European settlers transformed America’s Northeastern forests. From historic records and fossils, researchers know the landscape and plants are radically different today than they were 400 years ago. But little direct evidence exists to prove which tree species filled the forests before they were cleared for fields and fuel. Swamp-loving plants, like sedges and tussocks, are the fossil survivors, not delicate leaves from hardwood trees.
Now, thanks to a rare fossil discovery in the Pennsylvania foothills, scientists can tell the full story of America’s lost forests. The fossil site is a muddy layer packed with leaves from hardwood trees that lived more than 300 years ago along Conestoga Creek in Lancaster County, Pa. The muck was laid down before one of Pennsylvania’s 10,000 mill dams, called Denlinger’s Mill, was built nearby, damming the stream and burying the mud and leaves in sediment.
Researchers from Penn State University discovered the fossil leaves while investigating the lingering effects of milldams. The thousands of small dams — which powered mills, forges and other industry — changed the water table, altering the plants growing nearby and eventually changing the landscape from wetlands to deeply incised, quickly flowing streams.
Before Europeans arrived, American beech, red oak and sweet birch trees shaded Conestoga Creek, according to a study the researchers published on November 13 in the journal PLOS ONE. Some 300 years later, those trees are gone. The same spot is now home to mostly box elder and sugar maple trees, said Sara Elliott, the study’s lead author and a research associate at the University of Texas at Austin’s Bureau of Economic Geology.