Abstract
Satellite-derived vegetation indices (VIs) have been widely used to approximate or estimate gross primary productivity (GPP). However, it remains unclear how the VI-GPP relationship varies with indices, biomes, timescales, and the bidirectional reflectance distribution function (BRDF) effect. We examined the relationship between VIs and GPP for 121 FLUXNET sites across the globe and assessed how the VI-GPP relationship varied among a variety of biomes at both monthly and annual timescales. We used three widely-used VIs: normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and 2-band EVI (EVI2) as well as a new VI - NIRV and used surface reflectance both with and without BRDF correction from the moderate resolution imaging spectroradiometer (MODIS) to calculate these indices. The resulting traditional (NDVI, EVI, EVI2, and NIRV) and BRDF-corrected (NDVIBRDF, EVIBRDF, EVI2BRDF, and NIRV, BRDF) VIs were used to examine the VI-GPP relationship. At the monthly scale, all VIs were moderate or strong predictors of GPP, and the BRDF correction improved their performance. EVI2BRDF and NIRV, BRDF had similar performance in capturing the variations in tower GPP as did the MODIS GPP product. The VIs explained lower variance in tower GPP at the annual scale than at the monthly scale. The BRDF-correction of surface reflectance did not improve the VI-GPP relationship at the annual scale. The VIs had similar capability in capturing the interannual variability in tower GPP as MODIS GPP. VIs were influenced by temperature and water stresses and were more sensitive to temperature stress than to water stress. VIs in combination with environmental factors could improve the prediction of GPP than VIs alone. Our findings can help us better understand how the VI-GPP relationship varies among indices, biomes, and timescales and how the BRDF effect influences the VI-GPP relationship.
Highlights
Terrestrial gross primary productivity (GPP), the amount of carbon absorbed by terrestrial plants through photosynthesis, is the largest carbon flux between the terrestrial biosphere and the atmosphere
To evaluate the usefulness of the vegetation indices (VIs) for estimating GPP, we examined the relationship between moderate resolution imaging spectroradiometer (MODIS) GPP and tower GPP for each flux site
EVIBRDF, EVI2BRDF, and NIRV, bidirectional reflectance distribution function (BRDF) explained the same proportion of the variance in tower GPP as did MODIS GPP (Figure 2), the MODIS GPP is driven by incident photosynthetically active radiation (PAR), the fraction of PAR absorbed by vegetation canopies, temperature stress, and water stress
Summary
Terrestrial gross primary productivity (GPP), the amount of carbon absorbed by terrestrial plants through photosynthesis, is the largest carbon flux between the terrestrial biosphere and the atmosphere. No study has compared traditional VIs (i.e., VIs based on surface-reflectance that is not corrected with BRDF) against BRDF-corrected VIs for estimating GPP for a large number of sites across a wide variety of biomes at both seasonal and annual scales. The recent concurrent availability of FLUXNET and MODIS data for a large number of sites over a 15-year period (2000–2014) and the recent availability of the MODIS ASCII subsets for BRDF-corrected surface reflectance products provide an unprecedented opportunity to examine the relationships between satellite-derived VIs (both traditional and BRDF-corrected VIs) and tower GPP across a wide variety of biomes at various timescales. Our findings can reveal the performance of satellite-derived VIs for approximating GPP and the influence of the BRDF effect and informing future ecosystem functioning, vegetation productivity, and carbon cycle studies based on VIs
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