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

Open Access
Elliott, Sara Jean
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 05, 2012
Committee Members:
  • Peter Daniel Wilf, Thesis Advisor
  • Plant Macrofossils
  • Piedmont
  • Stream Restoration
  • Paleontology
  • Paleoecology
Valley bottoms throughout the Piedmont region in the northeastern U.S. consisted of stable, forested, wetland communities with small, interconnected channels and pools during the Holocene. However, as a result of historical European activities, especially milldam construction and the deposition of several meters of fine-grained sediments on the regional landscape, stream geomorphology and riparian vegetation have been significantly modified. The once-broad wetlands were converted into incised, meandering streams with nearly vertical, unstable banks that are currently contributing large quantities of nutrient-rich sediment downstream. Denlingers Mill, in southeastern Pennsylvania, is one of many derelict milldam sites composed of an altered, incised stream with a narrow terrace floodplain. However, this site is unusual compared to other previously studied legacy sediment sites because it has exposed bedrock slopes that support a modern secondary riparian forest adjacent to the stream, presumably growing atop the same substrate that the pre-settlement forest once did. The exceptionally prolific subfossil leaf mats at this site were deposited as the large, overhanging trees of the old-growth forest contributed prodigious quantities of leaves into the stream, similarly to the secondary forest today. Therefore, the leaf macrofossils from this site provide both a strong, novel, non-wetland signal, and an opportunity to directly compare the old-growth pre-settlement forest composition with the altered secondary forest. Very few Holocene paleoecological studies are based on leaves, opting instead for fruits, seeds, wood, or pollen, but due to their fragility and inability to be reworked, macrofossil leaves can supply local information not available from these other sources. The Denlingers Mill subfossil leaf assemblage, removed from a stream cutbank within an organic-rich hydric soil immediately underlying historical sediments, contains representative species from at least three different pre-colonial plant communities, including a novel non-wetland and upland hardwood forest assemblage supplied by the unusual upland slope areas at Denlingers Mill. Leaves and some fruits and seeds were cleaned of all siliciclastic and organic debris, and mounted on glass slides for identification. A total of eleven species or morphotypes were identified in the subfossil assemblage. Although the majority of the assemblage consists of facultative upland species, particularly American beech (Fagus grandifolia), sweet birch (Betula lenta), and species of red and white oak (Quercus spp.), some facultative wetland species such as willow (Salix sp.), red maple (Acer rubrum), and black ash (Fraxinus nigra) are also present. Integrating these results with previous studies, the pre-settlement landscape probably consisted of a suite of distinct communities with a continuum of overlapping species, namely a tussock sedge marshland in valley-bottoms, a transitional lowland red maple-black ash forested swamp, and a red oak-beech mixed hardwood forest on slopes and upland areas. When compared to the modern successional forest at Denlingers Mill, dominated by box elder (Acer negundo) and sugar maple (Acer saccharum), it is apparent that post-settlement riparian forests in the region have been significantly modified as a result of European colonization. Furthermore, this illustrates that, in addition to the loss of valley-bottom wetlands due mainly to milldam construction, another important aspect of colonial landscape alteration is the loss of riparian and slope old- growth forests attributable instead to deforestation and agriculture. Riparian vegetation influences fluvial hydrology, geomorphology, and channel bank stability. Therefore, successful restoration including the reestablishment of stream-accessible floodplains, and the creation of self-sustaining riparian buffer flora to reduce erosion and nutrient loading downstream, necessitates an understanding of the pre-European settlement plant communities, which can be greatly improved by incorporating leaf macrofossil studies.