Patterns of Overstory Composition in the Fir and Fir-Spruce Forests of the Great Smoky Mountains After Balsam Woolly Adelgid Infestation

Abstract

In 1957, the exotic balsam woolly adelgid (Adelges piceae Ratz.) was discovered in the southern Appalachians. It quickly spread throughout the spruce-fir forests, decimating adult Fraser fir (Abies fraseri (Pursh) Poir.) populations. In 1990 and 1991, a system of temporary and permanent plots was established on five mountains in the Great Smoky Mountains. Almost seventy percent of total standing fir basal area in the study area was dead. Mt. Collins, which experienced the major wave of fir mortality about 4 yr before sampling, had the greatest percentage (91%) of dead fir basal area of the five mountains. Despite catastrophic mortality, fir dominated (11–16 m2/ha) live stand basal area on the tops of the three higher elevation mountains, whereas red spruce (Picea rubens Sarg.) and deciduous taxa dominated the two lower mountains. Live fir density over the short term was positively associated with time since the major wave of mortality. Patterns of fir mortality, treefall rates and recruitment determined the proportion of dead fir stems in the overstory. These three factors in turn were dependent on time since major mortality. A logistic regression model predicted that the proportion of standing dead fir would increase until 5—8 yr after an episode of major mortality and decrease thereafter as dead trees fall and young fir reach the overstory.