THE INTERACTIVE EFFECTS OF MICROTOPOGRAPHY AND HYDROLOGY ON GROUND LAYER VEGETATION AND SOIL GAS FLUX RESPONSES TO A SIMULATED EMERALD ASH BORER INFESTATION IN BLACK ASH WETLANDS

Black ash (Fraxinus nigra Marshall) wetlands are at risk of significant ecological and functional changes due to the invasive emerald ash borer (EAB) (Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)), which kills trees in the Fraxinus (ash) genus. Simulated EAB infestations consisting of girdle treatments and ash cut treatments have been implemented in black ash wetlands to study the impacts of black ash canopy dieoff in these systems. Initial findings include ground layer vegetation shifts and impacts to carbon dioxide (CO2) and methane (CH4) release from soils, but these factors and their interactions with microtopography in these systems are not well understood. The objectives of this study were to explore how vegetation and greenhouse gas fluxes (GHGs) are currently responding to simulated EAB treatments in the Ottawa National Forest in the Great Lakes Region of North America six years after initial treatment implementation, and to determine how microtopography affects them as well. Tree seedling counts and diversity were not found to be affected by treatments alone. Microtopography, however, had a larger impact and showed more seedlings and a higher diversity of seedlings growing on hummocks. Seedlings over one year of age were also found in greater numbers and with more diversity on top of hummocks. Herbaceous species have continued their trend of increased cover in treated sites, with higher cover of obligate wetland species and graminoids in treated sites as well. Increased herbaceous cover was found on top of hummocks rather than in hollows. These findings suggest that herbaceous cover is influenced by both treatments and microtopography, while tree regeneration is more influenced by microtopography. The implications of the lack of response between treatments for seedlings is that the increase in water levels at our wetland sites may be buffered by microtopography and our sites could continue to stay forested following an EAB infestation. CO2 and CH4 fluxes showed different responses to treatment and microtopography. CO2 flux was highest in control sites and on top of hummocks, while CH4 was not found to be different between treatments or microtopography. The postulated reasons for these findings are depth to water table and root respiration. For CO2, the lower water levels allowed for more soil to be aerated and decomposed, and there is more root respiration in control sites because of a higher volume of living trees. For CH4, the low water levels created an environment where little CH4 was produced at all. When landscape-scale estimates of GHG fluxes were created using weighted fluxes from hummocks and hollows, CO2 fluxes were overestimated and CH4 fluxed were underestimated when the elevation-based flux differences due to microtopography were not factored in. These findings suggest that microtopography should be included when scaling up gas flux measurements to the landscape scale in order to get the most accurate estimates