Spread of beech bark disease in the eastern United States and its relationship to regional forest composition.
Beech bark disease (BBD) is an insect-fungus complex involving the beech scale insect (Cryptococcus fagisuga) and one of two canker fungi. Beech scale was introduced to Halifax, Nova Scotia (Canada) around 1890, presumably with the fungus Neonectria coccinea var. faginata. The disease has subsequently spread through a large portion of the range of beech. We used historical maps (1911-2003) of the extent of the advancing BBD front (defined by presence of scale insects) in North America to estimate its rate of spread as 14.7±0.9 km/year. This estimate did not account for stochastic "jumps" by the scale insects to several disjunct locations; therefore, this rate is a conservative estimate. Data collected from 93 611 plots located in 37 states in the eastern USA were used to characterize current forest composition in relation to historical and future BBD spread. Comparison of the year of scale colonization with beech density did not suggest a relationship between the scale spread rate and beech density. Our analyses also indicated that BBD has invaded less than 30% of regions where beech is present, but it has invaded most of the regions where beech is a dominant component of stands. Despite regional increases in beech mortality following invasion, considerable amounts of live beech remain in invaded areas. Moreover, the volume of beech has increased in most areas, though generally at lower rates than that observed for associated tree species.