A Global, 0.05-Degree Product of Solar-Induced Chlorophyll Fluorescence Derived from OCO-2, MODIS, and Reanalysis Data

Xing Li,Jingfeng Xiao

doi:10.3390/rs11050517

Abstract

Solar-induced chlorophyll fluorescence (SIF) brings major advancements in measuring terrestrial photosynthesis. Several recent studies have evaluated the potential of SIF retrievals from the Orbiting Carbon Observatory-2 (OCO-2) in estimating gross primary productivity (GPP) based on GPP data from eddy covariance (EC) flux towers. However, the spatially and temporally sparse nature of OCO-2 data makes it challenging to use these data for many applications from the ecosystem to the global scale. Here, we developed a new global ‘OCO-2’ SIF data set (GOSIF) with high spatial and temporal resolutions (i.e., 0.05°, 8-day) over the period 2000–2017 based on a data-driven approach. The predictive SIF model was developed based on discrete OCO-2 SIF soundings, remote sensing data from the Moderate Resolution Imaging Spectroradiometer (MODIS), and meteorological reanalysis data. Our model performed well in estimating SIF (R2 = 0.79, root mean squared error (RMSE) = 0.07 W m−2 μm−1 sr−1). The model was then used to estimate SIF for each 0.05° × 0.05° grid cell and each 8-day interval for the study period. The resulting GOSIF product has reasonable seasonal cycles, and captures the similar seasonality as both the coarse-resolution OCO-2 SIF (1°), directly aggregated from the discrete OCO-2 soundings, and tower-based GPP. Our SIF estimates are highly correlated with GPP from 91 EC flux sites (R2 = 0.73, p < 0.001). They capture the expected spatial and temporal patterns and also have remarkable ability to highlight the crop areas with the highest daily productivity across the globe. Our product also allows us to examine the long-term trends in SIF globally. Compared with the coarse-resolution SIF that was directly aggregated from OCO-2 soundings, GOSIF has finer spatial resolution, globally continuous coverage, and a much longer record. Our GOSIF product is valuable for assessing terrestrial photosynthesis and ecosystem function, and benchmarking terrestrial biosphere and Earth system models.

Highlights

Plant photosynthesis provides a wealth of ecosystem services that are essential for most life on Earth
Our analysis indicated that the model based on enhanced vegetation index (EVI), photosynthetically active radiation (PAR), vapor pressure deficit (VPD), and air temperature had a slightly higher performance than the model based on surface reflectance of the seven Moderate Resolution Imaging Spectroradiometer (MODIS) bands
The much smaller footprint of Orbiting Carbon Observatory-2 (OCO-2) provides the first opportunity to directly measure the ecosystem-scale photosynthesis at the ecosystem level, while its sparse global coverage imposes a strong limitation on the use of the OCO-2 solar-induced chlorophyll fluorescence (SIF)

Summary

Introduction

Plant photosynthesis provides a wealth of ecosystem services that are essential for most life on Earth. Satellite-based production efficiency models [3,4,5,6], which combine satellite measurements of vegetation and meteorological variables through a modeling approach; (2) process-based models [7,8,9,10], which simulate photosynthesis based on physiological and biogeochemical processes; and (3) data-driven models [11,12,13,14], which upscale the GPP estimates from eddy covariance (EC) towers combining machine learning techniques and a suite of gridded ecosystem variables. A newly emerging satellite retrieval, solar-induced chlorophyll fluorescence (SIF), has opened a new perspective to directly measure plant photosynthetic activity from space globally [17,18,19,20]

Methods

Results

Discussion

Conclusion