There is a growing body of research suggesting that the antidote to a significant portion of the modern mental health crisis is, in part, outside. Not in a vague "get some fresh air" way — in a specific, measurable, physiological way. Studies show that as little as 20 minutes in a green space lowers salivary cortisol. Birdsong activates the parasympathetic nervous system. The compounds trees release have been shown to reduce blood pressure. And the biodiversity of the soil children play in may shape the resilience of their immune systems for life.
The science has been accumulating for years. What hasn't kept pace is access — to parks, to walkable green space, to neighborhoods where trees are simply part of daily life rather than a destination that requires an effort to reach. Dr. Sheetal Rao, an internal medicine physician and co-founder of Nordson Green Earth, a Chicago-area urban forestry nonprofit, has watched that gap translate directly into patient outcomes. "Nature should be integrated into daily life," she said. "You shouldn't have to go somewhere, make an effort — because people are so busy." The communities that can't make that effort, she has found, are often the ones most in need of what green space provides.
What Trees Actually Do to Your Brain
The stress response is ancient, and so, it turns out, is the thing that quiets it. The prevailing theory among researchers is that humans evolved to read a green environment as a signal of safety — of food nearby, of threat absent. That still happens. A meta-analysis found that increased natural exposure consistently correlated with decreased salivary cortisol, with the greatest reductions occurring around 20 to 30 minutes of nature exposure. "Just being around greenery, studies show that our cortisol levels go down," Dr. Rao said. "And there are some elements that trees release that are shown to potentially reduce our blood pressure as well."
Those elements are phytoncides — volatile organic compounds emitted primarily by trees, particularly conifers, that have been shown in research to lower blood pressure, reduce stress hormones, and increase the activity of natural killer cells in the immune system. The practice of forest bathing, identified in Japan in the 1980s as shinrin-yoku, was built on this pharmacology: the idea that simply being among trees — breathing the air they produce — has a therapeutic effect that doesn't require any particular effort on the part of the person doing it.
Then there's sound. A 2022 study published in Scientific Reports found that listening to birdsong significantly reduced anxiety and paranoia in healthy participants. Researchers believe the mechanism is evolutionary — birdsong historically indicated that the environment was stable, predator-free, and safe to relax in. "The bird sounds that trees foster is another aspect that humans associate with feeling safe and calm," Dr. Rao said. "The dappled sunlight, the smells — these things can make us feel better." The parasympathetic nervous system, which governs rest, digestion, and recovery, responds to these cues in ways that the nervous systems of people living in dense, car-heavy, treeless environments rarely get to experience.
The Biodiversity Factor
The conversation about green space and mental health tends to focus on the visible — trees you can see, paths you can walk, parks you can sit in. But some of the most compelling recent research points underground. A landmark study published in Science Advances found that urban daycare children whose yards were renovated with forest floor material developed significantly more diverse microbiomes and better-regulated immune systems within a single month, compared to children playing on standard gravel. The implications extend beyond immunity. A dysregulated immune system and a dysregulated nervous system are not unrelated — chronic inflammation has been increasingly linked to depression, anxiety, and cognitive decline.
The biodiversity hypothesis, endorsed by the World Allergy Organization, formalizes what the data has been suggesting: reduced contact with natural microbial environments in early childhood disrupts the immune calibration that healthy development requires. "Biodiversity helps our health quite a bit," Dr. Rao said. The practical translation of this, for anyone involved in designing green spaces that serve communities rather than simply beautify them, is that the species mix matters. Native plants — the kind that belong to a specific ecosystem and support its insects, birds, and soil life — do more ecological and biological work than ornamental landscaping.
The practical translation, for anyone designing green spaces that serve communities rather than simply beautify them, is that the species mix matters. One Tree Planted's Urban Forestry Action Fund — which has supported projects across 60 cities — prioritizes native plantings in its community projects for exactly this reason: native species support local insects, birds, and soil biodiversity in ways that ornamental landscaping does not. The birdsong that the research keeps linking to calm, the soil microbiome diversity that shapes immune health — these are downstream effects of getting the plants right.
The Access Question
None of this is evenly distributed. Research published in npj Urban Sustainability analyzing 37 U.S. cities found that formerly redlined neighborhoods — those marked in the 1930s as poor investment risks, largely because they were home to Black, immigrant, and low-income residents — average roughly 23 percent tree canopy coverage compared to 43 percent in the highest-rated zones. Those same neighborhoods run up to 7°C hotter in summer. The communities with the highest rates of the chronic conditions that green space helps manage — hypertension, obesity, depression, diabetes — are often the ones with the least access to it. The correlation is not coincidental.
Walkability follows the same map. Neighborhoods built around cars, frequently those bisected by the highway construction of the 1950s that routed interstates through Black communities with deliberate frequency, score low on walkability indexes that research consistently ties to physical activity rates, mental wellbeing, and longevity. It is one of the clearest examples of infrastructure as health policy — and of health policy as something that happened to some communities and not others.
Nordson Green Earth's work in Markham, Illinois — a predominantly Black south Chicago suburb shaped by both redlining and highway fragmentation — operates at the neighborhood scale: a native prairie walking path beside the public library, with a birding program and a community wellbeing survey to track outcomes before and after. "Green space projects are more common in whiter, more affluent communities," said Christal Beyer, director of the Markham Public Library. "To be able to offer this is huge."
The urgency of that work is sharpened by a 2025 study published in Biological Psychiatry — one of the most significant recent findings in the field. Tracking more than 7,000 adolescents across the United States, researchers found that greater greenspace exposure at ages 9 and 10 was associated with measurably larger brain surface area, greater cortical volume, and better mental health and academic outcomes two years later. The brain, it turns out, develops differently depending on whether a child grows up with access to green space. Which means the question of who gets trees is not just a quality-of-life question. "We're so connected technologically, but so disconnected in many ways," Dr. Rao said. A place to walk, to hear birds, to be outside among living things without making a special effort — these are not small things. The research, increasingly, is unambiguous on that. Access though, still is.
