Brazil and Malaysia aren't the only ones removing trees faster than they're replacing them; we're doing the same thing to our urban forests, area arborists say.
Research by the American Forestry Assn. "shows us that cities are removing four trees for every one they're planting," says Tom Larson, a member of the group, president of Urban Forestry Consulting and founder of the nonprofit Tree Society of Orange County.
These are trees that we can ill afford to lose, he says. Our cities are already 5 to 9 degrees Fahrenheit warmer than surrounding areas, according to association statistics, and they're predicted to heat up another 6 to 12 degrees within the next few decades if present trends continue.
Trees cool cities off directly by providing shade and reducing energy consumption for air conditioning. Indirect cooling results from trees reducing carbon dioxide in the atmosphere. (Carbon dioxide traps heat that would otherwise radiate into space.) We need more trees, not fewer, Larson says.
But that doesn't mean we should all grab our shovels and go out and plant the biggest trees we can afford in our parkways. Putting the wrong trees in parkways in the first place--whether those plantings were done by citizens or municipalities--accounts in large part for the high incidence of tree removal that cities now face, tree experts say.
"Ninety percent of the trees available in the market do not belong in public parkways," says Daryl Smith, superintendent of parks, trees and landscapes in Huntington Beach. "Their trunks get too big, and their roots are too invasive."
If a tree is planted in an area too small for its root system, Smith says, it just seeks nourishment elsewhere. "The roots are attracted to the warmth of the soil under the sidewalk, the moisture and fertilizer in the lawn and the moisture and warmth in the rock and gravel under the asphalt in the street."
When roots burrow toward these natural magnets, he says, they begin popping up sidewalks and breaking up curbs, leaving cities with just two options: breaking up the roots, which endangers the life of the tree, or taking the tree out altogether.
"The minimum space any kind of shade tree needs to thrive is 10 feet--and most of our medians aren't even that big," says Steve Holcomb, an arborist at Irvine-based Golden Coast Environmental Services. "That's why cities find themselves having to cut down mature trees every 10 to 15 years. As soon as the trees get big enough to provide any environmental benefit, they tear up the hardscape."
Trees in cutouts fare even worse. "The average age of trees in these 2-foot-by-2-foot cutouts--which, personally, I think are an abomination--is only seven to eight years," says Alden Kelly, arborist consultant and member of Street Trees Seminar, a professional association of municipal workers, consultants and vendors dedicated to the promotion of street trees and improvement of street tree management.
"We're treating trees like big potted plants," he says. Or, as Holcomb puts it, "like long-term poppy beds."
Cities have been working with the nursery industry, public utility plants and other concerned parties to find trees that will have a longer life span in restrictive parkways, without causing problems to the hardscape. But the narrow limit of choices under these constraints--in Huntington Beach, for instance, just 10 tree species are approved--can create other problems.
"When you have only a few species to choose from and start building up large host populations, those host populations will inevitably attract insects and encourage them to multiply," Holcomb says.
Ornamental pear trees, for instance, which were planted heavily in many parts of the county over the last eight to 10 years, are now struggling with white fly infestations, he says. And palm trees, another widely used choice in narrow parkways, are developing "some real sophisticated diseases we don't know how to treat."
Pest problems are causing many cities to take a fresh look at their lists of street trees to see whether more can be added. Working with a committee of citizens, consultants and city staff people, Anaheim recently doubled its previous species list.
"We had maybe 25 varieties for the entire city before," says Al Epperson, tree services coordinator. "And lots of single varieties in certain areas.
"I worked in Chicago in the '60s when the Dutch elm disease struck, and we took out 10,000 trees a year, so I know what can happen. If you have only a few varieties, these diseases can wipe you out. The white fly infestation affecting the ash and evergreen pear is bad enough as it is."
Eliminating parkways as they now exist would allow for more options, and that's exactly what arborists would like to see.