Lessons from Soggy Leaves: A Pre-Settlement Flora from White Clay Creek, Chester County, Pennsylvania

Open Access
Grettenberger, Christen Lynn
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 08, 2011
Committee Members:
  • Peter Daniel Wilf, Thesis Advisor
  • fossil leaves
  • Holocene
  • subfossil flora
  • paleontology
Human activities, especially those associated with the use of milldams, greatly altered the geomorphology of the northeastern United States during the seventeenth and eighteenth centuries. Land clearing and the construction of milldams caused the deposition of several meters of fine-grained sediment above the pre-existing soils. Subsequent dam breaching converted broad wetlands to incised streams. Previous work shows that pre-settlement soils contain seeds from herbaceous wetland species, but these studies do not include leaf macrofossil evidence and provide little evidence of woody species. This study uses subfossil leaves to reconstruct the pre-European settlement flora of White Clay Creek, Chester County, PA and describes the influence of European settlement on the floral community at the site. Leaf subfossils were collected from a stream cut of the eastern branch of White Clay Creek, processed, and identified to the lowest possible taxonomic level. Leaf fragments spanning the subfossil layer were radiocarbon dated to accurately constrain the timing of deposition. Unlike a seed flora from a similar site, leaves found at the study site are generally from woody species and have a diverse range of wetland affiliations. Species found include obligate and facultative wetland species, specifically three morphotypes of willow (Salix sp.), swamp maple (Acer rubrum) and hazel alder (Alnus serrulata) and one facultative species, American beech (Fagus grandifolia). Leaves of species within the white and red oak subgenera (Quercus subgenus Quercus and Quercus subgenus Lobatae) were also found, but could not be identified to the species level. The large differences between the leaf flora and seed flora at a similar site show that the two plant organ types provide data about different portions of the community and indicate that future Holocene reconstructions should include both seeds and leaves. The diversity in wetland indicator statuses shows that leaf subfossils originate both from the riparian buffer and surrounding hill slopes. This flora, deposited between 217 and 368 bp, immediately preceding European settlement, contains more wetland species than the modern vegetation, suggesting that European settlement, specifically the construction of milldams altered floral composition.