Size- and age-class distributions of Fraser fir following balsam woolly adelgid infestation

Fraser fir (Abies fraseri (Pursh) Poir.) has suffered catastrophic mortalit y throughout most of its native range from an exotic insect, the balsam woo lly adelgid (Adelges piceae Ratz.). To assess the regeneration potential an d viability of fir populations, overstory and understory Fraser fir size an d age structure were analyzed. The data were collected from thirty-six 400- m(2) permanent plots, stratified into four stand canopy composition types, established near the summits of five mountains in the Great Smoky Mountains . We found that, where canopy mortality was severe, fir advance regeneratio n was re-entering the overstory. In seriously impacted stands, mortality of large fir and increased recruitment have produced distributions characteri zed by few large fir and relatively high densities of small fir. Densities of 0- to 10-year-old fir seedlings and fir seedlings less than or equal to 0.25 m tall were much lower in stands dominated by dead fir than in mostly intact fir stands. While the lack of reproducing adults appears to be the m ain cause, competition with invasive species and higher seedling mortality from environmental factors probably contribute. These results lead to a hyp othesis that Fraser fir will undergo a regeneration-mortality cycle with a decrease in the numbers of each successive generation.