It turns out the health gains of all that greenery add up. A recent study conducted in Portland, Ore., found that in neighborhoods where a nonprofit planted more trees, fewer people died. The paper, by researchers at the U.S. Forest Service, adds to a budding body of research into the health benefits of living around greenery. Its findings amount to a prescription for policymakers to plant more trees. “Urban trees are an essential part of our public health infrastructure, and they should be treated as such,” said Geoffrey Donovan, the Forest Service researcher who led the study published in the December issue of the journal Environment International. For three decades, the Portland nonprofit Friends of Trees planted nearly 50,000 oaks, dogwoods and other arboreal species around the city, giving Donovan and his colleagues granular data on how its canopy has changed over time.