Water table response to harvesting and simulated emerald ash borer mortality in black ash wetlands in Minnesota, USA
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 shallow-rooted 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.