Browsing by Subject "Transpiration"

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    Impacts of black ash stand transpiration on the hydrology of wetland forests in northern Minnesota, U.S.A.
    (2013-08) Telander, Andrew Clayton
    The possibility that the emerald ash borer (Agrilus planipennis) 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 ( Fraxinus nigra). 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.
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    Predicting the effects of climate change on water yield and forest production in the northeastern United States
    (1995) Aber, John D; Ollinger, Scott V; Federer, C. Anthony; Reich, Peter B; Goulden, Michael L; Kicklighter, David W; Melillo, Jerry M; Lathrop, Richard G Jr
    Rapid and simultaneous changes in temperature, precipitation and the atmospheric concentration of CO2 are predicted to occur over the next century. Simple, well-validated models of ecosystem function are required to predict the effects of these changes. This paper describes an improved version of a forest carbon and water balance model (PnET-II) and the application of the model to predict stand- and regional-level effects of changes in temperature, precipitation and atmospheric CO2 concentration. PnET-II is a simple, generalized, monthly time-step model of water and carbon balances (gross and net) driven by nitrogen availability as expressed through foliar N concentration. Improvements from the original model include a complete carbon balance and improvements in the prediction of canopy phenology, as well as in the computation of canopy structure and photosynthesis. The model was parameterized and run for 4 forest/site combinations and validated against available data for water yield, gross and net carbon exchange and biomass production. The validation exercise suggests that the determination of actual water availability to stands and the occurrence or non-occurrence of soil-based water stress are critical to accurate modeling of forest net primary production (NPP) and net ecosystem production (NEP). The model was then run for the entire NewEngland/New York (USA) region using a 1 km resolution geographic information system. Predicted long-term NEP ranged from -85 to +275 g C m-2 yr-1 for the 4 forest/site combinations, and from -150 to 350 g C m-2 yr-1 for the region, with a regional average of 76 g C m-2 yr-1. A combination of increased temperature (+6*C), decreased precipitation (-15%) and increased water use efficiency (2x, due to doubling of CO2) resulted generally in increases in NPP and decreases in water yield over the region.