When cities first planted trees it was to beautify their streets, but now we know that they also remove carbon from the atmosphere, helping to mitigate global warming. But not all trees are created equal, say researchers at the State University of New York (SUNY) at Syracuse. Some sequester more carbon than others; some emit more volatile organic compounds, contributing to the formation of ozone and thus diminishing urban air quality. So what you plant matters. For their own city, the researchers propose a mix of 31 species able to cope with the bitter winters of upstate New York.

Each city has its signature tree; for Los Angeles it's the spindly palm. But L.A.'s palms are succumbing to age and disease and are being replaced with oaks and sycamores. Tinseltown may lose a little of its identity, but the SUNY researchers would be pleased: These new species will be much greener, in every sense of the word.