It's enough to make Henry David Thoreau weep.

New England – the home of Vermont maple trees bursting with sweet syrup, and balsam fir and red spruce spread across New Hampshire's White Mountains – is losing its forests.

Of all America's forests under pressure from development, New England's are shrinking the fastest.

The problem is severe enough that some conservation groups say they have limited time to act.

"The window for conserving forests is closing," says Andy Swinton, director of field science with The Nature Conservancy, a nonprofit habitat conservation organization. But "there's really an opportunity here, because the next 20 years will determine the character of New England forests. This is a race against time, and the time to act is now."

The region's forests had made quite a comeback in the past two centuries: As agriculture declined, fields went back to wooded land. Now, however, those forests are under threat – from homeowners, this time. In their push to create more housing in an area where home prices are already through the roof, developers are moving into wooded land.

The numbers are stark, particularly in southern New England. By 2050, 70 percent of Rhode Island and 61 percent of Connecticut will be urbanized, according to a recent report in the Journal of Forestry by two researchers with the US Agriculture Department's Forest Service. Massachusetts is already losing 40 acres a day to development, estimates Mass Audubon. These three states will lose the highest percentage of forest of any state by mid-century, the Forest Service researchers say.

Part of the reason for the region's forest loss is its population density. Its urban areas are already so developed that they're pushing out, often into surrounding forests. The other factor is New England's development pattern and lifestyle.

Take long-distance commuting. The Southwest may be famous for its vast metropolises, but the trend is actually more pronounced in New England, says Kathy Sferra, a land protection expert at Mass Audubon.

For example: To be able to afford the cost of living, many workers live in less expensive housing far from the urban centers where they work. That leads to more crowded highways. In addition to the 40 acres the state loses every day to sprawling development, it loses an additional 38 acres to the "hidden" cost of development, such as road construction.

And, as in the rest of New England, most of Massachusetts' residential developments are low density, meaning few people living in large houses on big lots.

Residential lot sizes have increased 47 percent since 1970 in Massachusetts, according to Mass Audubon. New England's average lot size for new residential construction is the largest in the country at 1.3 acres, and its median lot size is three times the national average, says the Massachusetts Institute of Technology's Center for Real Estate and the Massachusetts Housing Partnership.

Meanwhile, the state's household size has shrunk 20 percent since 1970 to 2.5 people per household. Small wonder then that while New England's population increased 6.6 percent between 1990 and 2000, its total housing units grew 7.4 percent, according to the Rhode Island Department of Labor and Training.

Houses are also getting bigger. The National Association of Home Builders found that 40 percent of new homes in the Northeast have four or more bedrooms, making the region the national leader in terms of the size of homes.

These trends have spurred conservation groups to work more strategically, buying and protecting large plots of land in key areas rather than small, isolated locations.

"We've learned that doing conservation willy-nilly doesn't help because we end up with fragmented forests," says Mr. Swinton. "Since development is going to happen, we now know we need systematic, collaborative planning with the government, land trusts, and nonprofits to make sure that development and conserved forest area are intelligently designed."

For example: The Nature Conservancy worked with West Greenwich, R.I., and other conservation groups to purchase 1,700 acres of forest surrounding its town in an effort to protect the land. The conservancy's Borderlands Project is looking to accomplish a similar feat in another town in Rhode Island or Connecticut.

Conservation groups are also helping local governments improve their planning for infrastructure that leads to development, such as roads and Interstates, while minimizing sprawl and forest destruction.

State governments are also getting involved. Last month, Connecticut Gov. Jodi Rell (R) created a state office to "plot a new, antisprawl course." Rhode Island is now developing a land-use plan to encourage urban-center development. Massachusetts announced Aug. 4 that it would spend $50 million on conservation over the next five years, an increase of $20 million over its conservation spending during the past four years.

Forest-conservation groups next want to convince New England state governments to allocate some of the revenue generated through their greenhouse-gas reduction initiative to forest protection, according to Swinton.

Conservationists say that they'll need to bring all their tools to bear on the challenge of deforestation.

"Forests in this area made a comeback in the last century, but it looks like the pendulum is swinging in the opposite direction," he says.