Tree cover has diminished significantly in 20 U.S. metropolitan areas over the last 25 years, as roads, parking lots, and more buildings have taken root, according to a study released yesterday by American Forests. The enviro group says the loss is a shame — because trees not only look swell, but they also provide financial benefits to cities. For example, American Forests says that a well-shaded building costs between 20 and 50 percent less to cool each summer. A separate report by researchers at the University of Illinois at Urbana-Champaign suggests that tree cover may be linked to public safety. The two-year study of 98 buildings in a housing project in Chicago found that buildings with lots of vegetation nearby had 52 percent fewer total crimes, 48 percent fewer property crimes, and 56 percent fewer violent crimes than buildings with little vegetation.