Major cities are planting more trees to beat the heat

By Diane Dieta
Eugene, OR (July 10, 2007)- When you head out into the 100-degree blast furnace of a day today, remember that trees may be your best friend. That’s right: Big leaf maple, walnut, oak, and catalpa are out there on the city street corners, prepared to offer you a critical service, according to forestry researchers. Trees can help by dropping city temperatures, preserving pavement, foreshortening the release of gasoline vapors from parked cars, and offering city dwellers a scientifically validated peaceful and easy feeling. No kidding.


Their shade will be welcomed this week as a strong ridge of high pressure and offshore east winds will bring us a heat wave of 100 degrees today, 95 on Wednesday, 90 on Thursday and 85 for the four days after that, the National Weather Service predicts.
“Trees are one of those things that people take for granted,” said Paul Ries, the state’s urban forester. “We don’t think about all the value that trees give us.” On 100-degree days, trees need us, too, he said.
Asphalt island
Trees are the first line of defense from the familiar heat island effect in cities, where “the sun beats down on bare pavement – particularly dark pavement – and that energy is absorbed and then re-emitted,” said Kathy Wolf, a University of Washington research scientist. “So you have this cycle of ever-increasing temperatures.”
Major U.S. cities have felt the heat and are launching aggressive tree-planting efforts to bring temperatures down. Los Angeles, for instance, aims to plant 1 million trees over the next three decades. Chicago is up to 500,000 trees over the past two decades, and the shoveling continues apace.
Chicago officials produced a map of temperatures at discrete points across the city, and the city is working to green all the hot spots with cool, leafy trees. “They will be able to monitor over time what we hope will be diminished temperatures,” Wolf said.
Large- and small-scale studies have demonstrated the cooling power of trees, Ries said. The difference between a hot, bare parking lot and a wooded area is 10 to 15 degrees. City-scale studies have shown a nine-degree difference between less-vegetated urban centers and leafy suburban neighborhoods.
The temperature difference has economic implications, Ries said. A study out of the University of California at Davis found that pavement lasts longer on a shaded street than on an unshaded street. Researchers estimated the repair and replacement savings at 66 cents per square foot of roadway over 30 years, Ries said. “That doesn’t sound like a lot, but when you multiply that by the miles and miles of pavement, it adds up,” he said.
Parking in shade will save you gas money as well as reduce the amount of smog-causing gasoline vapors wafting into the atmosphere, another study showed. Cars continue producing air pollution after they are parked as the gasoline left in hoses and elsewhere evaporates. Hot cars on bare lots produce more than cooler cars under trees.
Researchers also have quantified the value of shade trees that cast their shadows on houses. Two 25-foot trees on the west side of a house save the homeowner $18 per year in cooling costs, Ries said. Trees can even help keep you from getting hot under the collar, Wolf said.
Researchers at Texas A&M University exposed research subjects to stress using gruesome pictures and measured their heart rate and blood pressure. Then, researchers showed subjects a series of other images. They found that tree pictures had a measurable calming effect. “It happens within seconds,” Wolf said. “They may have no awareness of it, but (the researcher) sees it in the measures.”
Tree care
Eugene has been in love with trees since the first settlers transplanted big leaf maple saplings from the riverbank to their front yards, said Phillip Carroll of the Eugene Tree Foundation. Eugene’s urban forest (counting just the trees along streets and in parks) is estimated at 100,000 trees. Better numbers will be available in coming years because the city is embarking upon a tree census. Through city-sponsored programs, paid crews and volunteers plant 1,600 new trees a year, and less than 200 of those are to replace old, dead trees, so the forest grows.
The trees will need help to prosper in this long stretch of hot, dry weather, officials said. Those less than five years old, in particular, need water – but so do some older, well-established trees. “A lot of people mistakenly think trees have these big tap roots, which they don’t,” Ries said. “Ninety percent of a tree’s root system is in the top 3 feet of soil. It’s important you get plenty of water into that top 3-foot zone.” Young trees need 15 gallons of water applied to the base of the tree once a week through the hot weather. Old trees needs a trickle of water to run from a hose for 20 minutes each at three places between the trunk and edge of its canopy.
This week, it’s a mutual aid pact between humans and trees. “Trees probably take the heat better than we do in many respects – but that’s not to say they couldn’t use a little help from us,” Ries said.
For the full article, visit the Eugene Register-Guard.