Effect of environmental conditions on the relationship between solar‐induced fluorescence and gross primary productivity at an OzFlux grassland site

Abstract

Recent studies have utilized coarse spatial and temporal resolution remotely sensed solar‐induced fluorescence (SIF) for modeling terrestrial gross primary productivity (GPP) at regional scales. Although these studies have demonstrated the potential of SIF, there have been concerns about the ecophysiological basis of the relationship between SIF and GPP in different environmental conditions. Launched in 2014, the Orbiting Carbon Observatory‐2 (OCO‐2) has enabled fine‐scale (1.3 by 2.5 km) retrievals of SIF that are comparable with measurements recorded at eddy covariance towers. In this study, we examine the effect of environmental conditions on the relationship of OCO‐2 SIF with tower GPP over the course of a growing season at a well‐characterized natural grassland site. Combining OCO‐2 SIF and eddy covariance tower data with a canopy radiative transfer and an ecosystem model, we also assess the potential of OCO‐2 SIF to constrain the estimates of Vcmax, one of the most important parameters in ecosystem models. Based on the results, we suggest that although environmental conditions play a role in determining the nature of relationship between SIF and GPP, overall, the linear relationship is more robust at ecosystem scale than the theory based on leaf‐level processes might suggest. Our study also shows that the ability of SIF to constrain Vcmax is weak at the selected site.

Sun‐Induced Chlorophyll Fluorescence, Photosynthesis, and Light Use Efficiency of a Soybean Field from Seasonally Continuous Measurements

“…This weak positive LUE:SIF y relationship was also found in the long‐term SIF measurements conducted at the Harvard Forest site (Yang et al, ). In contrast, though Verma et al () also reported a positive LUE:SIF y relationship in an Australian C4 grassland, they found that both LUE and SIF y were positively correlated with APAR. Under sunny conditions, our study observed negative LUE:APAR and positive SIF y :APAR relationships (Figure ), resulting in a negative LUE:SIF y relationship, contrary to both prior studies (Verma et al, ; Yang et al, ).…”

“…In contrast, though Verma et al () also reported a positive LUE:SIF y relationship in an Australian C4 grassland, they found that both LUE and SIF y were positively correlated with APAR. Under sunny conditions, our study observed negative LUE:APAR and positive SIF y :APAR relationships (Figure ), resulting in a negative LUE:SIF y relationship, contrary to both prior studies (Verma et al, ; Yang et al, ). These patterns derived from observations at the canopy scale or larger also differed from previous modeled results.…”

“…At the diurnal scale, Damm et al () also observed negative LUE:SIF y relationship at corn fields, which is consistent with our finding at the diurnal scale. At the seasonal scale, we compared our results with previous results reported for temperate forests (Yang et al, ) and a C4 grassland (Verma et al, ). Under cloudy conditions, our soybean field showed a weak positive LUE:SIF y relationship (derived from the relationship that both LUE and SIF y were negatively correlated with APAR, Figure ).…”

“…Previous studies have shown a quasi‐linear GPP:SIF relationship in different ecosystems observed from diverse sources and across multiple spatial scales (Damm et al, ; Frankenberg, Fisher, et al, ; Guanter et al, ; Perez‐Priego et al, ; Verma et al, ; Wood et al, ; Yang et al, ). Multiple applications have taken this empirical relationship forward, for example, using SIF to infer GPP directly, constrain the estimate of photosynthesis capacity, or derive electron transport rate proportionally (Guan et al, ; Guanter et al, ; Perez‐Priego et al, ; Zhang et al, ), but little justification has been made for the underlying processes associated with this relationship.…”

Assessment of SMAP soil moisture for global simulation of gross primary production

“…It suggests that SIF can be a powerful tool to track photosynthetic rates on the canopy and ecosystem scales [ Liming He et al , ]. Although the slope between GPP and SIF is higher for C4 crops than for C3 vegetation [ Liu et al , ; Wood et al , ] and environmental conditions also affect the relationship [Verma et al, ], the correlations between SIF and GPP appear to be robust.…”