By Susan Perry
“Our findings suggest that urban greening strategies with a remit for supporting community mental health should prioritize the protection and restoration of urban tree canopy,” the researchers concluded.
Many studies have found that living near a green space — land that is partly or completely covered with natural vegetation — is associated with health benefits, including lower blood pressure, reduced levels of stress and anxiety, and an increased sense of general wellbeing.
Research has even linked green space with lower Medicare expenditures.
What hasn’t been clear from these studies, however, is whether all types of green space confer the same benefits. Or are some green spaces potentially more healthful than others?
A new study from Australia, published recently in JAMA Network Open, offers an answer. It found that although residents of neighborhoods with plenty of leafy trees tend to have higher levels of psychological health and well-being, the same isn’t true for people living in neighborhoods where the green space consists primarily of open areas of grass.
In fact, people living in areas with higher percentages of bare grass tend to have higher levels of psychological stress, the study found. They also report being in poorer health.
“Our results suggest the type of green space does matter,” write the study’s authors, Thomas Astell-Burt and Xiaoqi Feng, in an online article for The Conversation. The two researchers are founding co-directors of the Population Wellbeing and Environment Research Lab at the University of Wollongong.
This finding doesn’t mean, however, that existing grassy areas should be removed or plans for new ones should be scrapped, they stress.
“Large open areas of grass can be awesome for physical activity and sport,” they write, “but let’s make sure there is also plenty of tree canopy too, while also thinking about ways to get more people outdoors in green spaces.”
Astell-Burt and Feng offer several possible explanations for their study’s findings. One has to do with the shade offered by trees:
Studies are linking high temperatures with heat exhaustion and mental health impacts. Research has suggested trees, rather than other forms of green space, may be best at reducing temperatures in cities. It may also simply be more comfortable to walk outside in cooler temperatures — not to mention going for a run or bike ride, both of which are good for mental health.
The biodiversity that trees offer may also be beneficial:
Research suggests tree canopy tends to be more biodiverse than low-lying vegetation. Increased biodiversity may support better mental health by enhancing the restorative experience and also via the immunoregulatory benefits of microbial “Old Friends” — microorganisms that helped shape our immune systems but which have been largely eliminated from our urban environments.
Open areas of grass, on the other hand, are not as inviting and therefore may impede rather than enhance health:
[L]arge areas of bare grass in cities can make built environments more spread-out and less dense. Without tree canopy to shield from the midday sun, this may increase the likelihood of people using cars for short trips instead of walking through a park or along a footpath. The result is missed opportunities for physical activity, mental restoration, and impromptu chats with neighbours. Previous work in the United States suggests this might be why higher death rates were found in greener American cities.
By Alissa Walker
On Dallas’s hottest summer days, Matt Grubisich would dispatch his colleagues at the Texas Trees Foundation to take manual air temperature readings across the city. “You think the number to go by is the weather station out at the airport,” he says, pointing to Dallas’s Love Field, five miles northwest of the city center. “But then we’d go to a parking lot downtown where it was 6 degrees warmer.”
As the director of operations and urban forestry for the Texas Trees Foundation, Grubisich was trying to demonstrate that Dallas needed to make major design changes to fortify itself against epic heat waves. “It’s easy to explain to people why they park under a tree when they drive to the grocery store,” he says. Getting local leaders and the population at large to take action to bring more trees to the city was a tougher sell.
Thanks to the city’s 2015 effort to map its urban forest, Grubisich and his team already knew that the city’s trees were not evenly distributed. Almost half of Dallas’s trees were located within the Great Trinity Forest, a 6,000-acre nature preserve. That didn’t leave a lot of trees for the rest of the city, where some neighborhoods only had tree canopy over 10 percent of their communities.
To make the case for rethinking the city’s approach to trees, Grubisich and the Texas Trees Foundation turned to data on a larger scale. The organization’s comprehensive urban heat study, released in 2017, showed that one-third of the city was suffering from a phenomenon called urban heat island effect. A full 35 percent of the city was covered by impermeable surfaces, like parking lots, roads, and buildings, which absorb sunlight and end up heating up the air around them.
“I knew we would have a rather robust urban heat island,” says Grubisich. “That number, that was the most alarming part. That was the catalyst.”
Dallas’s heat island was more than robust: Parts of the city were up to 15 degrees Fahrenheit warmer than their rural counterparts. The urban heat island was expanding so rapidly that the ninth-largest city in the country was warming faster than any other large U.S. city except Phoenix.