A Cool Dividend


Parking under shade trees does more than keep thighs from sticking to vinyl seats--it may curb pollution.

The U.S. Forest Service will meet today) with the environmental group TreePeople and Los Angeles City Council staff members to discuss a recent study that suggests lowering vehicle temperatures with something as simple as shade slows gasoline evaporation from parked cars.

“Parking lots are a wasteland,” said Greg McPherson, the study’s leader. “They’re pollution hot spots. Yet people haven’t focused on them as a component of the urban ecosystem.”


TreePeople is hopeful the study will bolster its efforts to increase tree plantings in Los Angeles and create the group’s envisioned “urban forest.” The city’s 2-year-old ordinance requiring 50% shade over parking lots within 10 years has been inconsistently enforced and only applies to the newest developments, as shopper Diane Wilson rediscovered Wednesday as she left an older--treeless--Northridge supermarket.

“The steering wheel and seat will be so hot,” said the 39-year-old mother from Eagle Rock as she loaded bags of ice into a cooler of drinks for her son’s Little League team. “I have to wait for it to cool down.”

Straight out of a sci-fi flick, noxious fumes waft from vehicle gas tanks under a searing sun. The scene plays daily in jammed parking spaces that checker the city. In fact, these harsh, heavily traveled environments take up an estimated 10% of cities, according to the Forest Service, which hopes to continue studying the benefits of trees in parking lots and encourage local groups to plant them.


The agency’s $30,000 government-funded study was the first to examine the relationship between shade and cars parked on asphalt, where temperatures can hit triple digits.

In suburban Sacramento, Forest Service researchers created weather stations within a typical parking lot in Davis to measure air temperature and wind speed. They compared temperatures in shaded and unshaded areas.

Then, using an index from the California Air Resources Board--which measures hydrocarbon emissions at different temperatures--the researchers calculated pollution reduction. They concluded that increasing canopy coverage to 50% could reduce hydrocarbon emissions by nearly a ton per day throughout the greater Sacramento area.

Researchers have long known that shade trees cool outside air temperature by as much as 10 degrees and car interiors by 40 degrees, all this confirmed in everyday life--just check out which spots are nabbed first. But less understood is that parked cars, not just careening jalopies, emit pollutants.

Like any liquid, gasoline evaporates as temperatures rise. The hotter the car, the higher the evaporation rate of gasoline and the release of hydrocarbons--the primary ingredient of ozone, a culprit linked to smog and global warming.

Most solutions have targeted auto makers. Since the ‘70s, carbon-activated canisters have been required on cars to capture and store gasoline vapors until the engine burns off the fumes.

Trees cannot completely clean up the problem, but they can help. Shade would tackle the problem at the next stage--after cars have rolled off the assembly line and into sizzling parking lots. Environmental experts are praising the strategy.

“It’s a constructive approach. It’s easy, as opposed to clamping down further on industry or the vehicles,” said Arthur Winer, director of UCLA’s environmental science and engineering program.

But others contend modest pollution reductions won by trees might be outpaced by more stringent standards in store for car manufacturers, a spokesman from the Clean Air Resources Board in El Monte said.


Parked cars account for nearly a third of hydrocarbon emissions from motor vehicles, and smog is at its worst during midday and summer. In the Southland, vehicles made up nearly half the total hydrocarbon emission in 1997, according to the South Coast Air Quality Management District.

Since April 1996, the city has required parking lots to include trees to reduce heat and glare: one tree to every four parking spaces. Within 10 years, the shade canopy must cover 50% of the parking lot stalls, City Planner Mike O’Brien said.

Still, a majority of the city’s parking lots were paved before that ordinance was enacted, which followed similar measures in the Central Valley by about two decades. And parking lots in Los Angeles and the Central Valley are rarely inspected after the design stage to ensure the promised trees have actually been planted. O’Brien acknowledged that under L.A.’s tree ordinance, inspectors are not supposed to check parking lots until they are a decade old.

Trees may be dead, stunted, removed or pruned into ornamental shapes that offer no leafy solace from the sun, said Dennis Pittenger, an environmental horticulturist at UC Riverside. At the Woodland Hills Promenade, for instance, just months before L.A.’s tree ordinance was adopted, mature trees were uprooted to make way for movie theater parking, a mall spokesman said.

TreePeople officials said more trees could make circling the lot seem like a walk in the park.

“Parking lots and other areas of extensive asphalt are not good for people,” said Andy Lipkis, the group’s president. “We want to break up the heat island, and create the notion of a park.”

Lipkis praised the parking lot at Santa Monica City Hall, with its concrete bumpers that protect trees. He gave a thumbs down to parking lots at Laurel Plaza in North Hollywood, and Dodger Stadium. “They’re equally challenged. You’re squinting and feeling terrible.”


Magnolia, fern pine and Chinese flame trees, with their spreading branches, provide good shading, TreePeople says. But the ubiquitous palm tree and the skinny eucalyptus--which grow up rather than out--are not effective, according to TreePeople.

And trees have not always been a priority for business owners because of the parking spaces they squeeze out and the cost of maintaining and watering them, Lipkis said. Costs for initial labor and the tree itself run about $100, while pruning and and other maintenance runs about $27 per tree annually, the Forest Service calculates.

Developer Jerry Katell said adding trees makes projects more attractive and cited his own Agoura Hills business park, where 1,000 trees are spread over 30 acres.

“That’s what people notice,” Katell said.


Cleaner Air in the Shade

A U.S. Forest Service study suggests that increasing tree canopy cover reduces pollutantsemitted by parked cars. Pollution measurements were taken over a two-week period in Sacrameto.


Amount of tree canopy coverage

Test case: 8%

Hypothetical cases: 25% / 50%


Average daytime air temperature in parking ot

Test case: 93.6’ F

Hypothetical cases: 92.6’ F / 91.3’ F


Vehicle hydrocarbons emitted citywide in Sacramento parking lots, in tons per day.

Test case: 25.36

Hypothetical cases: 25.24 / 24.78

Source: United States Forest Service