The forests of the northeastern US have been transformed by human activity for 400 years

A joint Harvard-Smithsonian study published today in the journal PLOS ONE reveals how much—and how little—Northeast forests have changed after centuries of intensive land use.

A hike in today's forest will reveal the same types of trees that a colonial settler would have encountered 400 years ago. But the similarities end there. Jonathan Thompson, research associate at the Smithsonian Conservation Biology Institute and lead author of the new study, explains, “If you were just looking at a list of tree species, you'd get the impression that the Northeast's forests haven't changed. But once you start mapping the trees and counting them up, a different picture emerges.” Thompson adds, “In some ways the forest has completely transformed.”

To reach these conclusions, Thompson and his colleagues compared colonial-era tree records with modern data from the US Forest Service in a 9-state region stretching from Pennsylvania to Maine.

Their results show stark contrasts between pre-colonial forests and today. Maple trees have exploded in the Northeast, their numbers increasing by more than 20 percent in most cities. Other types of trees have declined sharply. Beech, oak and chestnut trees are showing the most severe loss—a big problem, Thompson notes, for wildlife that depends on the nuts for winter survival.

Pine numbers have shifted more than any other tree type, increasing in some places, decreasing in others. Thompson attributes this variability to ecology and economics: “Pine is valuable for timber, but it bounces back quickly after being cut. It has a social and environmental dynamism.”

The nine states in the study share a similar—and remarkable—forest history: During the 18th and 19th centuries, more than half of the forest land was cleared for agriculture and cut for timber. Most of the farms were eventually abandoned and during the 20th century, the forests returned. Today about 80 percent of the northeast is forested.

Driving the study is a historical data source never before analyzed on such a massive scale: the maps and notes of the colony's first surveyors. These scribbles and dots in old records document individual “witness trees”—trees mapped between 1650 and 1850 to determine property boundaries when towns were first established.

Charlie Cogbill, an ecologist on the research team, spent three decades compiling these early records from the public records of nearly 1,300 cities. The massive geographic database he built pinpoints the location of more than 350,000 witness trees, which form the basis of this study.

David Foster, co-author of the study and director of the Harvard Forest in Massachusetts, explains: “It is critical to understand the nature and scale of forest change caused by human impacts. Charlie's effort to harvest the trees- witnesses provides a serene window into the region's first forests, allowing us to assess the cumulative effect of centuries of land use and climate change.”

Even 200 years later, the legacy of colonial agriculture remains the most powerful factor in determining modern forest composition—more powerful than regional climate, soil conditions, and many other factors. “To understand how much a forest has changed, the first question to ask is how extensively the area was cultivated over the past two centuries,” says Dunbar Carpenter, study co-author and research assistant at the University of Wisconsin. – Madison. If more than half of the city was cultivated, he adds, the local forests have probably diverged considerably from their pre-colonial state.

In general, the study says, these changes have made modern forests more homogeneous and less responsive to small changes in temperature and precipitation. Despite deforestation, extensive logging, wildfires, climate change, invasive pests and disease, the Northeast remains the most densely forested region in the country.

“The overriding theme of this forest area is resilience to multiple impacts,” notes Foster. “This is an important lesson for the future. If we don't replace forests with houses and sidewalks, they will withstand future challenges.”

Thompson echoes this theme of resilience. “The northeast wants to be a forest,” he says. “If you stop mowing your lawn, you'll have a forest. That's basically what we did in the mid-1800s—we stopped mowing fifty percent of the landscape. What happens ecologically after that? The Northeast is the first place we've watched it happen ».

And the story is still unfolding.