Separation of net ecosystem exchange into assimilation and respiration using a light response curve approach: critical issues and global evaluation

• Published in: Global change biology Vol. 16; no. 1; pp. 187 - 208
• Main Authors: LASSLOP, GITTA, REICHSTEIN, MARKUS, PAPALE, DARIO, RICHARDSON, ANDREW D, ARNETH, ALMUT, BARR, ALAN, STOY, PAUL, WOHLFAHRT, GEORG
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford, UK : Blackwell Publishing Ltd 2010; Blackwell Publishing Ltd; Wiley-Blackwell
• Subjects: Algorithms; Animal and plant ecology; Animal, plant and microbial ecology; Atmosphere; Biological and medical sciences; Carbon dioxide; Earth and Related Environmental Sciences; ecosystem respiration; Ecosystems; eddy covariance; Environmental changes; flux partitioning; FLUXNET; Fundamental and applied biological sciences. Psychology; General aspects; Geovetenskap och miljövetenskap; Geovetenskap och relaterad miljövetenskap; GPP; hyperbolic light; hyperbolic light response curve; Natural Sciences; Naturgeografi; Naturvetenskap; net ecosystem exchange; Photosynthesis; Physical Geography; Primary production; primary productivity; R-eco; Reco; Respiration; response curve; Synecology; temperature; uncertainty; Vapor pressure; Assimilation; GPP; Environmental factor; hyperbolic light response curve; uncertainty; FLUXNET; Exchange; R; Eddy covariance; Ecosystem; Light; flux partitioning; Respiration
• ISSN: 1354-1013; 1365-2486; 1365-2486
• DOI: 10.1111/j.1365-2486.2009.02041.x

The measured net ecosystem exchange (NEE) of CO₂ between the ecosystem and the atmosphere reflects the balance between gross CO₂ assimilation [gross primary production (GPP)] and ecosystem respiration (Reco). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light-response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of Reco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher Reco. Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 g C m⁻² yr⁻¹), while errors arising from the random error (median absolute deviation: ~2 g C m⁻² yr⁻¹) are negligible. Despite site-specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and Reco is not spurious but holds true when quasi-independent, i.e. daytime and nighttime based estimates are compared.