Measuring spatial and temporal shifts in forest structure and composition in T high elevation beech forests in response to beech bark disease in Great Smoky Mountains National Park

Exotic forest pests and pathogens are among the most serious environmental threats to millions of hectares of forested land worldwide. Beech Bark Disease (BBD) is a non-native, pathogenic complex consisting of associations between scale insects and fungi. First confirmed in Great Smoky Mountains National Park (GRSM) in 1986, this complex has since threatened local high elevation beech forests, which are G-1 ranked (critically imperiled) forest communities where American beech (Fagus grandifolia Ehrh.) is a foundational tree species. In 1994, GRSM initiated the BBD Monitoring Protocol at 10 high elevation beech forest plots in the Park. The plots were sampled biennially from 1994 to 2012 and again in 2017 to investigate infestation patterns and host mortality. Permutational multivariate analysis of variance (PERMANOVA) and nonmetric multidimensional scaling (NMDS) techniques were used to investigate shifts in forest structure and composition over the 23-year study period. Species-specific changes were analyzed using repeated measures linear mixed-effects models (RMLMM). High elevation beech forest communities are changing through time with trajectories becoming more influenced by Acer and Betula species. Despite great variability in plot location, aspect, slope, and beginning species composition, time was a significant factor, explaining ~10% of the variation in relative basal area. Species-specific shifts were highly variable. While the overstory Fagus grandifolia basal area declined significantly over the 23-year study period, no significant change in total basal area was observed, indicating that the loss of these mature trees was compensated by co-occurring species. Understory sapling and woody seedling abundance of F. grandifolia increased relative to most other species in the study. The loss of the foundational species, F. grandifolia (the dominant tree species which defines high elevation beech forests), will have broad consequences for associated biota, ecosystem function, and potentially, the long-term persistence of high elevation beech forests in GRSM