SUCCESSIONAL REPLACEMENT OF OLD-GROWTH WHITE OAK BY MIXED MESOPHYTIC HARDWOODS IN SOUTHWESTERN PENNSYLVANIA

Presettlement forests in southwestern Pennsylvania were dominated by Quercus alba L. A relatively undisturbed remnant of this forest type on a mesic site was surveyed to investigate the successional status of Q. alba in the region. This old-growth stand is now dominated by Fagus grandifolia Ehrh., Acer rubrum L., and Liriodendron tulipifera L., representing 62% of the importance value total. Oaks (Q. alba, Q. rubra L., and Q. velutina Lam.) totalled only 18% of the importance value total. The overstory mainly comprised L. tulipifera, F. grandifolia, Q. alba, and Nyssa sylvatica Marsh. Acer rubrum and Prunus serotina Ehrh. were the most abundant seedlings, and few saplings of any species were present. The oldest and largest trees were oak, with two Q. alba > 300 years. Over 90% of all trees were < 120 years old, including many F. grandifolia, L. tulipifera, A. rubrum, and A. saccharum Marsh., but few oaks. Logging of several trees in the 1930-1940s apparently accelerated the speed of obtaining dominance for these mixed mesophytic species. Radial growth patterns varied with species and canopy position. On average, the oldest oaks exhibited slow radial growth of < 0.75 mm/year. Many understory trees had average radial growth of > 1.5-2.0 mm/year and exhibited a release in growth associated with logging in the 1930-1940s. We believe this forest represents a primary example of an advanced stage of oak replacement by mixed mesophytic species, a process that seems inevitable in many eastern forests.