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The new finding fits with a host of evidence linking urban greenery to mental health.

By now you're probably familiar with at least some of the many psychological benefits of urban greenery. Parks, trees, and other forms of nature evidently possess the power to refresh tired minds and improve moods; they've also been associated with better mental health more broadly. This robust line of evidence suggests that city trees serve as leafy happiness ninjas, defending our brains against the stressors of urban life.

All of which is necessary background to make the findings of a new report in the journal Landscape and Urban Planning more palatable: Londoners who live near more street trees get prescribed fewer antidepressants. The largely U.K.-based research team—which includes some of the field's leading scholars—report that this association held true even when controlling for other local variables like socioeconomic status. They conclude (san citations):

Our analysis indicates that boroughs of London with a higher density of street trees tend to have lower antidepressant prescription rates. This effect remains after controlling for potential confounders and allowing for the influence of unmeasured area effects. These findings complement previous research suggesting the benefits of street greenery for mental health.

The study methods were straightforward: Researchers gathered data on antidepressant prescriptions across London in 2009-2010 and paired that with data on street trees in the same area. (They specifically focused on street trees as opposed to public parks, which suggests that their findings might even under-emphasize the effects of urban nature.) The research team also looked at other variables often associated with health outcomes, such as socioeconomic status, unemployment, smoking, and age.

The numbers revealed an average of 40 trees per kilometer across the boroughs of London, with antidepressant prescriptions ranging from about 358 to 578 per 1,000 people. But the places with higher tree densities had lower prescription rates: For every additional tree per kilometer of street, the researchers found 1.38 fewer prescriptions in the population. The link held true—if slightly weaker, down to 1.18 prescriptions—when they took the other variables into account.

Though the findings fit with previous research on nature's health benefits, the new study can't conclude that London trees directly caused the prescription trends. Plenty of other factors no doubt come into play. Streets with more trees might be generally more pleasant to live on, encourage physical activity, or reflect strong community ties—all of which could boost health and well-being. It's also possible that healthier people move to tree-lined streets in the first place.

Still, for cities weighing the pros and cons of public planting programs, it's one more data point in the plus column.

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