Southern Appalachian Fir and Fir-Spruce Forest Community Changes Following Balsam Woolly Adelgid Infestation

The southern Appalachian fir and fir-spruce forests are unique glacial relict communities that occupy 26,600 ha at the highest elevations of only seven mountain areas. Over the last three and a half decades an exotic insect, the balsam woolly adelgid (Adelges piceae Ratz.), has caused catastrophic mortality to populations of the endemic Fraser fir (Abies fraseri (Pursh) Poir.) throughout its entire native range. In 1990 and 1991, a set of temporary and permanent plots were established at the summits of five mountains in the Great Smoky Mountains to study processes of community change in overstory and understory composition and fir size and age distributions. Nearly 70% of standing fir basal area is dead over the entire study area. Patterns of overstory dominance are determined primarily by elevation and time since the major wave of mortality. Overstory fir mortality and the resulting increases in canopy openness have led to decreases in densities of 0-10 yr old fir seedlings and bryophyte cover. Herbaceous vegetation cover, densities of red spruce (Picea rubens Sarg.) seedlings, and small shrub densities have increased in some stands. Understory fir are growing and maturing more rapidly with greater insolation. Understory fir recruitment and selective mortality of larger fir are causing overstory size and age classes to become more skewed towards smaller and younger individuals. Future rates and spatial patterns of infestation and mortality will be determined by rates of fir recovery in patches subject to different biotic and abiotic stressors. Poorly reproducing fir may be eliminated in lower elevation mixed stands.