High-elevation ground-layer plant community composition across environmental gradients in spruce-fir forests

We examined the influence of vegetation structure and soil chemistry on post-adelgid, ground-layer plant communities in high-elevation forests of the southern Appalachian Mountains. Specifically, we hypothesized that post-disturbance community composition and diversity would vary along a gradient of soil acidity and other soil characteristics influenced by acid deposition. Ground-layer vegetation and soils were sampled on 60 randomly located nested vegetation plots in the spruce-fir (Picea abies) zone of Great Smoky Mountains National Park, TN and NC, USA. To capture a range of deposition levels, plot placement was stratified based on modeled acid deposition classes. Ordination and multiple regression results showed that ground-layer composition and diversity were negatively associated with acidity of the A horizon and the presence of ericaceous shrubs (i.e., Rhododendron spp.). A strong correspondence between soil acidity and ericaceous shrub cover was also observed, suggesting that soil acidity may be, in conjunction with overstory disturbance resulting from chronic acid deposition and adelgid induced mortality, an important driver of ericaceous shrub thicket expansion. Slow-decaying, acidic ericaceous litter may also induce a positive feedback resulting in enhanced acidification.