Urban ( 城市的 ) heat kills about 600 Americans every year, andsends another 65,000 to the emergency room, according to a recentreport from the Urban Land Institute. Climate change is going tomake these problems worse, the report's authors write.
Given that risk, Boston — like many cities around the country —plans to plant more trees. Urban trees have benefits beyond shadingand cooling. They clean the air, reduce noise pollution and helpprevent flooding by absorbing storm water."Great," you might be thinking, "let's plant many trees."
Unfortunately, the solution isn't that simple; a lot of street trees don'tmake it more than a few years in the big city. "It's tough to be a babystreet tree because their roots are really little. And the summers inBoston are quite hot, so drought alone can kill them," says AndrewTrlica, who recently earned an urban biogeochemistry doctorate fromBoston University.
An urban tree, especially one planted on a sidewalk bordering abusy street, has many factors against it. "Cars run into them. Bikesgetting locked to them is really surprisingly damaging when they'relittle because their bark (树皮) is weak," Trlica explains. "Road saltsare hard on their roots. Dogs would pee on them. It's just a toughenvironment."
Trlica says he began to wonder: If city officials want to increasetree canopy (树冠) cover to deal with climate change, should theyfocus on planting new trees or helping older ones survive? To figureit out, he looked at two conditions for Boston: spend the next twodecades planting saplings (树苗) in every available sidewalk location,or spend the time reducing themortalityrate of older trees by 50%.
For Trlica, the answer was clear. Yes, Boston should continueplanting trees, but the real canopy payoff will come from preservingbigger, leafier ones.
