Washington Post: Spurred by visions of their cities frying in a warmer world, mayors around the nation have grasped a green solution: trees. Like Johnny Appleseed, they have vowed to sow their seeds in great profusion, promising millions of new trees in the coming years. Arbor Day, that old fusty holiday, is getting a makeover. Cities once planted trees because they were beautiful. Now trees are part of the “green infrastructure” managed by “urban foresters” to work as powerful …