By Benjamin Brede (Germany)
The Amazon rainforests represent the largest connected forested area in the tropics and play an integral role in the global carbon cycle. In the last years the discussion about their phenology and response to drought has become intense. A recent study argues that greenness seasonality expressed as Enhanced Vegetation Index (EVI) is an artifact of variations in sun-sensor geometry throughout the year. In this study I aimed to reproduce these results with the Moderate-Resolution Imaging Spectroradiometer (MODIS) MCD43 product suite. With this data set modelling of reflectances at fixed sun and viewing geometry was possible. The derived EVI was spatially aggregated over large areas of central Amazon forests. The resulting time series of EVI contained distinct seasonal patterns, but was also correlated with the sun geometry expressed as Solar Zenith Angle (SZA). Second, I conducted a validation approach to test the performance of EVI for predicting canopy carbon uptake and structural changes. This involved field data from eddy covariance towers and forest phenology plots. I used the Vegetation Photosynthesis Model (VPM) – a carbon uptake model – to translate EVI into estimates for Gross Primary Productivity (GPP). Besides, I directly compared EVI dynamics with canopy leaf flush time series. In both cases EVI showed low predictive power for the ground processes, but was again correlated with SZA. Third, I assessed EVI’s sensitivity to drought. For that I analysed the relationship between Bidirectional Reflectance Distribution Function (BRDF)-adjusted EVI dry season anomalies and two drought indices. This analysis covered whole Amazonia and data from the years 2000 to 2013. The drought indices were derived from the Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis (TMPA) 3B42 V7 precipitation product. The results contained no general, meaningful connection between EVI anomalies and drought indices. Despite shortcomings in the individual assessment methods, the correlation of
the BRDF-adjusted EVI with the SZA is suspicious. It enforces the question if the MCD43 product can properly account for variations in Sun-Sensor Geometry Effectss (SSGEs). These results question the reliability of EVI in assessments of tropical forest canopy processes. Based on the presented results, an evaluation of the EVI’s strength for ecological applications seems reasonable. This should incorporate rigorous validation supported by ground plots.
Keywords: Amazon rainforests; Moderate-Resolution Imaging Spectroradiometer; Bidirectional Reflectance Distribution Function; Enhanced Vegetation Index; dry season leaf flush; drought.