Spring is here, and all across the country, the first pale green leaves are appearing on trees. But if you live in an urban area, you may be lucky to see such a sight: During the past 15 years, the quantity of trees in many U.S. cities has dropped by almost a third, while paved areas have increased by 20 percent, according to the conservation group American Forests. To turn those numbers around, urban areas would need to plant some 634 million trees — a daunting notion at a time when most municipalities are facing significant budget crunches. But the virtues of trees outweigh the costs, the group argues: They absorb carbon dioxide, return oxygen to the air, absorb stormwater runoff, provide habitat for local and migratory birds, muffle loud sounds, and cool things down on hot summer days. Together, those benefits can save cities millions of dollars.