Amazon rainforests green-up with sunlight in dry seasonAlfredo R. Huete,1,2Kamel Didan,1Yosio E. Shimabukuro,3Piyachat Ratana,1Scott R. Saleska,2,4Lucy R. Hutyra,5Wenze Yang,6Ramakrishna R. Nemani,7and Ranga Myneni6Received 23 December 2005; revised 6 February 2006; accepted 8 February 2006; published 22 March 2006.[1] Metabolism and phenology of Amazon rainforestssignificantly influence global dynamics of climate, carbonand water, but remain poorly understood. We analyzedAmazon vegetation phenology at multiple scales withModerate Resolution Imaging Spectroradiometer (MODIS)satellite measurements from 2000 to 2005. MODISEnhanced Vegetation Index (EVI, an index of canopyphotosynthetic capacity) increased by 25% with sunlightduring the dry season across Amazon forests, opposite toecosystem model predictions that water limitation shouldcause dry season declines in forest canopy photosynthesis.In contrast to intact forests, areas converted to pastureshowed dry-season declines in EVI-derived photosyntheticcapacity, presumably because removal of deep-rooted foresttrees reduced access to deep soil water. Local canopyphotosynthesis measured from eddy flux towers in both arainforest and forest conversion site confirm ourinterpretation of satellite data, and suggest that basin-widecarbon fluxes can be constrained by integrating remotesensing and local flux measurements.Citation: Huete, A. R.,K. Didan, Y. E. Shimabukuro, P. Ratana, S. R. Saleska, L. R.Hutyra, W. Yang, R. R. Nemani, and R. Myneni (2006), Amazonrainforests green-up with sunlight in dry season, Geophys. Res.Lett., 33, L06405, doi:10.1029/2005GL025583.1. Introduction[2] The mechanisms controlling tropical rainforest phe-nology and productivity are not well understood. Site-basedplot and flux tower results in old-growth Amazon rain-forests are ambiguous with some studies showing depressedphotosynthetic activity in the dry season due to moisturestress [Malhi et al., 1998] while other studies show dryseason increases in total canopy photosynthesis [Saleska etal., 2003] and species-specific shoot and leaf growth[Borchert, 1994] with little evidence of stress possiblybecause deep tree roots allow continuous access to deepsoil moisture layers, even during the dry season [Nepstad etal., 1994; da Rocha et al., 2004; Oliveira et al., 2005].Several prominent ecosystem models, by contrast, showsignificant ‘brown-down’ in the dry season as a conse-quence of increasing dry-season water stress [Botta et al.,2002; Tian et al., 1998] with other model studies suggestingAmazon tropical rainforests are light-limited [Nemani et al.,2003].[3] In the seasonally dry, central and eastern Amazonrainforests, photosynthesis begins to decline in the late wetseason, most likely a result of onset of senescence andlitterfall and envelopment of mature leaves by epiphylls(moss, algae) that inhibit photosynthesis [ Goulden et al.,2004; Roberts et al., 1998]. Litterfall peaks in the early dryseason followed by a period of rapid leaf tur nover withsignificant increases in leaf area and photosynthesis [Asneret al., 2004; Carswell et al., 2002; Saleska et al., 2003; Riceet al., 2004]. Leaf flushing in several rainforests, includingcentral Amazoˆnia, has been found to closely coincide withdry season peaks in photosynthetic active radiation (PAR)[Wright and van Schaik, 1994] which is mostly controlledby seasonal cloud cover variations [da Rocha et al., 2004].[4] At the basin scale, a more complex regional mosaic ofvegetation phenology can be expected as a result of foreststructural variations, land use activities, and associatedecological conditions [Asner et al. , 2004; Keller et al.,2004]. Photosynthesis has been found more tightly coupledwith water availability in the drier and transitional, southernAmazon rainforests that have lower biomass and leaf areaindex (LAI) [von Randow et al., 2004; Vourlitis et al.,2001]. In this study, we analyzed vegetation phenology andseasonal patterns in rainforest greening over the entireAmazon basin (7.5  106km2) using satel lite observations.2. Methods[5] Rainforest seasonality was a nalyzed at multiplescales, including (1) an extensive 2,000 km climate transectthrough eastern and central Amazoˆnia, (2) basin-wide, and(3) local forest and conversion (pasture or agricultural) sitesin eastern Amazoˆnia. We used Enhanced Vegetation Index(EVI) satellite data from the recently launched Terra Mod-erate Resolution Imaging Spectroradiometer (MODIS) sen-sor [Justice et al., 1998]. Previous satellite observationswith NOAA Advanced Very High Resolution Radiometer(AVHRR) measurements were constrained by cloud con-tamination and sensitivity to seasonally variable atmospherewater vapor and aerosol conditions, as well as poor spatialresolution (>4 km) [Kobayashi and Dye, 2005]. In contrast,the MODIS sensor offers new opportunities for basin-wideAmazon studies with state of the art calibration, atmospherecorrection, narrower spectral bands without water vaporGEOPHYSICAL RESEARCH LETTERS, VOL. 33, L06405, doi:10.1029/2005GL025583, 20061Department of Soil, Water and Environmental Science, University ofArizona, Tucson, Arizona, USA.2Institute for the Study of Planet Earth, University of Arizona, Tucson,Arizona, USA.3Instituto Nacional de Pesquisas Espaciais, Sa˜o Jose´ dos Campos,Brazil.4Department of Ecology and Evolutionary Biology, University ofArizona, Tucson, Arizona, USA.5Department of Earth and Planetary Sciences, Harvard University,Cambridge, Massachusetts, USA.6Department of Geography, Boston University, Boston, Massachusetts,USA.7NASA Ames Research Center, Moffett Field, California, USA.Copyright 2006 by the American Geophysical Union.0094-8276/06/2005GL025583$05.00L06405 1of4influences, finer resolution (250 m) observations that facil-itate cloud-filtering, and newly developed terrestrial prod-ucts [Justice et al., 1998].[6] Vegetation indices measure canopy greenness, a com-posite property of canopy structure, leaf area, and canopychlorophyll content [Myneni et al., 1995]. The EVI main-tains sensitivity even in high LAI canopies, such as foundthroughout the Amazon (LAI > 4), by relying on near-infrared canopy reflectance which is less prone to saturate,particularly with moderate resolution pixels [Gao et al.,2000; Huete e t al., 2002]. As has been no ted throughtheoretical analyse s, spectral indices more functional onthe near-infrared best describe area-averaged canopy pho-tosynthetic capacities and gross primary