Abstract
Beech bark disease (BBD) has adversely affected American beech and northeastern forests for over a century. Despite a long history of BBD in North America, disease susceptibility, interactions with non-BBD pathogens, and indirect effects on biodiversity are unexamined. BBD involves the fungal pathogens Neonectria faginata and N. ditissima which are believed to require predisposition by Cryptococcus fagisuga, a scale insect, to infect. Recent evidence suggests additional or alternative predisposing factors may operate in long-affected (aftermath) forests. BBD-related organisms commonly co-occur with other forest pathogens, such as decay fungi, that likely alter beech defensive chemistry and tree response to Neonectria and C. fagisuga. Dense thickets of understory beech arising from roots of dead overstory trees inhibit regeneration of desirable timber species and likely affect other forest plants and fauna as well. Here, I used case-control and observational studies to identify biotic and abiotic predisposing factors important to Neonectria infection and understand how factor combinations differ between N. faginata and N. ditissima, how BBD-related organisms respond to concurrent infections by decay fungi, and how dense thickets of understory beech affect forest biodiversity. Study results indicate (1) BBD development is more complex than previously believed and includes both abiotic and biotic factors other than C. fagisuga which differ between causal pathogens; (2) decay fungi on beech adversely affect C. fagisuga infestations possibly though bark chemistry changes; and (3) beech thickets reduce plant diversity but seemingly do not affect faunal diversity. These insights considerably expand our knowledge of BBD and have important implications for managing and predicting disease severity and impacts in aftermath forests.