Physiographic analysis of witness-tree distribution (1765-1798) and present forest cover through north central Pennsylvania

This study analyzed witness-tree data recorded from 1765 to 1798 with respect to landform in four major physiographic provinces represented through north central Pennsylvania. These data were also compared with present-day forest composition to evaluate broad changes that occurred 200 years after European settlement. In the Allegheny High Plateau, Tsuga canadensis (L.) Carr. represented 40-47% of witness trees in mountain coves and stream valleys, but only 9% on plateau tops, which comprised 45% Fagus grandifolia Ehrh. Pinus strobus L. represented less than or equal to 4% frequency across all landforms. The original forests of the Allegheny Mountains were dominated by mixed Quercus, Acer, Castanea dentata (Marsh.) Borkh., and Pinus and had significant T. canadensis only in stream valleys. The presettlement forests of the Allegheny Front and the Ridge and Valley provinces had a similar mix of Quercus, Pinus, Castanea, and Carya, with increased P. strobus the more mesic sites and Pinus rigida Mill. on the xeric ridges. Comparisons of presettlement with present-day forest composition indicate a dramatic reduction of T. canadensis (32% to 4%) and F. grandifolia (33% to 12%) in the High Plateau and increases in Acer (11% to 37%), Quercus rubra L. (0% to 10%), Prunus serotina Ehrh. (1% to 6%), and Betula (5% to 10%). Other units exhibited reductions in P. strobus, P. rigida, Quercus alba L., and Carya spp. and increases in Quercus prinus L., Q. rubra, Acer rubrum L., and P. serotina. Castanea dentata had its greatest abundance on higher elevation sites in each physiographic unit, and the elimination of this species this century apparently facilitated the increase in Q. prinus and Q. rubra on ridge sites. South of the Allegheny Plateau, increases in A. rubrum, P. serotina, and other mixed-mesophytic species may be in response to fire exclusion this century. The results of this study indicate the importance of landform and physiography on presettlement forest composition as well as the dramatic changes that have occurred as a result of altered disturbance regimes following European settlement.