Use of MISR/Terra data to study intra- and inter-annual EVI variations in the dry season of tropical forest

Remote sensing studies, based on the Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra Enhanced Vegetation Index (EVI), have shown controversial findings on the dry season and drought sensitivity of tropical forests. Here, we used Multi-angle Imaging SpectroRadiometer (MISR)/Terra data to study view-illumination effects on intra- and inter-annual EVI variability in the dry season of the Brazilian Xingu Indigenous Park between 2005 and 2010. In this period, the Amazon region experienced the two most extensive droughts of the 21st century, but the study area was not severely affected by the 2005 drought when compared to the 2010 episode. Bidirectional reflectance factor (BRF) was analyzed in Seasonal Semideciduous Forest. Intra-annual dry season EVI was evaluated as a function of the reflectance of MISR bands, MISR view zenith angles (VZA), view direction (forward scattering or backscattering), solar zenith angle (SZA), and the shade fraction from linear spectral mixture model. In a subsequent analysis, we used only the MISR nadir camera to eliminate the impacts of view angle and view direction on inter-annual EVI variability. Furthermore, we standardized the EVI monthly and quarterly using 2005 and 2010 as references. Results showed that view angle, view direction and solar illumination introduced significant intra- and inter-annual EVI variability not directly associated with canopy photosynthetic activity. Because of the stronger near-infrared (NIR) reflectance dependence, EVI was much more sensitive to view angle and view direction than the Normalized Difference Vegetation Index (NDVI), especially between ± 45.6° and nadir. EVI increased with view angle from forward scattering to backscattering. At MISR nadir viewing, EVI increased from the beginning to the end of the dry season due to increasing NIR reflectance resulting from a decreasing SZA from 43° (June) to 28° (September) and from a reduction in shade fraction. Monthly and quarterly inter-annual analysis using the MISR nadir camera showed the absence of well-defined patterns between the EVI, canopy greenness and droughts. Nadir EVI curves had similar patterns between the years when considering the standard deviation values. EVI differences across months were random and below the average standard deviation calculated between June and September of 2005 or 2010 (± 0.03). Due to the uncertainties, the quarterly standardized EVI mean was higher for 2005 (+ 0.349 ± 0.839) than for 2010 (+ 0.041 ± 0.754), but the opposite was observed when the quarter for EVI standardization was moved from July–August–September (JAS) to June–July–August (JJA). Our MISR results reinforce the strong need to take into account view-illumination effects when analyzing time series of MODIS EVI.

► View angle, view direction and solar illumination affect dry season EVI variability.
► EVI (NIR reflectance dependence) was more sensitive to angular effects than NDVI.
► Intra-annual dry season MISR nadir EVI was influenced by solar illumination.
► Inter-annual dry season MISR nadir EVI variation had similar magnitude (2005–2010).