TOWARD A REGIONAL GROWTH CURVE FOR LICHENOMETRY RESEARCH IN SE PENNSYLVANIA

A lichenometric survey of Tyler State Park (Bucks County, Pennsylvania) were used to establish a local growth curve that can be used for dating naturally exposed surfaces. Measurements of maximum thallus diameters of variably oriented crustose lichens (genus Rhizocarpon) were performed on historical structures at the park, which are constructed of Triassic sedimentary rocks quarried at nearby sites. Our findings indicate that growth rates of 0.5 mm/yr should be applicable to surfaces exposed for the past several centuries. In comparison, lichen growth curves on three buildings in nearby North Philadelphia are nearly an order of magnitude slower. Giving similar exposure orientation, such difference likely results from the influence of rock type, delayed establishment, or stunted growth on city buildings. Whereas it is tempting to use lichens from readily accessible and well established urban sites, locally available cultural landmarks will offer more accurate growth curves that can be used to assess the long-term or event-scale exposure histories of rock surfaces. Furthermore, dendrochronological, radiocarbon, optical luminescence, or exposure-age dating are often not applicable to establishing the duration of exposure or history of rockfall events, making lichenometry the only viable chronometric technique.