Influence of Vegetation Height, Plant Area Index, and Forest Intactness on SMOS L-VOD, for Different Seasons and Latitude Ranges

In this study, the spatial correlation between the vegetation optical depth at L-band (L-VOD) captured by the radiometer on-board the Soil Moisture and Ocean Salinity (SMOS) Satellite and the LiDAR products obtained by GEDI and ICESat-2 satellites is investigated at global scale. Suitable techniques, based on spatial and temporal averaging, are used to operate at the different spatial resolutions and to aggregate observations collected between May 2019 and April 2021. Then the correlation is investigated over four latitude ranges showing different peculiarities: in tropical latitudes, SMOS L-VOD is well correlated with the top of canopy height (RH100) recorded by both instruments and the plant area index (PAI) recorded by GEDI for all months of two years; in temperate northern latitudes the spatial correlations significantly decrease in cold months; in temperate southern latitudes, characterized by a high plurality of ecoregions, L-VOD proves to be better correlated with PAI than with RH100. Finally, the influence of forest intactness on L-VOD is also investigated. Intact forests markedly show the highest L-VOD values in tropical latitudes. The association disappears in boreal latitudes because of the extreme climatic factors that limit vegetation growing even in the case of intact landscapes.