Abstract
Abstract. Solar-induced chlorophyll fluorescence (SIF) has been shown to be a powerful proxy for photosynthesis and gross primary productivity (GPP). The recently launched TROPOspheric Monitoring Instrument (TROPOMI) features the required spectral resolution and signal-to-noise ratio to retrieve SIF from space. Here, we present a downscaling method to obtain 500 m spatial resolution SIF over California. We report daily values based on a 14 d window. TROPOMI SIF data show a strong correspondence with daily GPP estimates at AmeriFlux sites across multiple ecosystems in California. We find a linear relationship between SIF and GPP that is largely invariant across ecosystems with an intercept that is not significantly different from zero. Measurements of SIF from TROPOMI agree with MODerate Resolution Imaging Spectroradiometer (MODIS) vegetation indices – the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and near-infrared reflectance of vegetation index (NIRv) – at annual timescales but indicate different temporal dynamics at monthly and daily timescales. TROPOMI SIF data show a double peak in the seasonality of photosynthesis, a feature that is not present in the MODIS vegetation indices. The different seasonality in the vegetation indices may be due to a clear-sky bias in the vegetation indices, whereas previous work has shown SIF to have a low sensitivity to clouds and to detect the downregulation of photosynthesis even when plants appear green. We further decompose the spatiotemporal patterns in the SIF data based on land cover. The double peak in the seasonality of California's photosynthesis is due to two processes that are out of phase: grasses, chaparral, and oak savanna ecosystems show an April maximum, while evergreen forests peak in June. An empirical orthogonal function (EOF) analysis corroborates the phase offset and spatial patterns driving the double peak. The EOF analysis further indicates that two spatiotemporal patterns explain 84 % of the variability in the SIF data. Results shown here are promising for obtaining global GPP at sub-kilometer spatial scales and identifying the processes driving carbon uptake.
Highlights
Photosynthesis is the process by which plants and other organisms use sunlight to synthesize carbon dioxide (CO2) and water to glucose and oxygen
We find the California grasslands and pastures have a single peak that coincides with the first statewide peak in April; this is consistent with the seasonal cycle at California grassland sites in the AmeriFlux network (Fig. 5) that show a unimodal peak in the spring that ends in May
The blue region will contribute a negative anomaly in April and a positive anomaly in June. This empirical orthogonal function (EOF) arises because the grasslands and forests are both spatially separated and out of phase with each other, allowing the matrix factorization to place them into a single EOF that represents the processes driving the double peak in the timing of California’s photosynthesis
Summary
Photosynthesis is the process by which plants and other organisms use sunlight to synthesize carbon dioxide (CO2) and water to glucose and oxygen. Walther et al (2016) showed a decoupling of the photosynthesis and greenness dynamics in boreal evergreen forests by comparing SIF and EVI to model estimates of GPP, with SIF better capturing the seasonality of both deciduous broadleaf and evergreen needleleaf forests This is likely due to SIF capturing photosynthetic activity rather than photosynthetic capacity. This is the highest resolution SIF dataset from satellite measurements We compare this downscaled 500 m SIF data to AmeriFlux sites across the state of California to assess the relationship between SIF and GPP. We focus on California because there are a number of eddy flux towers and it encompasses a range of diverse ecosystems including deciduous and evergreen forests, irrigated croplands, and grasslands (see Fig. 1)
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