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Climate and Vegetation Effects on Temperate Mountain Forest Evapotranspiration
Citation:
Oishi, C., C. Miniat, K. Novick, S. Brantley, J. Vose, AND Johnt Walker. Climate and Vegetation Effects on Temperate Mountain Forest Evapotranspiration. Presented at 2015 American Geophysical Union Annual Meeting, San Francisco, CA, December 14 - 18, 2015.
Impact/Purpose:
Forest evapotranspiration (ET) can vary greatly at daily and seasonal time scales, but compared to carbon fluxes, often exhibits relatively consistent inter-annual behavior. The processes controlling ET involve the combined effects of physical and biological factors. Atmospheric conditions that promote high ET, consisting of high radiation and vapor pressure deficit (D), are often characterized by rainless periods when soil water supply to vegetation may be limiting and plant stomata may close to prevent excessive water loss. In contrast, periods of high ecosystem water availability require frequent precipitation and are characterized by low D. Thus, the combination of these contrasting conditions throughout a growing season may explain some of the consistency in ET. Additionally, vegetation composition is also an important factor in determining ET. In mixed species forests, physiological differences in water use strategies (e.g. isohydric/anisohydric species) can produce conservative water use throughout wet and dry phases of the growing season. Furthermore, transpiration by evergreen species may buffer phenological variability of deciduous species. We examine the respective roles of vegetation and climate on variability in ET in a mature, temperate forest, dominated by deciduous species with an evergreen rhododendron understory at the Coweeta Hydrologic Laboratory in the southern Appalachian Mountains of North Carolina. Since 2012, the site has experienced one year with a warm spring, leading to a two-week advance of leaf-out, and the wettest year in the 80-year climate station record. While eddy covariance-based estimates of net primary productivity was enhanced by earlier spring leaf phenology and depressed during a warm dry period, seasonal patterns in ET were consistent among years, leading to similar annual water vapor fluxes. We combine these data with estimates of soil evaporation from a sub-canopy eddy covariance system, and transpiration from sap flux measurements to quantify the daily and seasonal contributions of canopy, understory, and soil to inter-annual variability in ET.
Description:
Current forest composition may be resilient to typical climatic variability; however, climate trends, combined with projected changes in species composition, may increase tree vulnerability to water stress. A shift in forest composition toward tree species with higher water use has implications for biogenic emissions and deposition of reactive nitrogen and carbon compounds.