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Evaluating GPP and respiration estimates over northern mid-latitude ecosystems using solar induced fluorescence and atmospheric CO2 measurements

The terrestrial biosphere is currently a major sink of carbon dioxide (CO2), taking up roughly one quarter of anthropogenic emissions. The relative importance of different processes driving uptake by the terrestrial biosphere are poorly understood, and thus, the future of the land sink is highly uncertain. To refine our understanding of the processes driving uptake, better constraints on gross primary productivity (GPP) and ecosystem respiration (Re) are required. In this study, we employ Global Ozone Monitoring Experiment-2 (GOME-2) solar induced fluorescence (SIF) and total column CO2 (XCO2) from the Total Carbon Column Observing Network (TCCON) to evaluate GPP, net ecosystem exchange (NEE) fluxes produced by four terrestrial ecosystem models and FLUXCOM over northern mid-latitude ecosystems. We also investigate the prospects of constraining Re by combining constraints on GPP and NEE. We calculate an “optimized” Re by assuming that the NEE and GPP fluxes which show close agreement with SIF and TCCON are “true”. We find the inferred Re has a broader summer peak from early June through mid August than Re fluxes produced by FLUXCOM and ecosystem models. However, we also show that the precise shape of the optimized Re is sensitive to the magnitude of GPP, which is not well constrained. We discuss the limitations for constraining GPP and Re on large scales with the current constraints and what is needed to move forward.