After more than 150 years of natural regrowth, forest cover is declining across all six New England states, threatening the region's landscape and chipping away at a natural buffer against global warming, according to a study released Tuesday by Harvard University's laboratory for ecological research.
The study by Harvard Forest found that New England forests, having grown back after a spate of land clearings by European settlers, have come under increasing pressure from a new wave of commercial development, industrial use and invasive species.
Less than 20 percent of New England's 33 million acres of trees, waters and wetlands are permanently protected from development.
David Foster, director of the Harvard Forest program, said that the turning point for New England forests came about 20 years ago when the area once again began to lose forest cover.
That shift has happened more rapidly in densely populated southern New England states, but even more sparsely populated Vermont and Maine -- particularly southern Maine -- have seen troubling signs, he said.
"The trend is now downward in all of the states," he said. "There is great pressure on both forest and farm land."
In Massachusetts, he said, forest cover is down to just 60 percent of the land, with conservation efforts in a race against new development. He said an ambitious goal would be retain 50 percent of the state's forest land.
According to the report, there are three New England regions most threatened with loss of forest cover.
They include a band reaching from Rhode Island and Connecticut to coastal Maine that is vulnerable to dense development and sprawl; an area in central New England subject to increasing suburbanization; and a northern tier where rapid turnover in ownership could lead to more fragmented management.
The report makes a series of recommendations, including long-term conservation efforts to protect at least 70 percent of the region, or 30 million acres, as forest land permanently free of development.
The authors of the report say that the bulk of that -- about 23 million acres -- should be deemed "managed woodlands" that can be used for nature tourism and recreation, while providing critical habitats for plants and animals.
The remaining 7 million acres should be designated "wildland reserves" largely free of human intrusion.
The forest cover wouldn't be evenly distributed across the region.
In the south, only half the land may remain forested while large regions in northern Maine, New Hampshire and Vermont could remain up to 90 or 100 percent forested.
Another potential threat to New England's forests is renewed interest in renewable and alternative energy sources like biofuels.
In Massachusetts, activists are pressing a ballot question targeting wood-burning power plants even as state environmental officials announced last month they were dramatically increasing the amount of publicly owned forest land where logging is barred.
Cornell University Professor Tim Fahey said preserving forests isn't necessarily at odds with the pressure to harvest wood for biofuels.
"The potential is there if it's done carefully," he said. "The challenge is to make sure it's done in a sustainable way."
To avoid future loss of forest land, states should do more to encourage the redevelopment of existing urban areas like New England's aging mill towns, Foster said.
Robert Lilieholm, a professor of forest policy at the University of Maine at Orono, said perhaps the biggest threat to the region's forest is "hard deforestation." Unlike past deforestation efforts that eventually allowed trees to grow back, hard deforestation refers to human development that effectively prohibits regrowth.
Lilieholm also warned of the fragmentation of ownership of large tracts of forest land, particularly in northern Maine. He said as those large parcels of land are broken up and sold -- sometimes as temporary investments for pension funds or financial trusts -- it becomes harder to protect them because there are so many different owners.
"This fragmentation hinders coherent management," Lilieholm said.
One way to help encourage private owners to turn over their land to forest conservation projects is to bundle several parcels together to create a larger scale conservation area, according to Lynn Lyford, executive director of the New England Forestry Foundation.
Lyford said her group is working on a project in Western Massachusetts that already has amassed over 10,000 acres.
Sometimes, she said, the whole is greater than its parts, pointing to individual plots of land that may not seem worth saving until they are included as part of a larger parcel constituting a ridgeline or other key feature of the landscape.
"We can protect land that doesn't have special significant but is nonetheless important to protecting the green infrastructure," Lyford said.