More than 31 million acres of forest in the southern United States will disappear into the maw of urban development in the next four decades, according to a report released yesterday by the U.S. Forest Service in collaboration with other federal agencies. Wildlife and water and air quality will suffer as trees make way for sprawling growth and timber harvests increase by half, the report found. That’s bad news, but environmentalists say it’s not bad enough; they criticize the report as going too light on the timber and paper industries, noting that paper mills alone annually destroy 5 million acres of forests in the South. They also point out that the net loss of forested areas — which the report places at about 2 percent by 2040 — would be much greater if pine plantations and other tree farms were not counted as true forestland.