The loss of canopy ash trees (Fraxinus spp.) from North American forests may result in homogenization of understory flora

In North America, many canopy ash trees ( Fraxinus spp.) are being eliminated from forests by the Emerald Ash Borer ( Agrilus planipennis ). This is yet another case of a native tree species being functionally eliminated by non-native pests. Lasting effects on forests might be minimal if the species that replace ash are ecologically similar (functionally redundant). Alternatively, effects could be lasting if the vulnerable tree species have unique ecological attributes. As one of few ring-porous trees in north-temperate hardwood forests, ash leaves emerge and expand 1–2 weeks later than co-occurring deciduous trees our study region. This allows for higher light transmission into the subcanopy and could thereby benefit the understory plant community. In advance of their elimination, we tested for floristic differences beneath groves of canopy white ash compared to other canopy tree species. We studied understory flora with a factorial study that included four canopy tree species (white ash, American beech, yellow birch, sugar maple) on each of two soil types (Bh podzol and typical podzol). The subcanopy flora beneath white ash contained more plants of more species. Several species were more abundant on Bh podzol soil than typical podzol, and several species displayed effects of canopy tree species that depended upon soil type. There was limited evidence that the understory flora beneath ash was functionally unique. The flora beneath ash is apparently enriched due to greater radiation reaching the forest floor. The loss of canopy ash trees seems likely to reduce and homogenize the understory flora.