New Australian research finds that, when a neighborhood's green space leads to better health outcomes, it's the canopy of trees that provides most of the benefits.

"Shorn of tree canopy, sidewalk temperatures can be higher, sidewalks can seem noisier, and walkers along them are exposed to more air pollution," researchers write. (Photo: Tim Gouw/Unsplash)

In recent years, study after study has found that living in neighborhoods with abundant green space is linked to positive health outcomes. These include better heart health, stronger cognitive development, and greater overall longevity. No wonder these areas are also linked to lower levels of Medicare spending.

But when it come to promoting human health, not all green spaces are created equal. That's the conclusion of new Australian research, which finds higher levels of wellness in areas marked by one particular manifestation of the natural world: leafy trees.

"Protection and restoration of urban tree canopy specifically, rather than any urban greening, may be a good option for promotion of community mental health," write Thomas Astell-Burt and Xiaoqi Feng of the University of Wollongong in New South Wales. Their study, along with a commentary, is published in the journal JAMA Network Open.

They describe a large-scale longitudinal study featuring 46,786 mostly older residents of three Australian urban areas. The subjects were initially interviewed between 2006 and 2009; follow-up reports were taken between 2012 and 2015.

At both points, participants were asked to rate their overall health, and noted whether they have ever been diagnosed with, or treated for, anxiety or depression. In addition, they completed a 10-item questionnaire designed to measure their risk of psychological distress. Among other items, they noted how often in recent weeks they had felt "hopeless, rigid, or fidgety," "so sad that nothing could cheer you up," or "worthless."

Researchers compared the participants' answers to the natural features of the "mesh block" where their home is located (a geographical unit containing 30 to 60 dwellings). Using satellite imagery, the team calculated both the percentage of total green space and "separate green space types, including tree canopy, grass, or other low-lying vegetation."

After taking into account such variables as the participants' age, gender, education, and household income, the researchers were able to confirm the results of previous studies, finding that "total green space appeared to be associated with lower odds of incident psychological distress."

More intriguingly, they also found that exposure to low-lying vegetation was not consistently associated with any particular health outcome. Exposure to grass was, surprisingly, associated with higher odds of psychological distress. The wellness-boosting feature, then, appears to be the trees.

The researchers report that living in areas where 30 percent or more of the outdoor space is dominated by tree canopy was associated with 31 percent lower odds of psychological distress, compared to people living in areas with 0 to 9 percent tree canopy. "Similar results were found for self-related fair to poor general health," with tree-rich residents reporting better health overall, the researchers write.

Astell-Burt and Feng can only speculate on the reasons behind their findings, but they come up with some reasonable guesses. "Shorn of tree canopy, sidewalk temperatures can be higher, sidewalks can seem noisier, and walkers along them are exposed to more air pollution," they write.

In addition, they point to studies suggesting that "higher levels of biodiversity, rather than the amount of green space, were associated with more favorable levels of psychological well-being." Research shows that "tree canopy is more supportive of biodiversity than open grasslands," they add.

In an accompanying commentary, Dutch environmental researcher Sjerp de Vries cautions that these researchers are journeying into "uncharted territory," and suggests that a measure of per capita green space might be the best measure of its benefits. Additional research is clearly called for.

Nevertheless, these results provide evidence of the benefits of natural shade, and suggest that our love of trees may be biologically driven. A neighborhood's leafiness is worth keeping in mind when you're deciding where to put down roots.