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 increased moisture availability. Paleoecological records of pollen and charcoal have been developed in oak-dominated regions to provide a long-term perspective, but these records have largely focused on regional scales and most have been poorly resolved temporally and/or taxonomically. Stand-scale, taxonomically precise vegetation histories are relatively uncommon, and may provide insight into patterns and processes at scales particularly relevant to adaptive management. In this study we utilized pollen, charcoal, and plant macrofossils preserved in Turtlehead Rock Bog, a small floating peatland (~110 m2) occupying a forest depression in an oak and black birch (Betula lenta) dominated forest in southwestern Pennsylvania, to provide high-resolution records of vegetation and fire for approximately the last 900 years. Comparisons among wetland and upland vegetation, fire history, and regional paleoclimate records were used to examine long-term forest dynamics and their sensitivity to fire and climate variability.
Results/Conclusions
Results indicate that oak (likely Q. prinus and Q. rubra) and black birch have dominated the site for the last 900 years, and that fire frequency and oak abundance both gradually declined until European land-clearance. Although forest and wetland communities were relatively stable during this time, a widespread drought at 550 yr BP was associated with the largest fire in the record, the formation of the modern floating peat mat, and a modest increase in black birch abundance. These patterns support a historic linkage between oak recruitment and fire frequency, and show that forest composition remained relatively stable through temperature and moisture changes. However, European land-clearance and the pathogen-driven loss of American chestnut (Castanea dentata) were followed by dramatic shifts in the abundance of oak, birch, and other species. Interestingly, the pollen record also shows that oak populations have expanded within the last century, reaching abundances not observed for the past 400 years, despite fire suppression. Human modification of the landscape and/or recent climate trends appear to have substantially altered the long-term linkage between fire and oak abundance, warranting caution in using controlled burns to increase oak recruitment on the modern landscape.