Drought effects on litterfall, wood production and belowground carbon cycling in an Amazon forest: results of a throughfall reduction experimentIntroductionMaterial and methodsSite description and treatment designAboveground NPP and tree mortalitySoil CO2 effluxStatistical analysisResultsSoil waterMortality and necromassStem growth and wood productionLitterfall and canopy dynamicsANPPCorrelates of ANPP and LAISoil CO2 effluxDiscussionResponse and recovery of ANPPBelow ground carbon cyclingCarbon stock accumulation and forest recoveryThe authors thank J. Putz, M. Palumbo and an anonymous reviewer for commenting on this manuscript. Financial supported was provided by NSF (no. DEB-0213011), NASA and LBA-ECO (no. NCC5-332, NCC5-686, NCC5-700, NNG06GE88A), USAID, CNPq and the Gordon an...ReferencesDrought effects on litter fall, wood production andbelowground carbon cycling in an Amazon forest:results of a throughfall reduction experimentPaulo M. Brando1,2,3,4,*, Daniel C. Nepstad1,2, Eric A. Davidson2,Susan E. Trumbore5, David Ray2and Plı´nio Camargo61Instituto de Pesquisa Ambiental da Amazoˆnia (IPAM ), Avenida Rui Barbosa, 136,68005-080 Santare´m, Para´, Brazil2Woods Hole Research Cent er, 149 Woods Hole Road, Falmouth , MA 02543, USA3Depar tment of Botany, 214 Bartram Hall, University of Florida, PO Box 118526,Gainesville, FL 32611, USA4School of Natural Resources and Environment, University of Florida, Gainesville, FL 32611, USA5Depar tment of Earth Sys tem Science, University of California, Irvine, CA 92697-3100, USA6Centro de Energia Nuclear na Agricultura (CENA ), 13400-970 Piracicaba, BrazilThe Amazon Basin experiences severe droughts that may become more common in the future. Littleis known of the effects of such droughts on Amazon forest productivity and carbon allocation. Wetested the prediction that severe drought decreases litterfall and wood production but potentially hasmultiple cancelling effects on belowground production within a 7-year partial throughfall exclusionexperiment. We simulated an approximately 35–41% reduction in effective rainfall from 2000through 2004 in a 1 ha plot and compared forest response with a similar control plot. Woodproduction was the most sensitive component of above-ground net primary productivity (ANPP) todrought, declining by 13% the first year and up to 62% thereafter. Litterfall declined only in the thirdyear of drought, with a maximum difference of 23% below the control plot. Soil CO2efflux and its14C signature showed no significant treatment response, suggesting similar amounts and sources ofbelowground production. ANPP was similar between plots in 2000 and declined to a low of 41%below the control plot during the subsequent treatment years, rebounding to only a 10% differenceduring the first post-treatment year. Live aboveground carbon declined by 32.5 Mg haK1through theeffects of drought on ANPP and tree mortality. Results of this unreplicated, long-term, large-scaleecosystem manipulation experiment demonstrate that multi-year severe drought can substantiallyreduce Amazon forest carbon stocks.Keywords: above-ground net primary productivity; wood production; Amazon;drought; litterfall; global change1. INTRODUCTIONThefateofapproximately86–140Pgofcarboncontained in the forests of the Amazon (Saatchi et al.2007) will depend, in part, upon the effect of droughtepisodes on the amount and allocation of forestbiomass production. Several lines of evidences suggestthat Amazon drought episodes may be more commonand more severe in a warming world (Li et al. 2006).One of the most important ways in which severedroughts influence tropical forest carbon stocks isthrough tree mortality. Droughts associated withEl Nin˜o Southern Oscillation (ENSO) events elevatedtree mortality in East Kalimantan from a backgroundof 2 to 26% yrK1(Van Nieuwstadt & Sheil 2005), incentral Amazonia from 1.1 to 1.9% (Williamson et al.2000), Panama from 2 to 3% (Condit et al. 1995), andSarawak from 0.9 to 4.3–6.4% ( Nakagawa et al. 2000).(In one Panama forest, ENSO had no effect on treemortality; Condit et al. 2004.) Experimentally induceddrought in one hectare of an east-central Amazonianforest permitted the identification of a threshold ofdeclining plant-available soil moisture and cumulativecanopy water stress beyond which canopy treemortality rose from 1.5 to 9% ( Nepstad et al. 2007).Responses of net primary productivity (NPP) andits allocation during drought may have importantinfluences on carbon stocks, but they remain poorlyunderstood in moist tropical forests (Houghton 2005).Seasonal and interannual variation in the NPP of moisttropical forests is controlled mostly by changes in lightand soil moisture (Clark & Clark 1994). Hence, NPPincreases during ENSO events, when reduced cloudcover increases photosynthetically active radiationwithout provoking soil moisture deficits large enoughPhil. Trans. R. Soc. B (2008) 363, 1839–1848doi:10.1098/rstb.2007.0031Published online 11 February 2008Electronic supplementary material is available at http://dx.doi.org/10.1098/rstb.2007.0031 or via http://journals.royalsociety.org.One contribution of 27 to a Theme Issue ‘Climate change and thefate of the Amazon’.* Author and address for correspondence: Department of Botany,214 Bartram Hall, University of Florida, PO Box 118526,Gainesville, FL 32611, USA ([email protected]).1839 This journal is q 2008 The Royal Societyto inhibit photosynthesis. When soil moisture is inshort supply, NPP may decline and its relativeallocation among leaves, stems and roots may beaffected, but our understanding of these responses ispoor. Stem growth is expected to be the most sensitivecomponent of NPP to drought because it is low on thecarbon allocation hierarchy (Chapin et al. 1990).The response of root production to drought is lessclear, in part owing to substantial methodologicalchallenge of mensuration (Trumbore et al. 2006). Soilrespiration, which integrates CO2production from allbelowground sources, including root respiration andleaf litter decomposition, tends to be lower in the dryseason compared with the wet season (Davidson et al.2000, 2004; Saleska et al. 2003); this is probably largelydue to seasonal variation of heterotrophic respiration inthe litter layer. Responses to long-term droughttreatments may also include changes in root production.We conducted a 7-year, partial throughfall exclusionexperiment in east-central Amazonia to examine forestresponses to a 35–41%