Fourteen years of swamp forest change from the onset, during, and after invasion of emerald ash borer

Abstract

Since beginning its invasion of eastern North America by 2002, the Asian-origin beetle emerald ash borer (EAB; Agrilus planipennis) has decimated ash trees (Fraxinus spp.). In the Great Black Swamp in northwestern Ohio, USA, 90 km south of the epicenter of EAB invasion, we examined changes in forest communities during a 14-year period spanning EAB’s arrival in 2005 through 2018. No green ash (Fraxinus pennsylvanica) trees larger than 4 cm in diameter were alive on study sites in 2018, but ash saplings 1–4 cm in diameter (764 stems/ha) and seedlings (1.5% cover) were plentiful. Basal area growth of other tree species did not compensate for ash loss; total basal area averaged 8% lower in 2018 than in 2005. In the understory, non-native plants were negligible (< 2% cover, < 3 species/0.05 ha) among years and did not change significantly after EAB’s arrival. Native plant cover was twice as high in 2018 than when EAB arrived in 2005. Changes in understory cover primarily entailed reorganization of species already established when EAB arrived, rather than colonization by new species. New colonizers proportionally comprised only 0.05–0.07 of total cover among years. The shrub spicebush (Lindera benzoin) and forb Virginia creeper (Parthenocissus quinquefolia) dominated understories throughout the study. From a perspective of maintenance of native understories in EAB-aftermath forests, observed changes were probably as favorable as could be expected. Native species already present increased the most, neither floristic quality nor wetland indicator species declined, and non-native plants did not increase.