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

Abstract

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.

Riparian and valley-margin hardwood species of pre-colonial Piedmont forests: A preliminary study of subfossil leaves from White Clay Creek, southeastern Pennsylvania, USA

“…In contrast, another recent study from Denlingers Mill, also in Lancaster County and the Chesapeake Watershed, detailed valley-margin and upslope hardwood communities based on 11 recovered species of pre-settlement subfossil leaves and some fruits, dating to the early 1700s (Elliott et al, 2013). Subfossil leaves are rarely used by paleontologists because of their extreme fragility, but because they are minimally transported and temporally mixed compared to more commonly studied seeds and pollen, they can provide detailed, localized views of past landscapes that are not usually available otherwise (see Appendix 3 for additional information regarding the use of leaves versus fruits, seeds, and pollen in this research).…”

“…Deforestation, agriculture, and milldam construction during the seventeenth through nineteenth centuries in the Piedmont region profoundly altered fluvial geomorphology, resulting in the deposition of up to several meters of fine-grained "legacy" sediments above pre-existing wetland soils along many Piedmont valley-bottoms. Through subsequent dam breaching, the once-broad wetlands were converted into incised streams with unstable banks, primarily supporting weedy and non-native vegetation (Walter and Merritts, 2008;Hartranft et al, 2011; see Appendix 1 and Elliott et al, 2013 for additional information regarding historical background). To this day, Piedmont streams carry anomalously high concentrations of suspended sediments (Walter et al, 2007;Gellis et al, 2009;Pizzuto and O'Neal, 2009;Merritts et al, 2013;Voli et al, 2013), which contribute significantly to contemporary nutrient loading in downstream watersheds such as the Chesapeake Bay.…”

“…Damming, sediment accumulation, and later stream incision also significantly affected the floral assemblages in the riparian zone (Voli et al, 2009;Merritts et al, 2013; Appendix 1) and the valleymargin and upslope hardwood communities (Elliott et al, 2013), producing a landscape that contrasts starkly with the sluggish, low-gradient, pre-colonial wetlands (Hilgartner and Brush, 2006;Walter and Merritts, 2008). Ultimately, the increased distance to the water table, paired with the well-drained legacy sediments, excluded native wetland species whose roots could no longer reach the water table or extract moisture from the silty soils (Shafroth et al, 2002;Stanley and Doyle, 2003).…”

“…Subfossil leaves are rarely used by paleontologists because of their extreme fragility, but because they are minimally transported and temporally mixed compared to more commonly studied seeds and pollen, they can provide detailed, localized views of past landscapes that are not usually available otherwise (see Appendix 3 for additional information regarding the use of leaves versus fruits, seeds, and pollen in this research). Elliott et al (2013) was the first study that used subfossil leaves in this region. That work concluded that Red Maple-Black Ash Swamp forests and Oak-Beech mixed hardwood forests dominated the pre-settlement valley-margin and upslope hardwood communities, respectively.…”

“…That work concluded that Red Maple-Black Ash Swamp forests and Oak-Beech mixed hardwood forests dominated the pre-settlement valley-margin and upslope hardwood communities, respectively. Elliott et al (2013, figure 17) also provided a detailed artist's reconstruction of the riparian, valley-margin, and upslope pre-settlement landscape reconstructed from paleobotanical data. The results from Elliott et al (2013) demonstrated the significance of delicate, negligibly transported subfossil leaves for localized reconstructions of presettlement landscapes and the need for new subfossil leaf sites, such as White Clay Creek, to improve spatial understanding.…”