title: Confronting model predictions of carbon fluxes with measurements of Amazon forests subjected to experimental drought.
abstract: Considerable uncertainty surrounds the fate of Amazon rainforests in response to climate change. Here, carbon (C) flux predictions of five terrestrial biosphere models (Community Land Model version 3.5 (CLM3.5), Ecosystem Demography model version 2.1 (ED2), Integrated BIosphere Simulator version 2.6.4 (IBIS), Joint UK Land Environment Simulator version 2.1 (JULES) and Simple Biosphere model version 3 (SiB3)) and a hydrodynamic terrestrial ecosystem model (the Soil-Plant-Atmosphere (SPA) model) were evaluated against measurements from two large-scale Amazon drought experiments. Model predictions agreed with the observed C fluxes in the control plots of both experiments, but poorly replicated the responses to the drought treatments. Most notably, with the exception of ED2, the models predicted negligible reductions in aboveground biomass in response to the drought treatments, which was in contrast to an observed c. 20% reduction at both sites. For ED2, the timing of the decline in aboveground biomass was accurate, but the magnitude was too high for one site and too low for the other. Three key findings indicate critical areas for future research and model development. First, the models predicted declines in autotrophic respiration under prolonged drought in contrast to measured increases at one of the sites. Secondly, models lacking a phenological response to drought introduced bias in the sensitivity of canopy productivity and respiration to drought. Thirdly, the phenomenological water-stress functions used by the terrestrial biosphere models to represent the effects of soil moisture on stomatal conductance yielded unrealistic diurnal and seasonal responses to drought.
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
authors.forename: Thomas L
authors.initials: TL
authors.lastname: Powell
authors.affiliation: School of Geography, University of Leeds, Leeds, LS2 9JT, UK.|Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK.
authors.forename: David R
authors.initials: DR
authors.lastname: Galbraith
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Bradley O
authors.initials: BO
authors.lastname: Christoffersen
authors.affiliation: College of Engineering, Mathematics, and Physical Science, University of Exeter, Exeter, EX4 4QF, UK.|Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 80523, USA.
authors.forename: Anna
authors.initials: A
authors.lastname: Harper
authors.affiliation: Grupo de Pesquisas em Interação Atmosfera-Biosfera, Universidade Federal de Viçosa, Viçosa, CEP 36570-000, Minas Gerias, Brazil.
authors.forename: Hewlley M A
authors.initials: HMA
authors.lastname: Imbuzeiro
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh, EH8 9XP, UK.
authors.forename: Lucy
authors.initials: L
authors.lastname: Rowland
authors.affiliation: Museu Paraense Emilio Goeldi, Belém, CEP 66077-530, Pará, Brazil.
authors.forename: Samuel
authors.initials: S
authors.lastname: Almeida
authors.affiliation: Instituto de Pesquisa Ambiental da Amazônia, CEP 71503-505, Brasília, Distrito Federal, Brazil.
authors.forename: Paulo M
authors.initials: PM
authors.lastname: Brando
authors.affiliation: Centro de Geociências, Universidade Federal do Pará, Belém, CEP 66017-970, Pará, Brazil.
authors.forename: Antonio Carlos Lola
authors.initials: ACL
authors.lastname: da Costa
authors.affiliation: Grupo de Pesquisas em Interação Atmosfera-Biosfera, Universidade Federal de Viçosa, Viçosa, CEP 36570-000, Minas Gerias, Brazil.
authors.forename: Marcos Heil
authors.initials: MH
authors.lastname: Costa
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
authors.forename: Naomi M
authors.initials: NM
authors.lastname: Levine
authors.affiliation: Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK.
authors.forename: Yadvinder
authors.initials: Y
authors.lastname: Malhi
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Scott R
authors.initials: SR
authors.lastname: Saleska
authors.affiliation: Embrapa Amapá, Macapá, CEP 68903-419, Amapá, Brazil.
authors.forename: Eleneide
authors.initials: E
authors.lastname: Sotta
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh, EH8 9XP, UK.
authors.forename: Mathew
authors.initials: M
authors.lastname: Williams
authors.affiliation: School of GeoSciences, University of Edinburgh, Edinburgh, EH8 9XP, UK.
authors.forename: Patrick
authors.initials: P
authors.lastname: Meir
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
authors.forename: Paul R
authors.initials: PR
authors.lastname: Moorcroft
mesh_terms: D018533:Biomass; D001938:Brazil; D002244:Carbon / Q000378:metabolism*; D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000378:metabolism; D002940:Circadian Rhythm; D003681:Dehydration; D055864:Droughts; D017753:Ecosystem; D008954:Models, Biological; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology; D018515:Plant Leaves / Q000502:physiology; D012987:Soil; D014197:Trees / Q000254:growth & development / Q000502:physiology*; D014329:Tropical Climate; D014867:Water / Q000502:physiology*; D014934:Wood
chemical_list: D012987:Soil; D014867:Water; D002245:Carbon Dioxide; D002244:Carbon; D010100:Oxygen
keywords: Amazon; carbon cycle; drought; terrestrial biosphere model; throughfall exclusion; tropical rainforest