title: Leaf aging of Amazonian canopy trees as revealed by spectral and physiochemical measurements.
abstract: Leaf aging is a fundamental driver of changes in leaf traits, thereby regulating ecosystem processes and remotely sensed canopy dynamics. We explore leaf reflectance as a tool to monitor leaf age and develop a spectra-based partial least squares regression (PLSR) model to predict age using data from a phenological study of 1099 leaves from 12 lowland Amazonian canopy trees in southern Peru. Results demonstrated monotonic decreases in leaf water (LWC) and phosphorus (Pmass) contents and an increase in leaf mass per unit area (LMA) with age across trees; leaf nitrogen (Nmass) and carbon (Cmass) contents showed monotonic but tree-specific age responses. We observed large age-related variation in leaf spectra across trees. A spectra-based model was more accurate in predicting leaf age (R2= 0.86; percent root mean square error (%RMSE) = 33) compared with trait-based models using single (R2= 0.07-0.73; %RMSE = 7-38) and multiple (R2= 0.76; %RMSE = 28) predictors. Spectra- and trait-based models established a physiochemical basis for the spectral age model. Vegetation indices (VIs) including the normalized difference vegetation index (NDVI), enhanced vegetation index 2 (EVI2), normalized difference water index (NDWI) and photosynthetic reflectance index (PRI) were all age-dependent. This study highlights the importance of leaf age as a mediator of leaf traits, provides evidence of age-related leaf reflectance changes that have important impacts on VIs used to monitor canopy dynamics and productivity and proposes a new approach to predicting and monitoring leaf age with important implications for remote sensing.
authors.affiliation: Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Chavana-Bryant
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: Jin
authors.initials: J
authors.lastname: Wu
authors.affiliation: Department of Global Ecology, Carnegie Institution for Science, 260 Panama Street, Stanford, CA, 94305, USA.
authors.forename: Gregory P
authors.initials: GP
authors.lastname: Asner
authors.affiliation: College of Medicine, Swansea University, Singleton Park, Swansea, SA2 8PP, UK.
authors.forename: Athanasios
authors.initials: A
authors.lastname: Anastasiou
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Enquist
authors.affiliation: Sección Química, Pontificia Universidad Católica del Perú (PUCP), Avenida Universitaria 1801, San Miguel, Lima 32, Peru.
authors.forename: Eric G
authors.initials: EG
authors.lastname: Cosio Caravasi
authors.affiliation: Environmental Change Institute, School of Geography and the Environment, University of Oxford, Oxford, OX1 3QY, UK.
authors.forename: Christopher E
authors.initials: CE
authors.lastname: Doughty
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: Department of Global Ecology, Carnegie Institution for Science, 260 Panama Street, Stanford, CA, 94305, USA.
authors.forename: Roberta E
authors.initials: RE
authors.lastname: Martin
authors.affiliation: Centre for Ecology and Hydrology, Wallingford, OX10 8BB, UK.
authors.forename: France F
authors.initials: FF
authors.lastname: Gerard
mesh_terms: D055598:Chemical Phenomena; D017753:Ecosystem; D016018:Least-Squares Analysis; D008027:Light; D008962:Models, Theoretical; D010568:Peru; D018515:Plant Leaves / Q000033:anatomy & histology / Q000737:chemistry / Q000254:growth & development* / Q000502:physiology*; D058998:Remote Sensing Technology; D017215:Satellite Communications; D013045:Species Specificity; D014197:Trees / Q000502:physiology*
keywords: canopy trees; leaf age; leaf lifecycle; leaf spectral properties; leaf traits; phenology; tropical forests; vegetation indices (VIs)