Right now, 55 percent of the world’s population lives in urban areas. By 2050, that percentage is expected to grow to 68 percent, a forecast that presents city planners with a unique problem: How can urban dwellers stay healthy when scientific evidence shows that being in nature is better for their mental health?

In a study published Friday in JAMA Open Network, researchers at Australia’s University of Wollongong attempt to tackle that problem by figuring out precisely what types of green spaces provide the biggest benefits. In any given city, there’s an array of what could be considered “green”: big parks filled with trees, rooftop gardens, that patch of grass quilted onto the sidewalk. Each one comes with a different price tag — and a different likelihood of helping an urbanite for the better.

This study was supported by grants from Australia’s National Health and Medical Research Council, as well as Hort Innovation Ltd., a not-for-profit research and development corporation that supports Australia’s horticulture industry.

The team compared three types of green spaces: tree canopy (mature trees whose branches cover the ground when viewed from above), grass, and low-lying vegetation like shrubs. Nearly 47,000 city-dwelling Australian adults over 45 years old reported not only whether they lived near these different types of spaces, but also their self-reported mental and general health. They were surveyed twice, with the second survey six years after the first.

While these surveys didn’t expose many clear answers about the likelihood of experiencing depression and anxiety while living near these spaces, the researchers did find that exposure to more tree canopy was associated with a lower likelihood of psychological distress and better self-rated general health. Exposure to low-lying vegetation wasn’t consistently linked to any outcome, and exposure to grass was associated with a higher likelihood of reporting fair to poor general health, as well as “prevalent psychological distress.”

To show that comparison in numbers, exposure to 30 percent or more tree canopy, compared with 0 to 9 percent tree canopy, was associated with 31 percent lower odds of experiencing incidents of psychological distress. The people who reported that they were exposed to tree canopy lived within one mile of it.

Meanwhile, exposure to 30 percent or more grass was associated with 71 percent higher odds of prevalent psychological distress, after adjusting for age, sex, income, economic status, couple status, and educational level. 

While that is not a great look for grass, the study authors emphasize that “this finding ought not to be interpreted as evidence for removing existing grassy areas or defunding the planting of new open grassy areas because the result in this study may be confounded with other factors that are detrimental to mental health.” Because these results are based entirely on self-reported surveys, they may not represent the full complexity of a person’s mental wellbeing.

Despite this caveat, the authors have some ideas on why being around tree coverage results in people feeling healthier than being around grass. Without the shade of trees, sidewalk temperatures are hotter, sidewalks can seem noisier, and the walkers themselves are exposed to more air pollution. They also cite studies that show people are happier in green spaces that contain more stimuli — because trees are more complex than grass, it’s likely that we enjoy being around them more.

Previous research on a form of nature therapy known as “forest bathing” has also demonstrated a link between exposure to trees and healthiness. Spending time in forests has been shown to lower blood pressure, cortisol (stress hormone) concentrations, and pulse rate. When compared to people who experience forest bathing, those who stay in cities do not experience the same benefits.

The evidence consistently shows that being in nature offers positive health benefits, although more parameters are emerging. For example, a study released in June pinpointed the exact amount of time per week people need to spend in nature in order to achieve its benefits: 120 minutes. It doesn’t matter whether those two hours are spent all at once or broken up into moments throughout the week.

This new study provides a new sort of parameter — it’s clear that nature makes us happy, but certain forms of nature can help us feel the happiest.