Tree cover in the nation's Lower 48 states covers 659 million acres, more than one-third of the nation, according to a U.S. Forest Service study of national tree cover and impervious surfaces. New Hampshire leads the nation in percent tree cover (89 percent), followed by Maine (83 percent) and Vermont (82 percent). On the other end of the spectrum, North Dakota has the lowest percent tree cover (3 percent), followed by Nebraska (4 percent) and South Dakota (6 percent). Using aerial photograph interpretation of circa（大约） 2005 imagery, U.S. Forest Service researchers Dave Nowak and Eric Greenfield found that in urban and community areas, percent tree cover is highest in Connecticut (67 percent) and lowest in Nevada (10 percent). The study, "Tree and impervious（不受影响的） cover in the United States," was recently published in the journal Landscape and Urban Planning.

"Urban forests are a vital part of the nation's landscape," said Michael T. Rains, director of the Forest Service's Northern Research Station. "Forest Service science is supporting stewardship（管理工作） of urban forests with tools that communities, organizations and home owners can use to better understand the environmental benefits of trees."

Impervious cover in the conterminous United States is estimated at 2 percent, or 46 million acres. That percent goes up in urban areas, where impervious cover accounts for 25 percent of land cover. New Jersey leads the nation in impervious cover (12 percent) and Wyoming has the least statewide impervious cover (0.5 percent).

Both people and nature play a role in urban forestry, according to Nowak. "This research demonstrates how natural environments in concert with how we develop and manage communities significantly impacts tree cover in urban areas," Nowak said. "Cover data of a city or region can provide a baseline for developing management plans, setting tree cover goals, and for monitoring change through time, all of which are essential to sustaining urban forests."

The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation's forests and grasslands to meet the needs of present and future generations. The agency manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world. The mission of the Forest Service's Northern Research Station is to improve people's lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.