Trees are more than scenery. They’re a social justice issue.

The benefits of trees for human physiology and psychology run deep — but many in Massachusetts and beyond are left without access to green spaces. That needs to change.

Images from Adobe Stock/Photo illustration by Sam Steenstrup

A narrow concrete sidewalk is all that separates Emily Vides’s house in Somerville from four lanes of traffic on Mystic Avenue. When she used to take her kids to day care, she would walk through a nearby housing project. It was a more peaceful path — the buildings blocked noise and grime from the highway, and the area was well plowed in winter. Still, something was lacking: “Trees aren’t really a part of it,” she says.

Neighborhoods like Vides’s — one of Somerville’s lowest-income areas — are notorious for lacking canopy cover, and residents are missing out on much more than shady picnics and tire swings. The benefits of trees for human physiology and psychology run deep: decreased anxiety, better immune system functioning, and, studies have shown, improvements in children’s academic performance. Access to trees is a social justice issue, and local governments should take a hard look at who is able to enjoy these benefits — and who’s left without.

In a study published in 2018, researchers measured these benefits by revitalizing 37 clusters of empty lots across Philadelphia. Where there had been overgrown brush and illegally dumped trash, suddenly residents found parks with neatly trimmed grass and shade trees. The number of residents who reported feeling depressed dropped by more than 40 percent in neighborhoods with new parks — but there was no significant change where researchers had simply cleaned up trash or left empty lots alone. In a similar study, newly greened neighborhoods showed lower rates of many forms of crime, including gun assaults and burglaries.

Forested environments also improve physical health, studies have shown. Walking in nature can bring a person’s blood-sugar level from high to healthy, for example, a benefit not seen in urban environments.

According to Ming Kuo, director of the Landscape and Human Health Laboratory at the University of Illinois at Urbana-Champaign, trees have been tied to reductions in at least 16 categories of disease, including cancer, diabetes, and heart disease.

Many of these benefits are the products of small physiological changes that nature provides in a variety of ways, Kuo says. For example, exposure to plant-produced chemicals called phytoncides has been linked to healthy blood pressure, among other benefits.

Trees also help our brains to abandon fight-or-flight responses and take on a “tend-and-befriend” mind-set more conducive to giving and accepting goodwill, Kuo says. A green environment “makes us our better and healthier and smarter and nicer selves.”

For an article published in 2015, researchers analyzed seven American cities and found that high-income neighborhoods consistently had greater canopy cover than low-income neighborhoods. Unfortunately, this pattern can be hard to break. Trees tend to increase property values, leading to gentrification that forces out the neighborhood’s lower-income residents, the study reported. People who live in low-income neighborhoods may also resist tree-planting because they want to avoid increasing housing costs, tree maintenance costs, or both.

But some cities are working to break the cycle. Part of Somerville’s newly minted urban forestry plan is to start equalizing tree cover throughout the city. This means emphasizing planting in low-income and minority neighborhoods.

But such progressive plans are rare, says Robert Ricard, senior extension educator at the University of Connecticut’s College of Agriculture and Natural Resources. With limited budgets, most towns focus on removing trees that are in danger of falling and causing harm, leaving little money to manage trees proactively. “Trees or forests are low [on the list of priorities] after police, fire, schools, garbage disposal, plowing, and the like,” Ricard adds.

And challenges exist for even the most proactive towns. In addition to budget and time constraints, areas most in need of trees are often covered in pavement, making planting expensive and labor intensive. Planting on private and state-owned land requires special permission, which can further constrain a city’s efforts.

But for people such as Vides in Somerville, the presence of trees is imperative to making a place “feel more like a neighborhood.”

As we inch toward the spring planting season, local officials should keep people like Vides in mind. How long will we let financial and logistical barriers prevent cities from correcting the injustice of unequal tree planting? We need to give our cities the resources they need to treat trees as critical parts of our neighborhoods — all of our neighborhoods.

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Saima May Sidik is a freelance science journalist based in Somerville. Send comments to [e-mail].