Forested versus herbaceous wetlands: Can management mitigate ecohydrologic regime shifts from invasive emerald ash borer?

Abstract

Wetlands self-organize through reciprocal controls between vegetation and hydrology, but external disturbance may disrupt these feedbacks with consequent changes to ecosystem state. Imminent and widespread emerald ash borer (EAB) infestation throughout North American forested wetlands has raised concern over possible ecosystem state shifts (i.e., wetter, more herbaceous systems) and loss of forest function, calling for informed landscape-scale management strategies. In response, we employed a large-scale manipulative study to assess the ecohydrologic response of black ash wetlands to three alternative EAB management strategies: 1) a do-nothing approach (i.e., simulated EAB infestation via tree girdling), 2) a preemptive, complete harvesting approach (i.e., clearcut), and 3) an overstory replacement approach via group selection. We analyzed six years of daily water table and evapotranspiration (ET) dynamics in six blocks comprising black ash wetlands (controls) and management strategy treatments to quantify potential for hydrologic change and subsequent recovery. In both the do-nothing approach and complete harvesting approach, we found persistent changes in hydrologic regime defined by shallower water tables and lower ET rates coupled with increased herbaceous vegetation growth, indicating ecosystem state shifts driven by vegetation-water table interactions. The do-nothing approach showed the least hydrologic recovery after five years, which we attribute to reduction in overstory transpiration as well as greater shade (via standing dead trees) that reduces open water evaporation and herbaceous layer transpiration compared to complete harvesting. We found no evidence of ecohydrologic disturbance in the overstory replacement approach, highlighting its potential as a management strategy to preserve forested wetland habitat if periodically executed over time before EAB infestation. Although the scale of potential disturbance is daunting, our findings provide a baseline assessment for forest managers to develop preemptive mitigation strategies to address the threat of EAB to ecological functions in black ash wetlands.