The environmental health of a city is mirrored in the vigor of its trees, say urban forestry experts who will gather Sept. 16-18 in San Francisco for a summit on urban forestry issues. "We often hear about saving critical habitat for animals, but we'll be coming together to discuss the critical habitat of people," says Greg McPherson, a research scientist at the U.S. Forest Service's Western Center for Urban Forest Research and Education at UC Davis. Approximately 200 scientists, city officials and representatives of private tree organizations from 80 cities in 17 western states will meet to discuss successful tree-related programs in their communities. The summit also will include a demonstration of a portable mill that can be used by cities to saw downed trees into lumber for woodworking. Interest in urban forestry grew out of the ravages of Dutch elm disease, which devastated trees in midwestern cities in the 1970s, says McPherson, who, as a professional forester and landscape architect, is among just a handful of urban forestry scientists in the nation. He notes that urban trees, 80 percent of which are usually located on private property, improve local air quality, cut energy costs by providing cooling shade and have immeasurable aesthetic value.