Is Near Infrared Radiation Reflected from Vegetation a Better Proxy for Ecosystem Photosynthesis than solar-induced fluorescence?; A Comprehensive Test Across a Meso-scale Flux Network that Varies in Photosynthetic Capacity, Structure and Function.

There is much interest in measuring solar-induced fluorescence (SIF) and using it as a proxy to infer canopy photosynthesis. Yet, new papers are suggesting that near infrared radiation (NIR) reflected by canopies may be a better proxy. We are measuring carbon dioxide fluxes across a network of sites in California that differ in photosynthetic pathways (C4, corn; C3, alfalfa, grasslands and wetlands), that differ in canopy structure (repeated cutting of alfalfa; mix of dead and green vegetation of wetlands) and physiological capacity (native oaks and grasslands subjected to variation in soil moisture). At each of these sites we are measuring reflected red and NIR with light emitting diode sensors and canopy photosynthesis with eddy covariance. We base our analysis on one to ten years of data at the respective sites.

We have compared our carbon flux measurements with the amount of NIR reflected by the vegetation. We are observing good agreement between this index and carbon fluxes across all sites and their structural and functional conditions. This behavior can be explained with computations from the CANVEG model and seasonal measurements of hyperspectral reflectance.

NIR reflected by the vegetation is easier to measure, has a stronger signal and thereby has great potential for upscaling carbon fluxes across a landscape than solar induced fluorescence.