PALEOECOLOGY AND GEOMORPHOLOGY OF TURTLEHEAD ROCK BOG, A UNIQUELY VALUABLE PALEOENVIRONMENTAL ARCHIVE IN OHIOPYLE STATE PARK, PENNSYLVANIA

Paleovegetation records vary in source area and taxonomic resolution. For example, pollen records from large lakes provide regional vegetation histories at coarse taxonomic resolution. Stand-scale vegetation histories, comparable to modern ecological investigations, are relatively uncommon but have been derived from pollen and plant macrofossils preserved in forest hollows. We developed a stand-scale paleovegetation record with high taxonomic resolution from Turtlehead Rock Bog, a small peatland (~110 m²) occupying a forest depression in Ohiopyle State Park in Pennsylvania. The site is located on the eastern flank of the Laurel Hill Anticline within a rock city made up of large rectangular boulders of Homewood sandstone. The basin is artesian fed by down dip oriented joint openings that connect it to an aquifer with an estimated 15 m of hydrologic head and a 4-hectare recharge area. A floating peat mat surrounded by steep rock walls occupies the basin with an overflow spillway on the structurally down-dip side.

A 3.40-m long ¹⁴C dated sediment core indicates that boulder separation and sediment deposition began before ~9000 yr BP, with basal sediments containing mostly weathered-out sand. From 9000 until ~2000 yr BP, sand and charcoal-rich sediments accumulated slowly (0.17 mm/yr). Ferns and herbaceous plants occupied the wetland during this time, though forest vegetation was more dynamic. The area was dominated by hemlock (Tsuga canadensis) and birch (Betula) from 9000 to 8000 yr BP, and oak (Quercus) increased after 8000 yr BP. Black gum (Nyssa) and chestnut (Castanea dentata) became more common alongside oak after 7000 yr BP, and hickory (Carya) became more important after 3000 yr BP. A depositional hiatus occurs in the record at ~2000 yr BP, and a rapidly accumulating (2.1 mm/yr) peat mat established about 900 years ago. Diverse plant communities occurred on the mat and adjacent boulders by this time. Forest vegetation for the past 900 years was characterized by fluctuations in oak, black birch (B. lenta), and chestnut likely linked to fire dynamics until land-clearance in the mid 1700s. The paleoenvironmental record from this unique site is well-suited to examine stand-scale forest dynamics, and will be used to address management-relevant questions like the role of fire in maintaining oak forests of the Northeast.