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The little-known physical and mental health benefits of urban trees

Posted on 6 March 2023 by dana1981

This is a re-post from Yale Climate Connections. While predicated on accepted scientific findings, this article includes conclusions of the author and is presented to our readers as an informed perspective.

A2020 Pew poll found that 90% of Americans support planting trees as a method to curb climate change. The climate benefits of trees are simple to understand: About half of a tree’s dry weight is composed of carbon, which trees extract from the atmosphere as they grow.

According to the Environmental Protection Agency, forests in the United States remove about 800 million tons of climate-warming carbon dioxide from the atmosphere every year. That includes close to 45 million tons specifically from “urban forests” — a term that encompasses a wide variety of configurations of trees ranging from individual street trees to large parks and nature preserves.

Urban forests alone offset the climate pollution from nearly 10 million cars. And according to research led by scientists at the Nature Conservancy, planting more trees in suitable urban areas could remove another 70 million tons of carbon pollution per year, enough to offset the carbon pollution from 15 million more cars.

But you may not know that urban forests also benefit people’s health.

recent study of a 30-year tree-planting effort in Portland, Oregon, found that one premature death was avoided for every 100 trees planted. And researchers have identified a plethora of physical and mental health benefits that come along with planting more trees in urban areas.

For example, the cooling provided by urban forests can increase resilience to worsening heat waves. Access to trees can also help reduce individuals’ stress, improve mental health, strengthen immune systems, reduce crime, and improve student academic performance, among other benefits.

But as with many social issues, access to urban trees is highly inequitable in the United States, with wealthier and whiter communities enjoying substantially more tree canopy cover than poorer neighborhoods and Black and Brown communities.

What research shows about the benefits of urban forests

A 2020 paper published in the International Journal of Environmental Research and Public Health reviewed 201 studies on the various physical and mental health impacts of urban trees. These included a variety of scientific approaches, including different types of experimental, observational, and modeling studies.

The research also considered a wide variety of different health effects that the 2020 study authors grouped into three categories — reducing harm (such as by curbing air pollution, heat exposure, or crime), restoring peoples’ capacities (such as by reducing stress, restoring mental cognition and attention, or improving mental health), and building peoples’ capacities (such as by strengthening immune systems or motivating active living). For most of the potential benefits evaluated in these 201 papers, the majority of studies identified positive effects from an increase in urban trees.

Urban forests help communities stay cooler

The strongest evidence for the harm reduction from urban forests comes from their cooling services, which can help communities during extreme heat waves. In addition to providing shade, trees cool the air through a process known as evapotranspiration, in which their leaves release water into the atmosphere. Some heat energy in the surrounding air is used to evaporate that water from a liquid to a gaseous state, which leaves less heat to raise air temperatures. A new 2023 study in the Lancet found that doubling urban tree coverage in European cities from 15% to 30% could have saved over 2,600 lives during the continent’s extreme heat wave of 2015, reducing the death toll by nearly 40%. As a bonus, tree canopies also provide a measure of protection from the sun’s ultraviolet rays.

Six studies have investigated the impact of urban forests on crime. For example, a 2017 paper found that the presence of tree cover was associated with reduced gun assaults in Philadelphia, and trees located on public property were found to have a 40% greater crime reduction impact compared to trees on private property in a 2012 study of Baltimore.

Research has also found that urban forests reduce air pollution, although generally by less than 1%. Leaf surfaces can intercept some dangerous tiny particles from the air, and leaf pores absorb some gaseous pollutants. Despite their relatively small effects on air pollution, one study estimated that in 2010, the health benefits of urban forests in the U.S. were worth nearly $7 billion, preventing 850 premature deaths and 670,000 incidents of acute respiratory symptoms that year.

Urban trees do have one significant adverse health effect: Their pollen releases can trigger allergic reactions. A 2018 study noted that 30—40% of the world’s population is affected by some form of allergy, but tree allergies can be reduced by planting species that produce less pollen or preferentially selecting female plants of species that produce more pollen, like pine and oak.

Trees help people feel better

Research has linked exposure to trees to both physical and mental restoration. For example, a number of studies have found that exposure to urban forests generally reduces mental and physical stress, anxiety, and depression, and that they improve moods.

Studies of clinical populations with diagnosed mental health conditions also found mainly positive results from exposure to forests. For example, a 2015 study in London found that in boroughs with higher urban tree density, individuals diagnosed with depression required lower antidepressant prescription rates. And an influential 1984 study of postoperative patients in a Pennsylvania hospital found that those with views of a tree through their window had significantly shorter recovery times following gallbladder surgery.

Urban forests promote active lifestyles

Studies have also identified several positive effects from exposure to urban trees on individuals’ physical and mental health capacities. Nearly every study on the subject found that people live more active lifestyles when living in proximity to urban forests.

Six papers found that exposure to forests tends to result in healthier human immune systems, for example through boosted immune cell numbers and activity, though the underlying pathways are not completely understood. Numerous papers, for example, a 2015 study in Toronto, found lower incidences of cardiovascular disease in neighborhoods with higher tree density. Three papers also found that residents in communities with more trees feel a greater sense of connectedness, belonging, and trust.

2010 study in Michigan, a 2014 study in Massachusetts, and a 2018 study in Toronto all found that students in school campuses with greater tree cover perform better academically. The 2010 paper found that to be especially true when trees were visible through classroom and cafeteria windows.

But access to trees is unequal

In short, access to urban forests provides a plethora of physical and mental health benefits, allowing people in communities with better tree cover to live longer, happier, healthier lives on average.

But an analysis by the organization American Forests found that majority Black and Brown neighborhoods have 33% less tree canopy on average than majority white communities, and neighborhoods with the highest poverty rates have 41% less tree coverage than the wealthiest communities. American Forests also created a tree equity score tool with data about the level of tree inequity in every community around the country.

These findings suggest that efforts to reduce tree inequities by planting more urban forests in disinvested communities could provide the dual benefits of improving physical and mental health among residents of those neighborhoods — while also helping to slow climate change.

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  1. Little known? Ask any kid on a playground with one large tree on it where they want to play. Actually, you don't even have to ask: use your eyes!

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