Ecohydrology and self-organization of black ash wetlands
Diamond, Jacob Sullivan
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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 America has raised concern over possible ecosystem state shifts in forested wetlands (i.e., to wetter, more herbaceous systems) and loss of forest function, calling for informed landscape-scale management strategies. In this dissertation, I use black ash wetlands as a model system to understand complex ecohydrological dynamics, and I use these dynamics to explain the self-organization of observed patterns in vegetation, hydrology, and microtopographic structure. The combined inferences from the three research chapters strongly implicate black ash trees as autogenic ecosystem engineers, who, through the process of improving their local growing conditions, cause a cascade of environmental changes that result in a unique ecosystem structure. This unique ecosystem structure is under existential threat from the invasive EAB. Through experiment, I show that loss of black ash trees to EAB induces persistent shifts in hydrology that result from reduced evapotranspiration and subsequent changes to water table regime (Chapter 2). These results suggest the potential for catastrophic shifts of black ash wetlands from forested to non-forested, marsh-like states under a do-nothing EAB management scenario. However, research presented here suggests that preemptive management of black ash wetlands can potentially mitigate loss of desirable forested conditions. Forest management to replace black ash with other wetland canopy species may be a slow and steady path towards forest maintenance, and harvesting may facilitate establishment of alternative species. In the case of preemptive harvesting of black ash, I posit that maintenance of microtopographic structure, either through leaving downed woody debris or through physical creation, is paramount to forest recovery. Microtopography in these ecosystems provides crucial relief from anaerobic stress generated by higher water tables, allowing woody species to persist on elevated microsites (e.g., 30 cm above base soil elevation). Moreover, I show that microtopography in black ash wetlands has clear structure and pattern and that its presence arises from self-organizing processes, driven by feedbacks among hydrology, biota, and soils (Chapter 3). I further show that this structured and non-random microtopography has profound influence on biogeochemical processes in black ash wetlands, controlling plant richness and biomass, and soil chemistry gradients (Chapter 4). Based on this work, I propose that structured wetland microtopography is a diagnostic feature of strongly coupled plant-water interactions, and these interactions may be important for ecosystem resilience to disturbance.
- Doctoral Dissertations