COMPARING NOCTURNAL EDDY COVARIANCE MEASUREMENTS TO ESTIMATES OF ECOSYSTEM RESPIRATION MADE BY SCALING CHAMBER MEASUREMENTS AT 6 CONIFEROUSBOREAL SITES

During the growing season, nighttime ecosystem respiration emits 30-10 0% of the daytime net photosynthetic uptake of carbon, and therefore m easurements of rates and understanding of its control by the environme nt are important for understanding net ecosystem exchange. Ecosystem r espiration can be measured at night by eddy covariance methods, but th e data may not be reliable because of low turbulence or other methodol ogical problems. We used relationships between woody tissue, foliage, and soil respiration rates and temperature, with temperature records c ollected on site to estimate ecosystem respiration rates at six conife rous BOREAS sites at half-hour or 1-hour intervals, and then compared these estimates to nocturnal measurements of CO2 exchange by eddy cova riance. Soil surface respiration was the largest source of CO2 at all sites (48-71%), and foliar respiration made a large contribution to ec osystem respiration at all sites (25-43%). Woody tissue respiration co ntributed only 5-15% to ecosystem respiration. We estimated error for the scaled chamber predictions of ecosystem respiration by using the u ncertainty associated with each respiration parameter and respiring bi omass value. There was substantial uncertainty in estimates of foliar and soil respiration because of the spatial variability of specific re spiration rates. In addition, more attention needs to be paid to estim ating foliar respiration during the early part of the growing season, when new foliage is growing, and to determining seasonal trends of soi l surface respiration. Nocturnal eddy covariance measurements were poo rly correlated to scaled chamber estimates of ecosystem respiration (r (2) = 0.06-0.27) and were consistently lower than scaled chamber predi ctions (by 27% on average for the six sites). The bias in eddy covaria nce estimates of ecosystem respiration will alter estimates of gross a ssimilation in the light and of net ecosystem exchange rates over exte nded periods.