Recovery trends and predictions of Fraser fir (Abies fraseri) dynamics in the Southern Appalachian Mountains

Abstract

The endemic Fraser fir (Abies fraseri (Pursh) Poir.) is found in only seven montane regions in the southern Appalachians above ca. 1500 m elevation. Due to widespread insect-caused mortality from the invasive balsam woolly adelgid (Adelges piceae Ratzeburg), as well as possible impacts from climate change and atmospheric pollution, the future of Fraser fir populations remains uncertain. Long-term monitoring programs have been in place since the 1980s, and here we present the first predictive population models for endemic Fraser fir populations using the inventory data from Great Smoky Mountains National Park, which contains 74% of extant Fraser fir forests. Using two kinds of population data (understory density counts and overstory census data), we model Fraser fir population dynamics on five different mountaintops as a stage-structured matrix model with transition parameters estimated using hierarchical Bayesian inference. We predict robust recovery over the next several decades for some Fraser fir populations, particularly where mature overstory fir has persisted throughout the last two decades, and continued decline for populations at the lowest elevations. Fraser fir densities are already low at these lower elevations, suggesting that this population is vulnerable to local extirpation.