The tree is a beloved symbol of environmental health, but when it comes to combating pollution, not all trees are equal.
Just as some provide better shade than others, some types apparently are superior at filtering particulates from the air.
In the Central Valley, where particulates are a wintertime scourge, evergreens are considered more useful than trees that drop their leaves in fall.
Pines and deodar cedars, with their abundant needles, are favorites of Tom Cahill, a University of California, Davis, atmospheric physicist who has studied particle pollution around the world.
Deodars especially are suited for the dry summer climate of Sacramento, he said: "They have a low water requirement, a lot of surface area and are very pretty."
In keeping with the thinking that evergreens are better filters, Arden Middle School last fall planted a row of pines at the edge of the soccer field by Watt Avenue to shield students from traffic exhaust emissions.
The schools' PTA has set aside $10,000 for additional landscaping to be planted later this year.
The science behind trees' abilities to counter particle pollution is young. Greg McPherson at the UC Davis Center for Urban Forest Research said little work has been done here on the subject, though British scientists have published a few studies.
One, which appeared in the Journal of Arboriculture in January 2000, found bushy conifers were better than broad-leafed trees at capturing particles.
The ability to snag pollutants is just one factor in choosing the right tree type, McPherson said.
"You don't want trees that will drop big cones," he said. "You want to avoid trees with dangerous or messy fruit. And they also need to be pest and disease resistant. There's a whole long list of things."
Tree health is another consideration. "Picking species that can tolerate high levels of particulates and ozone (pollution) would be critical," McPherson said, adding that keeping a tree's soil environment healthy - with ample water and nutrients - may help it endure heat and foul air.
Besides acting as particle filters, it's possible trees could blunt people's exposure in another way: Cahill hypothesizes that trees planted tightly in a row might act like a wall, forcing airborne particles upward, away from where people breathe.
Such a system would be served best by yet another characteristic of some trees - the ability to grow tall fast, like, for example, certain cypresses and junipers.
Cahill and McPherson plan soon to test the idea by experimenting with various trees in a wind tunnel on campus.Sacramento Bee
