When an early Pennsylvania settler clawed his way through the woods to the top of a hill, he found disappointment. The view, he said, "is nothing but an undulating surface of impenetrable forest."

Penn's Woods was aptly named. When colonists arrived in the 1600s, it was 98 percent forest, causing another settler to declare that it "was not a land of prospects. There is too much wood."

Few complained over the next 200 years as the state's trees "came down like tall grass before a giant scythe," as a contemporary observer put it. By the time botanist Joseph Rothrock traveled through the northern tier in the late 1800s, he called it the "Pennsylvania desert."

Rothrock led a campaign to restore the state's forests. In 1895, he was named Pennsylvania's first commissioner of forestry. His program flourished. Over the next century, Pennsylvania's state forests grew to cover more than 2.1 million acres-one of the largest expanses of public forestland in the East, and one of the nation's most respected forestry programs.

For the last 11 years the independent Forest Stewardship Council certified that Pennsylvania's forests met or exceeded standards to maintain the sustainability of the woodlands. But they have added a caveat-they asked the state to study the long-term effects of the rapidly increasing gas drilling within the big woods.

Today, more than a million acres of the state's prized forestland sit on top of the Marcellus Shale-the gas-rich rock that's prompted a rush to drill in the Keystone State. Already, the state has leased more than 600,000 acres of its forestland for drilling and recently decided to open 200,000 more acres with the hope of raising $60 million to support its recession-strapped budget.

Gas company officials maintain they've been drilling in the forests for decades, with few problems. But legislators and biologists worry about the scale of the current boom.

"God isn't making any more land. He quit that a long time ago," said Rep. David K. Levdansky, a Democrat and chairman of the Pennsylvania House of Representatives' Finance Committee as well as an avid outdoorsman. "We ought to be very cautious about doing this."

What happens in the vast state forests between Interstate 80 and the New York border-places such as Tioga, Loyalsock and Tiadaghton, as well as the private forests nearby-is also of concern to the Chesapeake Bay, 200 miles downstream. According to a recent report by the U.S. Department of Agriculture's Forest Service and the Conservation Fund, the Bay watershed loses 100 acres of forests a day, and is likely to lose nearly 10 million acres by 2030.

That's of great concern, because forests have long been considered the best land use to protect the Bay. They absorb nitrogen, slow erosion, provide crucial fish and bird habitat and promote biodiversity.

The rush for gas has the potential to accelerate those losses and break up increasingly rare, large, unbroken blocks of forest. Each drill site requires at least five acres for a well pad, and miles of roads and pipelines that fragment the big woods.

"There's no doubt this is going to have an impact on interior-dwelling species," said Jerry Hassinger, a retired biologist from the Pennsylvania Game Commission. Fragmentation paves the way for invasive species and Hassinger fears that even if the gas companies do reclamation, the trees and grass they plant will be no match for the wily intruders that could hinder any forest recovery.

The affected area could extend up to 300 meters from the disturbance site, said Kim Van Fleet, a biologist with the Pennsylvania Audobon Society. This phenomenon is called the "edge effect." Because everything in an ecosystem interconnects, changes in sunlight, wind and vapor pressure brought about by cutting clearings have far-reaching consequences.

Van Fleet is worried about the scarlet tanager-17 percent of that songbird's breeding population lives in Pennsylvania forests. Cerulean warblers, wood thrush, ovenbirds, and forest raptors also top her list of affected birds.

"If there is fragmentation nearby, it will affect these birds," Van Fleet said, noting that they all require large forest tracts to thrive.

Matt Royer, a lawyer for the Chesapeake Bay Foundation, was concerned enough with the pace of drilling permit approvals in the Tioga Forest that he filed a legal challenge. By the time he won it, the earth had already been disturbed.

"What's it going to look like, in two or three years, when you do an aerial flyover of northeastern Pennsylvania, and it's sliced and diced with all these well pads and compressor stations, a contiguous forest area that doesn't have a contiguous forest anymore?" he asked.

Van Fleet and others point to lessons learned in the Allegheny National Forest in northwest Pennsylvania, where gas companies have been drilling shallow wells for decades, and where, the U.S. Forest Service reports, several species have lost habitat to gas drilling.

Ryan Talbott, executive director of the Allegheny Defense Project, said gas companies once drilled a few dozen shallow wells a year. In 2007, they drilled more than 1,200 wells. Today, more than 2,000 miles of roads support the gas industry. Talbott says he sees fewer cerulean warblers, northern goshawks, turtles and rattlesnakes, and more invasive plants.

"It was literally like watching a national forest be transformed in a couple of years," Talbott said.

Last year, Talbott's group sued the Forest Service for trying to open Pennsylvania's only national forest to more than 1,000 new wells without following the National Environmental Policy Act, which requires both public input and government reviews. In April, the Forest Service agreed to delay some drilling until it conducted a full environmental review with public comment.

Rep. Edward G. Staback, a Democrat and chairman of the Pennsylvania House of Representatives' Game and Fisheries Committee, opposed opening more forests to drilling and instead pushed for a gas tax to raise money. He lost.

But, Staback said he'll be watching the forests closely.

"Our concern is that the gas companies not walk away from their responsibilities the way the coal barons did years ago. They left land filled with slag piles. We're still paying the price for that today," he said. "We want to be absolutely, positively, sure that there's no way that happens again."