Impacts of black ash stand transpiration on the hydrology of wetland forests in northern Minnesota, U.S.A.

Abstract

The possibility that the emerald ash borer (<italic>Agrilus planipennis</italic>) will spread to northern Minnesota and cause widespread mortality of black ash is an issue of concern for land managers. Northern Minnesota contains many wetland forests dominated by black ash (<italic> Fraxinus nigra</italic>). Given the importance of evapotranspiration of overstory tree species, such as black ash, to the hydrologic budgets of forest systems, transpiration was approximated at multiple ash forests of northern Minnesota. In 2012, sap flux rates were monitored at three stands of black ash with differing moisture regimes within the Chippewa National Forest, Minnesota, USA. Sap flux rates for black ash were within the expected range of values. Black ash exhibited small sapwood area across all of the trees monitored. When sap flux was converted to sap flow and scaled from the sample trees to site level estimates of transpiration, transpiration values were low due to small sapwood area. Results were compared to soil moisture and potential evapotranspiration data to determine the efficacy of the methodology employed for estimating sap flux and to corroborate transpiration results. In the summer of 2012, transpiration by black ash only accounted for 16% to 21% of total potential evapotranspiration. Despite the common flooded status of black ash wetland forests, moisture regime and moisture limitation did impact sap flux and transpiration in black ash stands as there was variability in sap flux and sap flow among sites of varying moisture regime. Potential evapotranspiration and vapor pressure deficit were only important when moisture was not limited spatially or temporally. Belowground impacts to sap flux from soil moisture only became perceivable during periods of moisture limitation. Sources of error associated with the transpiration determination process were explored and while the results of this study indicate trends in transpiration of black ash forests, the multiple stages of transpiration determination contain error that needs to be acknowledged when considering implications of transpiration results. Black ash mortality caused by emerald ash borer could be problematic in terms of hydrologic influence, impacts on vegetation community structure and composition, and alterations to forest management regimes; however, black ash contribution to total evapotranspiration was less than expected and indicates that loss of black ash may result in fewer alterations to hydrologic processes than previously anticipated.