Trees — A different shade of green

Cities look to urban forests as a natural utility

  • The pollution cycle and how trees and sunlight affect it.

    USDA Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research, WW/FS.FED.US/PSW/PROGRAMS/CUFR
  • Los Angeles Mayor Antonio Villaraigosa is joined by TreePeople Founder Andy Lipkis at a tree planting. The mayor campaigned on a promise to make L.A. the 'greenest' city in the US by planting a million trees

During his 1980 presidential campaign, Ronald Reagan famously blamed trees for emitting 93 percent of the nation’s nitrogen oxide pollution. Trees were worse for the environment than automobiles, he said, a statement that fueled decades of "killer tree" jokes.

Twenty-six years later, cities in Reagan’s home state of California are trying to live down his dendrophobic reputation. In October, Los Angeles kicked off an effort to plant 1 million trees, part of Mayor Antonio Villaraigosa’s election campaign promise to become "the biggest, greenest city" in America. Civic leaders in the Sacramento area are considering an even more ambitious effort: planting 4 million trees over the next 40 years.

While tree-planting in cities is nothing new, the scale and intent of the efforts in L.A., Sacramento and a handful of other Western cities mark a significant shift in the way urban forests are perceived. Once seen merely as a way to beautify the concrete jungle, mass tree-plantings have become major components of efforts to combat global warming by sucking up carbon dioxide from the air. And where roots and canopies were once viewed as hindrances to sewer lines, sidewalks and power lines, trees are now being treated as part of the civic infrastructure, on par with other public utilities.

"Trees in cities have long been undervalued," says Greg McPherson, a research forester at the Center for Urban Forest Research in Davis, Calif. "Cities have done easy things to be green and conserve energy. And it’s not working. So almost out of desperation they are looking at trees as green or bio technology."

In Denver, Colo., the mayor wants to plant 1 million trees over the next 20 years. Seattle, Wash., plans to add 695,000 trees over the next 30 years. And Albuquerque, N.M., is adding 10,000 over the next five years.

Benefits of bark

This trend began in the ’90s, when the Forest Service came out of the rural woods to study the value that cities can get from their trees. The agency’s Urban and Community Forestry Program puts its research into terms politicians can understand — money. Trees aren’t cheap: They need care to get established, they use water, and they can crumple sidewalks and knock out power lines. Cities budget for tree-related expenses such as picking up leaf litter, but rarely look at what the trees provide in return, McPherson says. According to the Center, every dollar spent in Los Angeles on a tree yields $2.80 worth of benefits.

"A tree in an urban setting is more valuable than one in a rural forest," says Hashem Akbari, staff scientist at the Department of Energy’s Lawrence Berkeley National Laboratory. "Both can help with carbon sequestration. But an urban tree can do much more. It helps save energy because it shades buildings and decreases air temperatures, thus reducing the amount of air conditioning needed and the amount of fuel that is burned."

Properly positioned around a house in a city like Los Angeles, trees can cut air conditioning costs by 30 percent. Sacramento neighborhoods with high canopy cover are typically 5 to 8 degrees cooler than neighborhoods with low cover, according to the Sacramento Tree Foundation. And the Department of Energy says that just three trees planted around a house can save between $100 and $250 annually in cooling and heating costs. "Our utility gives away 30,000 trees a year," says Rob Kerth, executive project leader for the Sacramento Tree Foundation. "They say it is the cheapest power plant they’ll ever build."

Trees also help reduce stormwater runoff by slowing down rainfall and absorbing water that would otherwise flow quickly over impervious pavement. Los Angeles officials predict that 1 million trees will save $5 million each year in stormwater runoff costs. "It costs so much money to build the infrastructure for a city to deal with storm water. It’s so much cheaper to just use the tree canopy," says Deborah Gangloff, executive director of American Forests.

Money talks

All this, however, takes time — most trees don’t provide full benefits until they are 25 to 30 years old — and it takes upfront capital, which many cities lack. "It is damned expensive," says Ray Tretheway, executive director of the Sacramento Tree Foundation, noting that the Sacramento Municipal Utility District pays $2 million a year for its tree program.

Los Angeles plans to spend $70 million on its tree program. The city has partnered with five nonprofits that pledged to plant 875,000 trees, with the city planting the remainder. About half the trees will be planted on private land. The mayor promised to focus on low-income communities, which tend to have fewer trees; one neighborhood, for example, has only a 5 percent canopy cover, compared to the national average of 27 percent.

The Forest Service’s Urban and Community Forestry Program is picking up some of the expenses through grants to cities. But even as more cities embrace trees, the federal program’s budget is shrinking, dropping by 25 percent in just the past four years.

McPherson hopes his research inspires more communities to jump on the bandwagon. In fact, his group is part of a Sacramento-area study examining how an urban forest affects the amount of local smog; the results may put to rest any lingering belief in Reagan’s "killer trees." Future research will focus on psychological benefits, including stress reduction and workplace productivity. "By trying to monetize everything and put it in dollars and cents for policy makers, one could argue we are really minimizing trees," McPherson says. "But money talks."


The author is an HCN intern.

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