Silvicultural Options to Promote Resilience of Northern Great Lakes Fraxinus nigra (Black Ash) Forests following Emerald Ash Borer (Agrilus planipennis) Invasion

Abstract

Fraxinus nigra (black ash), a dominant species of northeastern North American wetland forests, is threatened by the invasive insect, emerald ash borer (Agrilus planipennis, EAB). The potential for one or more tree species to replace F. nigra and approaches for facilitating this replacement have not been explored. To fill these knowledge gaps, this research examined: (1) the influence of competition and climate on the growth of mature F. nigra in association with Tilia americana (basswood), Populus tremuloides (quaking aspen), and Thuja occidentalis (northern white-cedar) at the mesic, moist, and wet F. nigra forest margins, respectively; (2) the survival and growth of planted seedlings of 12 potential F. nigra replacement species under four canopy treatments: unharvested control, clearcut, Fraxinus girdling to emulate EAB, and group selection; and (3) the response of F. nigra forest groundlayer plant communities to treatment-induced canopy changes. The research goals were to determine (1) how the influences of climate and competition vary by species in F. nigra forests and whether these factors shift in importance along a gradient of waterlogging stress; (2) the most promising combinations of species and treatment for silvicultural efforts to maintain post-EAB forest function; and (3) the extent to which natural regeneration of associated tree species can be expected to aid in maintaining post-EAB forest cover, as well as the potential effects on this regeneration of EAB- and treatment-induced shifts in groundlayer community composition. The main findings were that mature F. nigra had slower growth and appeared more sensitive to competition than T. americana and P. tremuloides, while having similar growth and appearing less sensitive to competition than T. occidentalis. Fraxinus nigra growth showed greater sensitivity to precipitation and temperature across the gradient of waterlogging stress relative to the three associated species. Canopy treatment type interacted with species to influence the survival and relative growth rate (RGR) of planted seedlings. Planted seedlings of shade-intolerant species generally had moderate-to-low survival and high RGR across treatments, while shade-tolerant species had higher survival with lower RGR. Natural tree regeneration varied by treatment and fell short of F. nigra forest stocking recommendations. Graminoid cover increased in the clearcut treatment, while bottomland shrubs and wetland indicator species were more numerous in the control and girdle. Overall, the findings suggest that planting seedlings of non-Fraxinus tree species with or without overstory treatment as EAB approaches is a potentially viable and necessary management option for sustaining ecosystem services in F. nigra wetlands. Site, species, and canopy treatment selection should be coordinated to maximize artificial regeneration success.