Response of Forest Phenology, Evapotranspiration, and Net Ecosystem Exchange to Climatic Drivers in a Southern Appalachian Forest

Abstract:

The southern Appalachian region is experiencing increasing air temperature, CO₂, and precipitation variability, but the magnitude and direction of the forest carbon and water cycling responses are unknown. Examining the interannual variability of these environmental drivers can offer insight into the mechanisms that are likely to dominate the longer-term responses. Higher temperatures in these mature, energy-limited, deciduous forests can lengthen the growing season, leading to higher gross primary productivity; however, these gains may be offset by increased respiration rates. Longer growing seasons may also increase evapotranspiration, particularly if warmer temperatures also increase vapor pressure deficit. On the other hand, evapotranspiration may decrease if more variable precipitation patterns lead to more frequent water-limited and energy-limited conditions. We examine the effects of climate on phenology and fluxes of water and carbon using several years of data from the Coweeta Hydrologic Laboratory. In an attempt to constrain and identify sources of these fluxes, we combine data from a vertical profile eddy covariance system with a subcanopy eddy covariance system and soil CO₂ efflux measurements. Analysis of temporal variability in fluxes is examined at daily to interannual scales. Higher air temperature in February and March of 2012 corresponded with leaf expansion occurring approximately two-weeks earlier than in 2013 and 2014. Overall, seasonal variability in climatic drivers appears to have a greater effect on net carbon exchange than on evapotranspiration.