Buried Fossil Leaves Reveal Precolonial US Forests: Guiding Stream Restoration

Hundreds of years ago, leaves from ancient trees fell into streams where they were buried. Now, a team of geoscientists has uncovered leaves buried by sediment behind milldams, revealing the truth behind what ancient forests and streams in America were like before Europeans established first contact.

"Milldams were built from the late 1600s to the late 1800s in Pennsylvania and other parts of the east," said Peter Wilf, one of the researchers, in a news release. "We can't get information from historic records on what the area looked liked before the dams because recording of natural history didn't really begin until the 1730s and was not detailed."

In this case, the researchers examined a mill site in Pennsylvania. There, they discovered a dark paleo-wetland soil layer containing fossil leaf deposits, with four plus meters of historical sediment buildup on top. They then looked at samples of 300-year-old leaves that were buried in the sediment.Because sediment quickly covered the leaf layer, the leaves that dated from before the dam remained intact. This helped the scientists understand the makeup of the forest near the water's edge before milldams were built and forests were cleared.

"We expected to see evidence for single stream channels that meandered back and forth across the valley bottom landscape for millennia," wrote the authors in a news release. "Instead, we found that most of the valley bottoms at the time of European contact were dominated by wetland ecosystems and numerous small, stable 'anastomosing' streams."

These branching and reconnecting streams were far different from the steep-banked meandering streams that characterized the landscape after European settlement. In addition to finding out about the waterways, though, the researchers managed to discover that the precontact forest was overwhelmingly American beech, red oak and sweet birch. That's a similar makeup to modern red oak/beech forests today. However, box elder and another maple are the trees that now dominate the forest that grows above the stream.

The findings reveal that researchers can reconstruct ancient landscapes, showing what existed before the 1700s. This could help with restoration efforts and establishment of precolonial-like habitats might decrease the amounts of nutrients from the legacy sediments that currently flow into the Chesapeake watershed.

The findings are published in the journal PLOS One.