Trees are incredibly smart. They run on sunshine, provide shade in summer and ever so kindly drop their leaves to allow the winter sun through. And now a team from the University of Exeter has determined that they are good for our mental health, too. Londoners who had more trees on their street popped fewer antidepressant pills. Eric Jaffe of Citylab reports:

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. ... 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.

So what is it about trees that makes us happier people? Perhaps they know they are going to live longer, as discovered by Geoffrey Donovan who looked at comparable death rates in areas where the emerald ash-borer killed all the trees. In other research Donovan found that trees prevent crime, correlating tree coverage with crime statistics; the more trees, the lower the crime rate.

Research by the USDA's Forest Service also showed that people who live around trees are physically healthier: "About 850 lives are saved each year, the number of acute respiratory symptoms is lower by about 670,000 incidents each year, and the total health care savings attributed to pollution removal by trees is around $7 billion a year."

A tale of two cities. At top: Poor Somerville, Massachusetts; at bottom: Rich Cambridge, Massachusetts. Google Earth via Per Square Mile

Over at Per Square Mile, Tim de Chant spent some quality time with Google Earth, comparing neighborhoods and finding that (surprise!) wealthy neighborhoods have more trees. He also quotes research on the subject:

They found that for every 1 percent increase in per capita income, demand for forest cover increased by 1.76 percent. But when income dropped by the same amount, demand decreased by 1.26 percent. That’s a pretty tight correlation. The researchers reason that wealthier cities can afford more trees, both on private and public property. The well-to-do can afford larger lots, which in turn can support more trees.

Indeed, piles of of research show that people who live around trees are richer, healthier, safer, live longer and just generally lead happier lives. In America, where the poor don't have as easy access to health care and prescriptions, it might skew the pill-poppers up the economic ladder. Since the National Health System in the U.K. gives pretty much everyone equal access to care and prescriptions, it shouldn't be surprising that poorer, sicker and less happy people would be taking more antidepressants.

The authors of the study claim that they have adjusted for socioeconomic status, employment status, prevalence of smoking and age. There is so much statistical noise here that it's impossible to tell what factor is actually causing the difference, but the conclusion is clear: "Street trees may have a role to play in supporting neighborhood mental health."

Amen to that.