areas Conservancy defines “natural area forests” as places that look and feel like natural, untouched wilderness forests.⁵³ They are forested areas in the city that lack regular mowing.⁵⁴ Natural area forests can be planted and planned by humans, as we see in cases such as Central Park in Manhattan and Prospect Park in Brooklyn.⁵⁵ Natural area forests are characterized by natural regeneration, diversity in structure and are typically at least half an acre in size.⁵⁶ Some natural area forests in cities are rather large and can span up to a few hundred acres, such as Seton Park in the Bronx. A healthy forest is characterized by its native canopy and multiple horizontal layers, including shrubs and groundcover, while an unhealthy forest is characterized by the dominance of invasive vines that can topple trees and disturb the canopy.⁵⁷

Natural area forest in New York City

A recent interview with Dr. Clara Pregitzer, Deputy Director of Conservation Science at Natural Areas Conservancy, showcased the importance of the type of tree coverage when it comes to heat mitigation strategies. This past summer, Dr. Pregitzer and some colleagues conducted a research study with the aim of capturing variations in air and surface temperatures using heat sensors placed across the city in areas with different types of tree canopy. The study showed that the type of tree cover can have significant effects on heat mitigation in cities – on average, natural area forests are four to six degrees Fahrenheit cooler than other types of tree canopy. Additionally, during high heat index days (above 103 degrees Fahrenheit), these differences can be magnified. For example, in the study, on a hot summer day in Seton Park in the Bronx, temperatures in a natural area forest were 11 degrees cooler as compared to the tree canopy in the streetscape nearby. The research study also found that the larger the forest patch, the greater the cooling benefits for surrounding neighborhoods. Dr. Pregitzer explains that as cities invest more in tree canopy, they must also focus on investing in protecting and expanding existing tree canopy in urban parks, which can provide not only some of the coolest shade but also an opportunity for residents to experience the sights and sounds of nature.

Although city governments seem to place increasing priority on streetscape canopy,⁵⁸⁵⁹⁶⁰⁶¹⁶²⁶³⁶⁴⁶⁵⁶⁶ Natural Areas Conservancy’s research suggests that we should be investing in natural forest areas as well. Natural areas within a city need care and resources to maximize their benefits. As cities enact policies with the goal of increasing tree equity, they should be mindful of what investment and tree planting strategies will maximize the benefits of trees for all residents. While planting new trees may seem like the shiniest option, maintaining existing parks can have immense benefits for the numerous neighborhoods they serve.

