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

Kai Morgenstern, T. Andrew Black, Elyn R. Humphreys, Timothy J. Griffis, Gordon B. Drewitt, Tiebo Cai, Zoran Nesic, David L. Spittlehouse, Nigel J. Livingston

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The annual net ecosystem productivity (FNEP) 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 FNEP 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 FNEP 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 FNEP 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 FNEP (270 g C m -2 per year) of the 4 years. For 1999, a cool and cloudy La Niña year, FNEP 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.

Original languageEnglish (US)
Pages (from-to)201-219
Number of pages19
JournalAgricultural and Forest Meteorology
Issue number3-4
StatePublished - Jun 20 2004

Bibliographical note

Funding Information:
This research was initially funded by Forest Renewal, BC and continued by Operating and Strategic grants from the Natural Science and Engineering Research Council of Canada. Significant contributions from Fluxnet-Canada and the Canadian Foundation for Innovations are also appreciated. Doug Worthy, Meteorological Service of Canada, provided valuable assistance with calibration gases. Bill Grutzmacher, Steve Lackey, and Al Aalgaard of TimberWest Forest Corp, and Bill Beese of Weyerhaeuser Canada made the operation of our research site on private land possible. Altaf Arain, John Warland, Mike Novak, and David Gaumont-Guay contributed many helpful insights during discussions. Gilbert Ethier, Eva Jork, Scott Krayenhoff, Isla Myers-Smith, Leslie Dampier, and Paul Jassal made many helpful contributions through their work at the site. Last but by no means least the technical assistance provided by Dwaine Young, Andrew Sauter, and Rick Ketler was crucial in keeping this long-term monitoring project going.


  • Carbon balance
  • Douglas-fir
  • Ecosystem photosynthesis
  • Ecosystem respiration
  • Eddy covariance
  • Error analysis
  • Net ecosystem productivity


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