Investigating the usefulness of satellite-derived fluorescence data in inferring gross primary productivity within the carbon cycle data assimilation system

E N Koffi,A J Norton,C Frankenberg,M Scholze,P J Rayner

doi:10.5194/bg-12-4067-2015

Abstract

Abstract. Simulations of carbon fluxes with terrestrial biosphere models still exhibit significant uncertainties, in part due to the uncertainty in model parameter values. With the advent of satellite measurements of solar induced chlorophyll fluorescence (SIF), there exists a novel pathway for constraining simulated carbon fluxes and parameter values. We investigate the utility of SIF in constraining gross primary productivity (GPP). As a first test we assess whether SIF simulations are sensitive to important parameters in a biosphere model. SIF measurements at the wavelength of 755 nm are simulated by the Carbon-Cycle Data Assimilation System (CCDAS) which has been augmented by the fluorescence component of the Soil Canopy Observation, Photochemistry and Energy fluxes (SCOPE) model. Idealized sensitivity tests of the SCOPE model stand-alone indicate strong sensitivity of GPP to the carboxylation capacity (Vcmax) and of SIF to the chlorophyll AB content (Cab) and incoming short wave radiation. Low sensitivity is found for SIF to Vcmax, however the relationship is subtle, with increased sensitivity under high radiation conditions and lower Vcmax ranges. CCDAS simulates well the patterns of satellite-measured SIF suggesting the combined model is capable of ingesting the data. CCDAS supports the idealized sensitivity tests of SCOPE, with SIF exhibiting sensitivity to Cab and incoming radiation, both of which are treated as perfectly known in previous CCDAS versions. These results demonstrate the need for careful consideration of Cab and incoming radiation when interpreting SIF and the limitations of utilizing SIF to constrain Vcmax in the present set-up in the CCDAS system.

Highlights

The terrestrial carbon flux has been identified as the most uncertain term in the global carbon budget (Le Quéré et al, 2013)
Under the studied configurations solar induced chlorophyll fluorescence (SIF) increases with Vcmax when the gross primary productivity (GPP) is controlled by the carboxylation enzyme Rubisco, and remains almost constant when photosynthesis is limited by electron transport
The analyses show that a robust linear relationship between SIF and GPP can be inferred for each plant functional types (PFT)

Summary

Introduction

The terrestrial carbon flux has been identified as the most uncertain term in the global carbon budget (Le Quéré et al, 2013). The gross primary productivity (GPP), which is the flux of CO2 assimilated by plants during photosynthesis, is the input to the system used to characterize carbon flux so its variation can significantly contribute to the uncertainties in terrestrial CO2 fluxes. Complex systems have been built to reduce the uncertainties in GPP. These algorithms are either based on up-scaling or atmospheric inverse modelling methods. Up-scaling methods estimate GPP at global scale by establishing relationships between local GPP measurements and environmental variables using these variables to calculate GPP globally (e.g., Jung et al, 2011; Beer et al, 2010 and references therein). The inverse modelling approach uses CO2 concentration observations at a global scale to constrain the pro-

Results

Discussion

Conclusion