U.S. urban forests are losing ground as tree cover in urban areas is declining at a rate of about 4 million trees per year, a Forest Service study indicates. Tree cover in 17 of the 20 cities analyzed declined while 16 cities saw increases in impervious cover such as pavement and rooftops, a USFS release said Wednesday. Land that lost trees was for the most part converted to either grass or ground cover, impervious cover or bare soil, it said. The greatest percentage of annual loss in tree cover occurred in New Orleans, Houston and Albuquerque, researchers said. The findings from New Orleans were expected due to the devastation of Hurricane Katrina in 2005, they said. Tree cover ranged from a high of 53.9 percent in Atlanta to a low of 9.6 percent in Denver, while cities with the greatest annual increase in impervious cover were Los Angeles, Houston and Albuquerque, the study found. \"Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces,\" U.S. Forest Service Chief Tom Tidwell said. Forest researchers at the USFS Northern Research Station used satellite imagery to measure tree cover. \"Trees are an important part of the urban landscape,\" Michael T. Rains, director of the Northern Research Station, said. \"They play a role in improving air and water quality and provide so many environmental and social benefits. As our Forest Service Chief says, \' … urban trees are the hardest working trees in America.\' This research is a tremendous resource for cities of all sizes throughout the nation.\"