Solar Induced Fluorescence measured from satellite sensors captures mostly structural than physiological responses to environmental stressors

A deep understanding of the responses of terrestrial vegetation to environmental forcing is crucial for understanding the global carbon dynamics, especially under a changing climate. Vegetation responses to stress can manifest first as plant physiological responses, and at later stages through changes in canopy structure. Remote sensing of vegetation has been proven very valuable in providing such understanding. One of the major breakthroughs has been the use of multi and hyper spectral sensors on board satellites that can retrieve Solar Induced Fluorescence (SIF), which is closely linked with plant photosynthesis.

In this study we assess whether (SIF), as observed by instruments on board of satellites, can adequately capture both of those responses. Using 7 different global SIF products, water and CO2 flux data from 120 eddy-covariance towers and climate reanalysis products, we found a good agreement between the 16-day responses of flux tower observed gross primary productivity (GPP) and SIF to soil moisture and light, and a weaker agreement regarding the responses to temperature and atmospheric humidity. Overall, we found that current satellite SIF responses to environmental stressors mostly reflect structural changes in vegetation structure, and that satellite SIF has limited skill in capturing early stress plant physiological responses except for the light availability response at higher latitudes. While satellite SIF significantly outperforms more traditional vegetation indices (EVI, NDVI), as it does not saturate unlike the latter ones, it does not provide major additional information regarding vegetation physiological responses to either hydrological or atmospheric droughts.