Eighth-year survival and growth of planted replacement tree species in black ash (Fraxinus nigra) wetlands threatened by emerald ash borer in Minnesota, USA

Black ash (Fraxinus nigra) is native to lowland forests of the western Great Lakes region, USA, where it often comprises a majority of trees. Like all native ash in North America, black ash is threatened by emerald ash borer (EAB; Agrilus planipennis), but the impacts from EAB mortality may be particularly severe in these forests given the foundational role of black ash at regulating ecosystem function. Compounding the problem is that associated tree species occur in low abundance and their abundance may be further reduced as habitat declines with climate change. These converging threats point to the need for silvicultural intervention to establish replacement tree species in anticipation of EAB invasion. Here we report on a large-scale management experiment from Minnesota, USA that includes different silvicultural approaches for establishing replacement tree species in black ash forests. Specifically, we examined eighth-year survival and growth of planted seedling in treatments that included clearcutting, group selection, uncut forest, and emulation of EAB mortality by girdling black ash. Species included nine that are native to the ecosystem, two from the next southern climate zone, and one exotic species, Manchurian ash (Fraxinus mandshurica). Among species and treatment combinations, survival was highest for American elm (Ulmus americana), averaging around 81% in uncut, group selection, and girdle treatments. Swamp white oak (Quercus bicolor), a species from the next southern climate zone, also had high survival in these treatments (ranging from 61% to 79%). Both species had survival under 60% in the clearcut treatment. Most native southern boreal species, as well as Manchurian ash, had low survival (0% to less than 40%) in most treatments. In the clearcut, girdle, and group selection treatments relative diameter and relative height growth was highest for balsam poplar (Populus balsamifera), averaging, respectively, around 0.25 mm mm(-1) yr(-1) and 0.20 cm cm(-1) yr(-1), followed by swamp white oak and red maple (Acer rubrum). Non-native Manchurian ash had consistently low growth in all treatments compared to other species. All species had low growth rates in the uncut treatment. An integration of survival and diameter growth pointed to group selection as the treatment that provides the best balance between survival and growth. Our results indicate promising survival and growth of at least some replacement species, including several predicted to be future climate-adapted, as well as a silvicultural approach in group selection that is an effective method to regenerate these species.

Automated summary

Black ash forests in the USA are threatened by emerald ash borer, leading to a need for silvicultural intervention to establish replacement tree species. Results from a large-scale management experiment in Minnesota show that American elm and swamp white oak have promising survival rates in treatments like uncut, group selection, and girdling, suggesting these species may be viable replacements.