Black ash wetlands are seriously threatened because of the invasive emerald ash borer (EAB). Wetland hydrology is likely to be modified following ash mortality, but the magnitude of hydrological impact following loss via EAB and alternative mitigation harvests is not clear. Our objective was to assess the water table response to simulated EAB and harvesting to determine if management actions will be needed to maintain ecosystem functions following EAB infestation. We applied four replicated treatments to 1.6 ha plots as follows: (1) control, (2) girdling of all black ash trees to simulate loss via EAB mortality, (3) group selection harvests (20% of stand in 0.04 ha gaps), and (4) clear-cut harvest. Water table (WT) elevations were monitored for 1 year pre-treatment and two years post-treatment. Clear-cutting delayed WT drawdown in both years of the study, and the WT was significantly higher than the control treatment, predominantly when WT depth was below 30 cm. The effect of the group selection treatment on WT response was muted compared to clear-cutting and also limited to periods when the WT depth was below 30 cm. These responses were attributed to establishment of shallowrooted vegetation in cut areas, which would have limited influence on WT dynamics as depth increased. There was little effect of girdling on the WT in the first year post-treatment, but effects on the WT were very similar to clear-cutting in the second year and more pronounced when the WT was within 30 cm of the soil surface. These effects were attributed to reduced transpiration coupled with the presence of a partial canopy following girdling, which would have reduced vegetation establishment and evaporation compared to clear-cutting. Given the large influence of WT depth on vegetation dynamics and associated feedbacks to altered hydrology, these early results indicate a greater risk of ecosystem alteration following EAB mortality compared to clear-cut harvesting. Depending on local hydrologic regime, variation in precipitation patterns, and time for complete canopy loss, it may be necessary for managers to implement active mitigation strategies (e.g., group selection coupled with planting of alternative species) prior to EAB infestation to maintain ecosystem processes in these forested wetland systems.Résumé : Les milieux humides occupés par le frêne noir (Fraxinus nigra Marshall) sont sérieusement menacés à cause de l'agrile du frêne (AF) (Agrilus planipennis Fairmaire). Les caractéristiques hydrologiques des milieux humides seront probablement modifiées à la suite de la mortalité du frêne mais l'ampleur de l'impact hydrologique causé par les pertes dues à l'AF et les coupes alternatives visant à réduire cet impact n'est pas claire. Notre objectif consistait à évaluer la réaction de la nappe phréatique (NP) à l'action simulée de l'AF et à la coupe pour déterminer si des interventions d'aménagement sont nécessaires pour maintenir les fonctions de l'écosystème à la suite d'une infestation de l'AF. Nous avons appliqué les quatre traitements s...

2Fraxinus nigra (black ash) wetland forests in northern Minnesota, USA, are threatened by the 3 invasive insect, emerald ash borer (Agrilus planipennis, EAB). A potential management option is 4 promoting regeneration of non-EAB-host tree species to maintain ecosystem functions. Using an 5 operational-scale field experiment, we examined the survival of 12 alternative tree species in 6 response to different canopy treatments. We planted the seedlings in 1.6 ha plots assigned to four 7 replicated canopy treatments: untreated control; group selection (0.04 ha gaps, 20% of stand); 8 black ash girdling to emulate EAB-induced mortality; and clearcut. Fall and spring plantings 9 were used to compare the effects of spring ponding. Control (32.9%), group selection (34.5%) 10 and girdling treatments (33.3%) had comparable overall seedling survival. Survival in the 11 clearcut treatments was significantly lower (22%). Species selection, overstory treatment, and 12 season of planting together resulted in survival rates ranging from 0.08% to 94.1%. Conifer 13 species had low overall rates of survival (10.7%), while some species with native ranges not 14 presently overlapping with northern F. nigra forests, e.g. Quercus bicolor (75.5%), had high 15 survival rates. If growth is light-limited, group selection may be effective in promoting 16 recruitment and supporting a larger variety of species. 17 18 19

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