Paving the road to FLEX and Biomass: a multi-frequency study of the vegetation in three regions of Europe

Abstract

To understand the vegetation status and evolution, it is necessary to use a multi-frequency Earth Observation approach. Visible/near-infrared indices are sensitive to green components of the vegetation as they are related to the photosynthetically active parts of the vegetation from widely used indices such as NDVI (Normalized Difference Vegetation Index), to FAPAR or the more innovative solar-induced fluorescence (SIF). FAPAR quantifies the fraction of the solar radiation absorbed by living leaves for the photosynthetic activity while SIF represents an emission of energy emanating from chlorophyll molecules that is a function of the activity of competing pathways for de-excitation (photochemistry versus nonphotochemical quenching). These indices can be used as proxy for the Gross Primary Production (GPP) but they saturate quickly even for moderate biomass values ( < 80 Mg/ha).

On the other hand, active microwave (MW) observations allow to map the Above Ground Biomass (AGB) distribution using the backscattered signal of Synthetic Aperture Radars (Bouvet et al. 2018). Active and passive MW observations also give access to the hydric state of the vegetation via the optical depth created by the water molecules contained in the plants/trees (hereafter VOD, Vegetation Optical Depth). Depending on the frequency, MW radiation is sensitive to the water content in distinct parts of the vegetation, from the leaves, to the branches and the trunk (with small water-containing elements being transparent for long wavelength radiation). Liu et al. (2015) showed that passive MW VOD with frequencies higher than 6 GHz can also be used to estimate AGB while Rodriguez-Fernandez et al. (2018) showed that passive L-band (1.4 GHz) VOD (L-VOD) is even more sensitive to AGB, without significant signs of saturation for high AGB values (300 – 400 Mg/h).

In this contribution we will discuss a panchromatic analysis of the vegetation using: • Passive MW VOD at different frequency bands: SMOS L-VOD (Kerr et al. 2012), AMSR-2 C-VOD, X-VOD (van der Schalie et al. 2017) • Active MW VOD at C-Band: ASCAT VOD (Vreugdenhil et al. 2016) • Sentinel 3 FAPAR (Reyes Muñoz et al., this symposium) • Sentinel 5P SIF (Guanter et al. 2021) • ESA Climate Change Initiative AGB maps (Cartus et al., this symposium)

Three 500 x 500 km2 regions in the Iberian Peninsula, Northern Finland and Central Europe were studied using time series in the period from 2010 to 2021.

The VOD data from different sensors were compared showing a significant complementarity. Bias and temporal correlation maps of ASCAT and AMSR-2 VOD with respect to SMOS L-VOD show regions with positive values and regions with negative values. These maps were compared to land cover maps but no clear relationship was found. The temporal correlation of SIF and FAPAR was found to vary between the focus areas with values from R=0.59 in Northern Finland, to 0.71 in the Iberian Peninsula and 0.81 in Central Europe. FAPAR and SIF seasonal cycles are similar but FAPAR increases in early spring and SIF only in late May or June. A comparison between microwave and visible data was performed by comparing SMOS VOD and Sentinel 5P SIF. Temporal correlation maps show regions with negative values, in particular in the Iberian Peninsula.

This study is part of the ESA Land Carbon Constellation (LCC, Scholze et al., this symposium), whose goal is to constrain Carbon-cycle models using the assimilation of multi-frequency vegetation-related data (Kaminski et al. this symposium). The LCC project helps to pave the road to studies that will be done in the future with dedicated SIF observations by the ESA FLEX mission (Moreno, 2021) and high resolution AGB maps from the ESA Biomass mission (Le Toan et al. 2011).