Long-term observations of energy, water vapor and CO2 fluxes over a temperate deciduous forest in Tennessee

Long-term flux measurements are a powerful tool to elucidate the factors controlling the ecosystem energy, water and carbon balance and their trends. These datasets are critical for testing and improving the parameterizations in various climate models. In this study, we present over a decadal (2005-2015) record of continuous eddy correlation flux measurements of surface energy, CO₂ and water vapor over the Chestnut Ridge (35.9311N, 84.3323W), NOAA Surface Energy Balance Network (SEBN) flux tower site, located in the national deciduous forest reservation near Oak Ridge in eastern Tennessee, USA. During the study period the mean annual temperature and precipitation (P) at this site were ~15 °C and 1334 mm, respectively. Drastic changes in albedo, vegetation growth and energy fluxes typically occurred following the onset of the growing season in April. Seasonally, all shortwave radiation components peaked during the end of May, while longwave radiation peaked in August. During the early growing season in April-May, sensible heat flux was the largest component of energy balance, whereas latent heat flux dominated during warm and wet periods of the summer. Daily total evapotranspiration (E) rates reached up to 7 mm d^-1 in July-August while leaf area index, calculated using normalized difference vegetation index estimates peaked up to ~6 mm^-2 in May. The mean annual E over the site was 697±150 mm y^-1. The daily mean net ecosystem exchange (NEE) was negative during the growing season, which spanned from April to November, with peak values of >-8 g C m^-2 observed in June-July. The forest was a carbon sink during the entire period with -NEE = 499 ± 206 g C m^-2 y^-1. The earliest onset of the growing season was observed in 2012, the year with the highest March air temperature. Even though 2007 was the warmest year of the record, with the second warmest March, an anomalous late spring freeze event in April 2007, delayed the vegetation recovery resulting in lowest annual E (543 mm y^-1) and CO₂ uptake (-NEE =170 g C m^-2 y^-1) of the record. Annual P, NEE and E showed an increasing trend during the study period. The interannual variations in annual E and NEE were mostly controlled by annual P and growing season length