O green-up da estação seca na Amazônia Central é consistente em três diferentes escalas espaciais

Abstract

Dry season green-up of Central Amazon upland forest, as detected in the Enhanced Vegetation Index (EVI) of the Moderate resolution imaging spectro-radiometer (Modis) has been a subject of recent controversy. Progressive decrease in solar zenith angle during the dry season reduces sub-pixel shade content, causing artefactual increase in NIR reflectance, with consequent increase in EVI. Cloud contamination has also been difficult to detect and to filter given the large Modis pixel. To reduce these artifacts, we used 16y of 1km Modis-MAIAC (Multi-Angle Implementation of Atmospheric Correction) images, corrected to nadir view and 45o solar zenith angles and having an improved cloud filter. We then obtained seasonal 30m Landsat 8 Operational Land Imager (OLI) surface reflectance at two Central Amazon sites, to provide independent corroboration of Modis-MAIAC seasonality for EVI, Near Infra-red (NIR) and Green chromatic coordinate (Gcc). We controlled for sun-sensor geometry effects in OLI by using dates with similar solar elevation angles and by comparing pixels with the same phase angles. For three different dates (Feb, May, Aug) at one site, OLI and Modis-MAIAC showed the same temporal ranking for EVI, for NIR and for Gcc. For two dates (May, Dec) at the second site -- having nearly identical solar elevation angles and near-nadir view angles -- EVI and NIR seasonal changes were also corroborated. We used Central Amazon tower-mounted RGB cameras, running for 4y at the Amazon Tall Tower (ATTO, 2° 8'36"S and 59°0’2"W) and 7y at the Manaus k34 tower (2°36'33"S, 60°12'33"W), to obtain landscape-scale and crownscale Gcc, monthly Leaf Area Index (LAI, based on the fraction of upper canopy crowns that are leafy) and monthly leaf age class abundances by crown (based on time since last flush) in the camera view. These were compared to seasonal patterns of Gcc and EVI in 88 pixels of Modis-MAIAC centered on each tower. Modis-MAIAC Gcc was not always consistent with mean camera-derived Gcc but was positively correlated with the inter-crown variance of camera-derived Gcc (R2 = 0.68 and 0.53 at ATTO and k34, respectively). At both sites, EVI was well correlated with the abundance of mature leaves 2-7 months old (R2=0.82 and 0.80, respectively), but was poorly explained by LAI (R2 = 0.20 and 0.15, respectively), which show that leaf demography and not leaf amount, is the main driver of seasonal EVI and of dry season green-up. After 6-7 months of lower-than-expected rainfall in 2015/16, we saw a small precocious positive flush anomaly in Feb-Mar, then a large negative flush anomaly in Jun-Jul, when a peak of flushing crowns would occur in normal climate years. These two flush anomalies were evident at both tower sites in Modis-MAIAC EVI and Gcc.