PS 30-165 - Changes in species composition at two forested sites in the Cumberland Plateau over 40 Year Period

Abstract: Declining regeneration of Quercus, a foundational species in upland hardwood forests, has been observed over the past several decades as part of a larger trend of shifting species composition in the Eastern United States. Several potential drivers of this non-equilibrium dynamic have been studied including climate change, fire exclusion, and mesophication, but the effect of symbiotic mycorrhizae on adult recruitment in these forests has not yet been investigated. We hypothesize that the importance values of fire tolerant, shade intolerant and ectomycorrhizal fungi associated tree species have decreased between the 1970s and 2023. This study examines changes in species composition using data collected over several decades from two sites in the Cumberland Plateau – Fall Creek Falls State Park (FCF) and Franklin State Forest (FSF). Five 1-hectare permanent plots at FSF were surveyed in 1977, 2015 and 2023, and 3 of these plots were additionally surveyed in 1998 and 2005. Three 1-hectare plots at FCF were surveyed in 1976, 2016 and 2023. Every woody stem > 3cm diameter at 1.5 m breast height (DBH) was identified to species and measured to the nearest 0.1 centimeter. Importance values were calculated as the sum of relative frequency, relative density, and relative dominance, and each tree species received a fire tolerance, shade tolerance ranking and assigned to a mycorrhizal association based on established literature. In general, Acer, Kalmia, Liquidambar, Liriodendron, and Nyssa increased in abundance whereas Carya, Cornus, Oxydendron and Quercus species decreased in abundance. We found a significant increase in importance values of mesophytic species across both sites as pyrophytic species importance values declined (P = 0.04). In addition, species associated with arbuscular mycorrhizal fungi increased in abundance as species associated with ectomycorrhizal fungi declined (P = 0.02), but only at the FCF site. Our analysis provides evidence that composition of historically oak-hickory dominated upland forests in the Cumberland Plateau has shifted in the past 45-years, and the shifts are closely related to species’ fire tolerance and at some sites mycorrhizal association.