Natural area forest in New York City

References

Nowak, D. J., & Crane, D. E. (2002). Carbon Storage and sequestration by urban trees in the USA. Environmental Pollution, 116(3), 381–389. https://doi.org/10.1016/s0269-7491(01)00214-7
Xiao, Q., & McPherson, E. G. (2002). Rainfall interception by Santa Monica’s Municipal Urban Forest. Urban Ecosystems, 6(4), 291–302. https://doi.org/10.1023/b:ueco.0000004828.05143.67
Oke, T. R. (n.d.). Urban heat islands. The Routledge Handbook of Urban Ecology. https://doi.org/10.4324/9780203839263.ch11
Kim, W., Lim, S.-K., Chung, E.-J., & Woo, J.-M. (2009). The effect of cognitive behavior therapy-based psychotherapy applied in a forest environment on physiological changes and remission of major depressive disorder. Psychiatry Investigation, 6(4), 245. https://doi.org/10.4306/pi.2009.6.4.245
Kim, W., Lim, S.-K., Chung, E.-J., & Woo, J.-M. (2009). The effect of cognitive behavior therapy-based psychotherapy applied in a forest environment on physiological changes and remission of major depressive disorder. Psychiatry Investigation, 6(4), 245. https://doi.org/10.4306/pi.2009.6.4.245
Urban trees for equitable cities: Tools to realize SDG goal 11. Nature4Climate. (n.d.). Retrieved February 7, 2023, from https://nature4climate.org/cop-27-event/urban-trees-for-equitable-cities-tools-to-realize-sdg-goal-11/
American Forests. (2021, October 27). Tree equity score. American Forests. Retrieved January 20, 2023, from https://www.americanforests.org/tools-research-reports-andguides/tree-equity-score/
Locke, D.H., Hall, B., Grove, J.M. et al. Residential housing segregation and urban tree canopy in 37 US Cities. npj Urban Sustain 1, 15 (2021). https://doi.org/10.1038/s42949-021-00022-0
Flocks, J., Escobedo, F., Wade, J., Varela, S., & Wald, C. (2011). Environmental justice implications of urban tree cover in Miami-Dade County, Florida. Environmental Justice, 4(2), 125–134. https://doi.org/10.1089/env.2010.0018
Tree equity score. Tree Equity Score. (n.d.). Retrieved February 12, 2023, from https://treeequityscore.org/methodology/
Mujahid, M. S., Gao, X., Tabb, L. P., Morris, C., & Lewis, T. T. (2021). Historical redlining and cardiovascular health: The multi-ethnic study of Atherosclerosis. Proceedings of the National Academy of Sciences, 118(51). https://doi.org/10.1073/pnas.2110986118
Rubinstein, D. (2022, February 12). A million more trees for New York City: Leaders want A greener canopy. The New York Times. Retrieved January 23, 2023, fromhttps://www.nytimes.com/2022/02/12/nyregion/trees-parks-nyc.html
Two Arizona cities take major steps towards realizing tree equity. American Forests. (2021, October 17). Retrieved February 1, 2023, from https://www.americanforests.org/article/two-arizona-cities-take-major-steps-towards-realizing-tree-equity/
Greene, M., & Nahr, J. (2022, June 15). In extreme heat, trees can cool neighborhoods. But a Tribune investigation found a city has planted more trees in wealthier, whiter areas . Chicago Tribune.
Kaylah.Nance@raleighnc.gov. (n.d.). Street Tree Equity Project. Raleighnc.gov. Retrieved February 1, 2023, from https://raleighnc.gov/parks/street-tree-equity-project
Robinson, L. (2022, October 17). $7.7m grant delivers 40k new trees to 36 underserved communities. WBAL. Retrieved February 1, 2023, from https://www.wbaltv.com/article/tree-solutions-now-act-of-2021-grants/41651180
Briggs, R., & Jaramillo, C. (2019, July 24). Have you ever wondered why some Philly blocks are absolutely barren of trees? WHYY. Retrieved February 1, 2023, from https://whyy.org/articles/phillys-low-income-neighborhoods-have-fewer-trees-and-the-citys-free-tree-program-isnt-helping/
Salabert, S., Singer, R., Hansman, H., Joyce, S. M., & Noone, G. (2022, July 1). How Los Angeles became the leader of a tree-planting revolution. Outside Online. Retrieved February 1, 2023, from https://www.outsideonline.com/outdoor-adventure/environment/urban-tree-planting-los-angeles/
NYC Health . (2021). 2021 New York City Heat-Related Mortality Report. NYC Environmental Health . Retrieved January 23, 2023, from https://nyccas.cityofnewyork.us/nyccas2021v9/report/
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Gruskin, A. (n.d.). The city may be in for a million more trees. www.ourtownny.com. Retrieved February 12, 2023, from https://www.ourtownny.com/news/the-city-may-be-in-for-a-million-more-trees-EK2279699
Ding, J. (2022, June 22). La needs 90,000 trees to battle extreme heat will residents step up to plant them? Phys.org. Retrieved February 12, 2023, from https://phys.org/news/2022-06-la-trees-extreme-residents.html
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Conservancy, N. A. (n.d.). Natural Areas Conservancy. Retrieved February 1, 2023, from https://naturalareasnyc.org/

Not all tree canopy is created equally

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