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.”
For the study, Astell-Burt and Feng analyzed data collected from 46,786 adults, aged 45 and older, who lived in three Australian cities: Sydney, Wollongong and Newcastle. The participants were interviewed between 2006 and 2009 and then again six years later, between 2012 and 2013. At both interviews, they were asked to rate their general health and if they had ever been diagnosed by a physician with anxiety or depression. They were also asked questions designed to assess their risk of psychological distress.
Astell-Burt and Feng then looked to see what associations, if any, existed between the participants’ health and the green space where they lived. They used satellite images to calculate the percentage of total space, including its type (trees, low-lying vegetation or grass), within a one-mile walk of the participants’ homes.
After accounting for factors that can influence general health and psychological well-being, including age, household income, educational level and marital status, the researchers found that “total green space appeared to be associated with lower odds of incident psychological distress.”
But that was true only for green spaces with significant amounts of trees.
“Adults with 30% or more of their neighbourhood covered in some form of tree canopy had 31% lower odds of developing psychological distress,” compared to people living in areas with 0 to 9 percent tree canopy, the researchers explain. “The same amount of tree cover was linked to 33% lower odds of developing fair to poor general health.”
Adults living in neighborhoods with 30 percent or more of the land covered in grass, however, were 29 percent more likely to have psychological distress. And they were 53 percent more likely to report being in fair or poor general health.
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.
Limitations and implications
This is an observational study, so it can’t prove any direct cause-and-effect relationship between green spaces and psychological health. Also, the participants self-reported much of the data the study relies on, including whether they had ever been diagnosed with anxiety or depression. Such self-reports can be unreliable, particularly given the stigma often associated with mental illness.
Still, the study’s findings are interesting and suggest that future studies on the effects of green space on wellness and well-being should look at the types, not just the amount, of those spaces.
“Population growth and the demand for more housing, amenities, and infrastructure in Sydney, Newcastle, and Wollongong is a challenge experienced in many other cities worldwide,” Astell-Burt and Feng write in their study. “Street trees in prime building locations are at a particular risk of being cut down. Shorn of tree canopy, sidewalk temperatures can be higher, sidewalks seem noisier, and walkers along them are exposed to more air pollution.”
“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,” they conclude. “In addition, the promotion of equal access to tree canopy may provide greater equity in mental health.”
FMI: The study can be read in full on the JAMA Network Open website.