Subfossil Leaves Reveal a New Upland Hardwood Component of the Pre-European Piedmont Landscape, Lancaster County, Pennsylvania

PLOS ONE, November 2013

24236120

Sara J. Elliott, Peter Wilf, Robert C. Walter, Dorothy J. Merritts

Widespread deforestation, agriculture, and construction of milldams by European settlers greatly influenced valley-bottom stream morphology and riparian vegetation in the northeastern USA. The former broad, tussock-sedge wetlands with small, anastomosing channels were converted into today's incised, meandering streams with unstable banks that support mostly weedy, invasive vegetation. Vast accumulations of fine-grained "legacy" sediments that blanket the regional valley-bottom Piedmont landscape now are being reworked from stream banks, significantly impairing the ecological health of downstream water bodies, most notably the Chesapeake Bay. However, potential restoration is impaired by lack of direct knowledge of the pre-settlement riparian and upslope floral ecosystems. We studied the subfossil leaf flora of Denlingers Mill, an obsolete (breached) milldam site in southeastern Pennsylvania that exhibits a modern secondary forest growing atop thin soils, above bedrock outcrops immediately adjacent to a modified, incised stream channel. Presumably, an overhanging old-growth forest also existed on this substrate until the early 1700s and was responsible for depositing exceptionally preserved, minimally transported subfossil leaves into hydric soil strata, which immediately underlie post-European settlement legacy sediments. We interpret the eleven identified species of the subfossil assemblage to primarily represent a previously unknown, upland Red Oak-American Beech mixed hardwood forest. Some elements also appear to belong to a valley-margin Red Maple-Black Ash swamp forest, consistent with preliminary data from a nearby site. Thus, our results add significantly to a more complete understanding of the pre-European settlement landscape, especially of the hardwood tree flora. Compared with the modern forest, it is apparent that both lowland and upslope forests in the region have been modified significantly by historical activities. Our study underscores that generally overlooked subfossil leaves can provide important, local, temporally constrained paleoecological data, with much potential value in this case for stream and wetland restoration decisions in the mid-Atlantic region.