A High-Resolution Paleoecological Perspective on Temperate Oak Forest Dynamics: Implications for Understanding Contemporary Oak Decline

Recruitment failure of oak (Quercus) during the past century has raised concerns about ongoing changes in temperate forests of the eastern United States. Although the causes of oak decline and the associated forest changes are widely debated, most arguments have focused on the effects of fire suppression or moisture availability. Paleoecological records have provided long-term perspectives on oak forest dynamics, but these pollen and charcoal data are often limited by poor spatial, temporal, or taxonomic resolution. In this study I utilized pollen, charcoal, and plant macrofossils preserved in Turtlehead Rock Bog, a small floating peatland occupying a unique depositional basin in an oak-black birch (Betula lenta) forest in southwestern Pennsylvania to 1) reconstruct the depositional and wetland history of this unusual system, 2) compare the timing of wetland and upland changes with fire history and regional paleoclimate records, 3) develop a high-resolution record of fire and oak forest dynamics, and use these data to discuss oak decline and current management strategies.Results indicate contemporaneous shifts in the arboreal pollen and wetland macrofossil records, along with changes in sediment accumulation rate, soil bulk density, organic matter content, and overall macrofossil preservation, suggesting a climate or disturbance driver for most vegetation changes. From 9000 to 2000 cal yr BP, sandy, charcoal-rich sediment accumulated slowly and preserved evidence of a changing upland forest and fern-dominated wetland. Then a >1000 year depositional hiatus occurred, likely caused by regional aridity. At 800 cal yr BP a sedge marsh occupied the basin, followed by the establishment of a diverse sedge peat mat around 550 cal yr BP, associated with an increase in black birch and likely wetter conditions. Oak (likely Q. prinus and Q. rubra), American chestnut (Castanea dentata), and black birch have dominated the surrounding forest for the past 900 years, while fire occurrence and oak abundance both gradually declined. Comparisons of charcoal accumulation and oak pollen support a historical fire-oak linkage influenced by overall forest composition; however, oak has expanded in the past 75 years during a period of fire suppression. Recent oak recovery at the site may be attributable to a local fire event around 1930 and/or the eradication of American chestnut. Therefore, although prescribed burns likely increase oak recruitment in some forest types, human modifications to the landscape appear to have altered the historical relationship between fire and oak, warranting caution in using prescribed burns to increase oak recruitment on the modern landscape.