Climate and Vegetation Effects on Temperate Mountain Forest Evapotranspiration

Abstract:

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 affecting ET involve physical and biological factors. Atmospheric conditions that promote high ET, consisting of high radiation and vapor pressure deficit (D), often occur during rainless periods when soil water supply may limit vegetation water use. In contrast, high soil water availability often coincides with frequent precipitation and low D. In mixed species forests, physiological differences in water use strategies (e.g. isohydric/anisohydric species), leaf habit (e.g., evergreen/deciduous species), and leaf phenology can produce conservative water use throughout wet and dry phases of the growing season and the year. Thus, the combination of these factors may explain some of the observed consistency in ET.

We examine the respective roles of climate and vegetation 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 one year with the greatest amount of precipitation in the 80-year climate station record. While eddy covariance-based estimates of net primary productivity increased with earlier spring leaf phenology and decreased during a warm dry period, seasonal patterns in ET were consistent among years, leading to similar annual water vapor fluxes (coefficient of variation = 0.05). 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. These results will improve our ability to predict forest responses to future climatic variability and species composition, and have important implications for managing forest and water resources.