CANOPY TREE TURNOVER IN OLD-GROWTH MESIC FORESTS OF EASTERN NORTH AMERICA

Abstract

I studied the dynamic nature of old-growth, eastern U.S. forests by addressing the following questions: (1) How much do stand density, basal area, and size structure vary over time within several old-growth remnants? (2) How do mortality and growth rates vary with stem size? (3) How much does the importance of individual species vary over time and space? (4) At what rate do snags and stumps form and deteriorate? In 1990–1991, I resampled canopy stems within several old-growth remnants in the southern Appalachians, in Hueston Woods State Park, Ohio, and in the Tionesta Scenic and Research Natural Areas, Pennsylvania. I had previously sampled those sites in 1976–1977 using the point-centered quarter method. I remeasured the same trees and measured new trees if the old ones had died or if a new stem closer to the point than the old stem for that quarter had grown to ≥25 cm in diameter at breast height. Density and basal area changes were small. Density changes equaled −0.33%, −0.52%, and 0.00%/yr for the southern Appalachians, Hueston Woods, and Tionesta sites, respectively. Corresponding basal-area changes were 0.03%, −0.22%, and 0.45%/yr. Mortality increased consistently with stem size in all three areas. However, growth rates of smaller stems more than compensated for the higher mortality rates of larger stems, so that overall stem size increased between samples. Most species changed little in relative density or basal area. Overall Fagus showed the largest changes between samples, with a small decrease in the southern Appalachians, a larger decrease in Hueston Woods, and an increase in Tionesta. Trees usually died standing, breaking off at a variety of heights. For example, only 6–27% of trees that died were uprooted, depending on region, whereas 16–31% broke at a height of ≥16 m. Total snag densities were 15–18 snags/ha. In general these stands were marked by slow changes toward fewer, larger stems, even after centuries in which no major disturbances had occurred. However, changes in pathogens, climate, and atmospheric chemistry could change these trends in the future.