Overstory treatment and planting season affect survival of replacement tree species in emerald ash borer-threatened Fraxinus nigra forests in Minnesota, USA
Fraxinus nigra (black ash) wetland forests in the northwestern Great Lakes region of North America are threatened with extirpation by the invasive insect, emerald ash borer (Agrilus planipennis, EAB). A potential management option is promoting regeneration of non-EAB-host tree species to maintain ecosystem functions. Using an operational-scale field experiment, we examined the survival of 12 alternative tree species in response to different canopy treatments. We planted the seedlings in 1.6 ha plots assigned to four replicated canopy treatments: untreated control; group selection (0.04 ha gaps, 20% of stand); adult black ash girdling to emulate EAB-induced mortality; and clearcut. Fall and spring plantings were used to compare the effects of spring ponding. Control (32.9%), group selection (34.5%) and girdling treatments (33.3%) had comparable overall seedling survival. Survival in the clearcut treatments was significantly lower (22%). Species selection, overstory treatment, and season of planting together resulted in survival rates ranging from 0.08% to 94.1%. Conifer species had low overall rates of survival (10.7%), while some species with native ranges not presently overlapping with northern F. nigra forests, e.g. Quercus bicolor (75.5%), had high survival rates. If growth is light-limited, group selection may be effective in promoting recruitment and supporting a larger variety of species.