Zusammenfassung
The relationship between vegetation optical depth (VOD) retrieved by L-band SMOS radiometer and forest above
ground biomass (AGB) was investigated in tropical areas of Africa and South America. VOD was retrieved from
the latest version of level 2 SMOS algorithm, while reference AGB was obtained from a pantropical database,
encompassing a large number of ground plot data derived from field surveys conducted on both continents. In
Africa and South-America, VOD increased with AGB, reaching saturation at about 350 Mg ha−1. The strength of
the relation was improved selecting VOD data in appropriate seasons, characterized by a higher dynamic range
of values. The capability of VOD data to estimate AGB was further investigated using Random Forest decision
trees, adding to VOD selected climate variables from the Climatic Research Unit (temperature, potential evapotranspiration,
and precipitation) and water deficit data, and validating regression tests with ground data from
the reference AGB database. The results for the five analyzed years indicate that the best estimates of AGB are
obtained by the joined use of VOD and potential evapotranspiration input data, but all climate variables brought
an improvement in AGB estimates. AGB estimates were relatively stable for the considered period, with limited
variations possibly due to changes in biomass and to data quality of VOD and of climate variables. The VOD
signal and estimated AGB were also analyzed according to ecological homogeneous units (ecoregions), evidencing
data clusters, partially overlapped to each other, in the VOD - AGB plane.
