Sensitivity and uncertainty of the carbon balance of a Pacific Northwest Douglas-fir forest during an El Niño/La Niña cycle

The annual net ecosystem productivity (F_NEP) of a second-growth Douglas-fir stand established in 1949 on the Canadian West Coast varied considerably over the 4-year period between 1998 and 2001. This period included the El Niño/La Niña cycle during the northern hemispheric winters of 1997/1998 and 1998/1999, offering a unique opportunity to study how a typical forest ecosystem in the Pacific Northwest reacts to interannual climate variability. This was possible even though annual F_NEP values calculated from eddy covariance (EC) measurements of CO₂ fluxes were subject to biases. These were largely due to the failure of the EC method to accurately measure losses of CO₂ under low turbulence conditions at night, which caused F_NEP overestimates of as much as 90 g C m ^-2 per year. As these biases were largely unaffected by interannual climate variability, it was possible to reliably quantify interannual differences in F_NEP estimates if they were larger than random variability, which was estimated to be ±30 g C m^-2 per year at most. Interannual differences were mainly due to differences in ecosystem respiration (R) between the 4 years. In the year following the 1997/1998 El Niño, high air temperatures led to the highest annual R of the 4 years, while annual gross ecosystem photosynthesis (P) was only slightly higher than normal. This resulted in 1998 having the lowest F_NEP (270 g C m ^-2 per year) of the 4 years. For 1999, a cool and cloudy La Niña year, F_NEP was 360 g C m^-2 per year, much higher than 1998, but somewhat lower than the last 2 years, for which F _NEP values were 390 and 420 g C m^-2 per year, respectively.