"I think that I shall never see/A poem lovely as a tree," wrote Joyce Kilmer. The poet, no doubt, was talking about aesthetics. If he wrote those lines today, chances are he would also have urban blight and community health in mind.
As well as bringing arboreal charms to inner cities, trees help improve air quality by reducing air temperature, removing air pollutants and providing shade that lowers energy use in buildings, thus reducing air-polluting emissions from power plants.
In New York City alone, it's estimated that trees remove about three times more lung-choking air pollutants than do shrubs. It's little wonder, then, that the city has embarked on an ambitious program to plant 1 million new trees by 2030, and Boston is planning to plant 100,000 new trees over 20 years.
A major reason why cities increasingly are turning to trees as a public health measure is because of the alarming rise in respiratory and cardiovascular diseases in urban areas. Asthma, in particular, now affects 20 million Americans. The culprit in many researchers' view is particulate matter (PM) spewed out by diesel combustion, power plants and smelters, to name just a few sources. But trees, according to an increasing body of research, are a perfect antidote by capturing fine particles in the air and potentially deflecting them away from high-population areas.
On June 2, Cornell Cooperative Extension-New York City and Cornell's Department of Horticulture focused on this thesis at a conference at Weill Cornell Medical College funded by the U.S. Department of Agriculture.
The participants heard from urban planners about their tree-planting programs, from horticulturists about designing sites to mitigate the effects of particulate pollution, and from medical experts about tiny particles in the air that can get into the lungs and interact with lung cells. Then they heard from Tom Whitlow, a researcher in Cornell's Urban Horticulture Institute.
His "inconvenient conclusion": Increasing tree cover as a strategy for reducing asthma "is unlikely to work." It might be disingenuous, he said, "to suggest that planting more trees might help a community's health" in a directly measurable way.
Whitlow described his as-yet unpublished research into the way fine particulate matter, called PM2.5 (meaning the particles are 2.5 micrometers in diameter and smaller), is deposited on leaves. These particles are among the U.S. Environmental Protection Agency's designated six so-called criteria pollutants. The agency warns that PM2.5 pollutants are unhealthy to breathe and are associated with premature mortality and other serious health effects.
Using a wind tunnel, Whitlow circulated a plume of particles through bunches of foliage. After measuring the change in the plume, he concluded that there was "no leaf-area effect" and that leaves are not good filters of PM2.5. In fact, he noted, "Deposition [on a surface] reaches a minimum in the PM2.5 zone."
Although more research is needed, Whitlow said he has so far monitored 16 different size fractions, both larger and smaller. What's important, he later added, is to use "the appropriate currency to establish cause and effect."
Participant Max Zhang, Cornell assistant professor of mechanical and aerospace engineering, who has developed a computer model of the changes in size of particulates in relation to their distance from roads in New York City's South Bronx, noted the increasing evidence of the impact on human health by the smallest particles in the air, or nanoparticles. Planting trees near roadways has the potential to reduce particles at the nano level, he said. "Canopy has the potential to protect those living near roads." Zhang noted that 36 million Americans live within 300 feet of a four-lane highway, railroad or airport.
Among those advocating on-ground studies was Nina Bassuk, Cornell professor of horticulture. She suggested that a research program be tied to areas in New York City where the air quality in similar streets with and without trees could be compared to determine if trees were able to deflect or remove a wide range of pollutants from the air.
Jennifer Greenfeld of New York City's Department of Parks and Recreation offered the view that "public health benefits do not trump other considerations. Tree planting has a role to play in some of these problems of neighborhood ills."
But Charles Lord of Boston's Urban Ecology Institute voiced a fear that was in many minds. "I hope this is not the silver bullet that kills tree-planting programs," he said.