Runoff and Stream Water Chemistry Responses to Simulated Emerald Ash Borer Invasion in Black Ash Wetlands in Northern Michigan

Abstract

Black ash (Fraxinus nigra Marsh) is the dominant hardwood species in many northern forested wetlands, especially in the Great Lakes Region. Black ash is subject to extremely high rates of mortality following the infestation of Emerald ash borer (EAB, Agrilus planipennis Fairmaire). Our research expands upon previous work examining the hydrologic impacts of EAB on black ash wetlands by examining changes in baseflow and response to precipitation using a paired watershed design. To simulate anticipated long-term impacts, all ash stems greater than or equal to 2.54 cm in diameter at breast height were felled and left on site. We hypothesize that 1) the treatment watershed will become more responsive to rainfall events and have higher water yield relative to pre-treatment conditions; and 2) chemical (dissolved organic carbon (DOC), total dissolved nitrogen (TDN), chloride, and sulfate) and isotopic (2H and 18O) tracers in stream water will show a reduced wetland water signature relative to precipitation and local upland groundwater. During the two-year pre-treatment period we observed median water yield to rainfall ratios of 0.033 and 0.022 on an event basis in the control and treatment watersheds, respectively. During the four-year post-treatment period the ratio was 0.013 (-62.3%) for the control watershed and 0.018 (-17.1%) for the treatment watershed. We did not observe an increase in treatment watershed responsiveness relative to the pre-treatment period as expected. However, we did observe a significantly smaller reduction in responsiveness in the treatment watershed relative to the control. Climatic differences and a shift in hydrologic regime in the pre- and post-treatment periods are the likely explanation for the decreased responsiveness of both watersheds to rainfall. Results also show that the relationship between DOC and TDN concentrations in stream water and wetland water were weaker following the treatment. The slope of the relationship between stream water and both wetland surface water and soil pore water in the treatment watershed was reduced by approximately 60% for both DOC and TDN. The relationship did not significantly change within the control watershed. These findings suggest that the loss of black ash will lead to greater responsiveness to rainfall events relative to undisturbed wetlands.