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UA ECOL 596L - Amazon forests did not green‐up during the 2005 drought

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ClickHereforFullArticl eAmazon forests did not green‐up during the 2005 droughtArindam Samanta,1Sangram Ganguly,2Hirofumi Hashimoto,3Sadashiva Devadiga,4Eric Vermote,5Yuri Knyazikhin,1Ramakrishna R. Nemani,6and Ranga B. Myneni1Received 11 December 2009; accepted 26 January 2010; published 5 March 2010.[1] The sensitivity of Amazon rainforests to dry‐seasondroughts is still poorly understood, with reports ofenhanced tree mortality and forest fires on one hand, andexcessive forest greening on the other. Here, we repor tthat the previous results of large‐scale greening of theAmazon, obtained from an earlier version of satellite‐derived vegetation greenne ss data ‐ Collection 4 (C4)Enhanced Vegetation Ind ex (EVI), are irreproducible, withboth this earlier version as well as the improved, currentversion (C5), owing to inclusion of atmosphere‐corrupteddata in those results. We find no evidence of large‐scalegreening of intact Amazon forests during the 2005drought ‐ approximately 11%–12% o f these drought‐stricken forests display greening, while, 28%–29% showbrowning or no‐change, and for the rest, the data are no tof sufficient quality to characterize any changes. Thesechanges are also not unique ‐ approximately similarchanges are observed in non‐ drought years as well.Changes in surface solar ir radiance are contrary to thespeculation in the previously published report of enhancedsunlight availability during the 2005 drought. There wasno co‐relation between drought severity and greennesschanges, which is contrary to the idea of drought‐inducedgreening. Thus, we conclude that Amazon forests did notgreen‐up during the 200 5 drought.Citation: Samanta, A.,S. Ganguly, H. Hashimoto, S. Devadiga, E. Vermote, Y. Knyazikhin,R. R. Nemani, an d R. B. Myneni (2010), Amazon forests did notgreen‐up during the 2005 drought, Geophys. Res. Lett., 37, L05401,doi:10.1029/2009GL042154.1. Introduction[2] The Amazon forests store significant amount of car-bon, by some estimates as much as 100 billion tons [Malhiet al., 2006], in their woody biomass. Should these forestsdie due to moisture stress in a progressively warming cli-mate and savannas replace them, as some studies havesuggested [e.g., Cox et al., 2004; Salazar et al., 2007;Huntingford et al., 2008], the carbon released to the atmo-sphere will act to accelerate global climate changes signif-icantly [Cox et al., 2000]. However, the drought sensitivityof these forests is poorly understood and currently underdebate. Extreme droughts such as those associated with theEl Niño Southern Oscillation (ENSO), when the plant‐available soil moisture stays below a critical threshold levelfor a prolonged period, are known to result in higher rates oftree mortality and increased forest flammability [Nepstad etal., 2004, 2007]. The drought of 2005, however, was unlikethe ENSO‐related droughts of 1983 and 1998 ‐ it wasespecially severe during the dry season in southwesternAmazon but did not impact the central and eastern regions[Marengo et al., 2008]. There are varying reports of forestresponse to this drought ‐ higher tree mortality and declinein tr ee growth fr om ground observations [Phillips et al.,2009] and more biomass fires [Aragao et al., 2007], onthe one hand, and excessive greening from satellite ob-servations [Saleska et al., 2007, hereafter SDHR07], on theother. Reconciling these reports remains a priority.[3] The availability of a new and improved version ofsatellite‐derived vegetation greenness data set ‐ Collection 5(C5) Enhanced Vegetation Index (EVI) ‐ facilitates a con-ciliation of the aforesaid reports for two reasons. First, theC5 EVI data were generated from significantly improvedalgorithms and input‐data filtering schemes related to cloudsand aerosols that otherwise corrupt EVI data [ Didan andHuete, 2006] ‐ aerosols from biomass burning are wide-spread in the Amazon during the dry season [e.g., Eck et al.,1998; Schafer et al., 2002], and aerosol loads were signifi-cantly higher, compared to other years, during the dry sea-son of 2005 [Koren et al., 2007; Bevan et al., 2009].Second, this data set spans a longer time period (2000–2008). Our analysis here is focused on answering the fol-lowing five questions: (a) are the results published bySDHR07 reproducible with both the current and previousversions of EVI data? (b) What fraction of the intact forestarea impacted by the drought exhibited anomalous greeningin year 2005? (c) Is there evidence of higher than normalamounts of sunlight during the 2005 drought, which mayhave somehow caused the forests to green‐up, as speculatedby SDHR07? (d) If drought caused the forests to green‐up,is there a relationship between the severity of drought andthe spatial extent or magnitude of greening? (e) Aregreenness changes during the 2005 drought unique com-pared to changes in non‐drought years?2. Data and Methods[4] Detailed information on data and methods is providedin the auxiliary material.7“Amazon forests” in this report1Department of Geography and Environment, Boston University,Boston, Massachusetts, USA.2BAERI, NASA Ames Research Center, Moffett Field, California,USA.3Department of Science and Environmental Policy, California StateUniversity, Monterey Bay, Seaside, California, USA.4Sigma Space Corporation, at NASA Goddard Space Flight Center,Greenbelt, Maryland, USA.5Department of Geography, University of Maryland, College Park,Maryland, USA.6Biosph eric Science Branch, NASA Ames Research Cen ter, MoffettField, California, USA.Copyright 2010 by the American Geophysical Union.0094‐8276/10/2009GL042154$05.007Auxillary materials are available in the HTML. doi:10.1029/2009GL042154.GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L05401, doi:10.1029/2009GL042154, 2010L05401 1of5refer to intact (i.e., undisturbed) Amazon forests, south ofthe Equator, affected with drought during the dry season(July–August–September) of 2005 (Figure S1).3. Results and Discussion[5] SDHR07 claim to have filtered out cloud, shadow andaerosol contaminated data from their analysis. However,their published patterns cannot be reproduced when suchdata are filtered out (Figure 1a compared to Figure 1B ofSDHR07) and the spatial extent of greening decreased by35% (Table S1). SDHR07’s patterns can be reproduced onlyif no data are screened from analysis (Figure 1b). SDHR07’ spatterns also cannot be reproduced with the newer C5 EVIdata, irrespective of whether


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