Comparison of SIF, MODIS GPP, and NDVI measurements over western United States grasslands

Current methods for measuring gross primary production (GPP) rely on models that make assumptions about how several physiological, climatic, and environmental interactions affect vegetation productivity. Observation-based approaches are limited, as data collected at eddy-covariance flux towers covers a small spatial footprint and is not representative globally. Solar-Induced Chlorophyll Florescence (SIF), an energy flux emitted from plants as a byproduct of photosynthesis, is a promising approach for a data-driven GPP measure. However, further characterization of the relationship between SIF and GPP across different biomes is necessary. In this analysis, we examine this relationship through comparison of total aboveground grassland plant production (ANPP) and seasonally integrated values of MODIS Normalized Difference Vegetation Index (NDVI), MODIS GPP, and GOME-2 SIF in ten grassland study areas across the western United States from 2007 to 2017. SIF has also been shown to more precisely capture the physiological cycle of a growing season than vegetation index or leaf area index thresholds, which more closely relate to plant structure than photosynthetic activity. Therefore, we use SIF to define the period over which all values are integrated. The first objective is to evaluate the utility of SIF for defining the start and end of the seasonal cycle in each of the ten study areas. Second, we compare the seasonal productivity estimates among the several commonly used datasets and assess the degree of similarity between them. Our preliminary results suggest that seasonally integrated NDVI and SIF are closely related to GPP across this region.