Intercomparison of methods to estimate gross primary production based on CO₂ and COS flux measurements

Abstract

Abstract. Separating the components of ecosystem-scale carbon exchange is crucial in order to develop better models and future predictions of the terrestrial carbon cycle. However, there are several uncertainties and unknowns related to current photosynthesis estimates. In this study, we evaluate four different methods for estimating photosynthesis at a boreal forest at the ecosystem scale, of which two are based on carbon dioxide (CO2) flux measurements and two on carbonyl sulfide (COS) flux measurements. The CO2-based methods use traditional flux partitioning and artificial neural networks to separate the net CO2 flux into respiration and photosynthesis. The COS-based methods make use of a unique 5-year COS flux data set and involve two different approaches to determine the leaf-scale relative uptake ratio of COS and CO2 (LRU), of which one (LRUCAP) was developed in this study. LRUCAP was based on a previously tested stomatal optimization theory (CAP), while LRUPAR was based on an empirical relation to measured radiation. For the measurement period 2013–2017, the artificial neural network method gave a GPP estimate very close to that of traditional flux partitioning at all timescales. On average, the COS-based methods gave higher GPP estimates than the CO2-based estimates on daily (23 % and 7 % higher, using LRUPAR and LRUCAP, respectively) and monthly scales (20 % and 3 % higher), as well as a higher cumulative sum over 3 months in all years (on average 25 % and 3 % higher). LRUCAP was higher than LRU estimated from chamber measurements at high radiation, leading to underestimation of midday GPP relative to other GPP methods. In general, however, use of LRUCAP gave closer agreement with CO2-based estimates of GPP than use of LRUPAR. When extended to other sites, LRUCAP may be more robust than LRUPAR because it is based on a physiological model whose parameters can be estimated from simple measurements or obtained from the literature. In contrast, the empirical radiation relation in LRUPAR may be more site-specific. However, this requires further testing at other measurement sites.

Technical note: Novel estimates of the leaf relative uptake rate of carbonyl sulfide from optimality theory

“…PAR availability, together with VPD, the major driver of short-term variability in LRU, typically decreases with canopy depth, and, since LRU is negatively related to PAR (e.g. Kooijmans et al, 2019;Kohonen et al, 2022), canopy-integrated LRU is expected to be larger than leaf-scale LRU at the top of the canopy (Sun et al, 2022). This in turn suggests the difference to the values by Maignan et al (2021) to diminish if their values were expressed at leaf scale and the top-of-the-canopy environmental conditions.…”

“…While the magnitude and drivers of g s i are poorly understood, g s s and the C c i /C c a ratio are well known to vary over short timescales in response to diel changes in environmental drivers, as well as along large-scale bioclimatic gradients (Lloyd and Farquhar, 1994). With regard to the former, recent work by Kohonen et al (2022) and Sun et al (2022) demonstrated that contrasting leaf gas exchange theories are able to reproduce and explain the observed short-term response of LRU to key drivers such as incident PAR or the vapour pressure deficit (VPD).…”

Diurnal variability of atmospheric O2, CO2, and their exchange ratio above a boreal forest in southern Finland

“…(1) when NEE EC measurements are available, GPP is calculated as the difference between the NEE EC measurements and the respiration flux, which is calculated using a temperature function; or (2) when NEE EC measurements are not available, GPP is calculated using an equation that is based on the air temperature and light (photosynthetically active radiation, PAR). A more detailed description of these calculations is given by Kulmala et al (2019) and Kohonen et al (2022). By estimating ER r and ER a of this boreal forest, we created the opportunity to apply atmospheric O 2 measurements to separate NEE into GPP and TER (the O 2 method).…”

Carbon and Water Fluxes of the Boreal Evergreen Needleleaf Forest Biome Constrained by Assimilating Ecosystem Carbonyl Sulfide Flux Observations

“…The eddy covariance COS flux data from Hyytiälä used for ecosystem COS flux assimilation and evaluation in this study are available in Kohonen, Tramontana, and Kolari (2022) via https://doi.org/10.5281/zenodo.6940750. The soil COS flux from chamber measurements at Hyytiälä used for assimilation are published in Sun et al.…”