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

• Published in: Agricultural and forest meteorology Vol. 123; no. 3; pp. 201 - 219
• Main Authors: Morgenstern, Kai, Andrew Black, T., Humphreys, Elyn R., Griffis, Timothy J., Drewitt, Gordon B., Cai, Tiebo, Nesic, Zoran, Spittlehouse, David L., Livingston, Nigel J.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 20.06.2004; Oxford Elsevier; New York, NY
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agronomy. Soil science and plant productions; Biological and medical sciences; Carbon balance; Douglas-fir; Ecosystem photosynthesis; Ecosystem respiration; Eddy covariance; Error analysis; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Net ecosystem productivity; Ecosystem photosynthesis; Eddy covariance; Error analysis; Ecosystem respiration; Net ecosystem productivity; Douglas-fir; Carbon balance; Biometeorology; Productivity; Pseudotsuga menziesii; Ecosystem; Softwood forest tree; Gymnospermae; Coniferales; Spermatophyta; Photosynthesis; Respiration
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2003.12.003

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 CO 2 fluxes were subject to biases. These were largely due to the failure of the EC method to accurately measure losses of CO 2 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.