Amazon forests maintain consistent canopy structure and greenness during the dry season.
The seasonality of sunlight and rainfall regulates net primary production in tropical forests. Previous studies have suggested that light is more limiting than water for tropical forest productivity, consistent with greening of Amazon forests during the dry season in satellite data. We evaluated four potential mechanisms for the seasonal green-up phenomenon, including increases in leaf area or leaf reflectance, using a sophisticated radiative transfer model and independent satellite observations from lidar and optical sensors. Here we show that the apparent green up of Amazon forests in optical remote sensing data resulted from seasonal changes in near-infrared reflectance, an artefact of variations in sun-sensor geometry. Correcting this bidirectional reflectance effect eliminated seasonal changes in surface reflectance, consistent with independent lidar observations and model simulations with unchanging canopy properties. The stability of Amazon forest structure and reflectance over seasonal timescales challenges the paradigm of light-limited net primary production in Amazon forests and enhanced forest growth during drought conditions. Correcting optical remote sensing data for artefacts of sun-sensor geometry is essential to isolate the response of global vegetation to seasonal and interannual climate variability.
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
Douglas C
DC
Morton
1] University of Maryland, College Park, Department of Geographical Sciences, College Park, Maryland 20742, USA [2] Global Land Cover Facility, College Park, Maryland 20740, USA.
Jyoteshwar
J
Nagol
1] NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA [2] Sigma Space Corporation, Lantham, Maryland 20706, USA.
Claudia C
CC
Carabajal
1] NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA [2] University of Maryland, College Park, Department of Geographical Sciences, College Park, Maryland 20742, USA [3] Swansea University, Department of Geography, Singleton Park, Swansea SA2 8PP, UK.
Jacqueline
J
Rosette
Earth System Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA.
Michael
M
Palace
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
Bruce D
BD
Cook
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
Eric F
EF
Vermote
NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
David J
DJ
Harding
Swansea University, Department of Geography, Singleton Park, Swansea SA2 8PP, UK.
Peter R J
PR
North
D016477:Artifacts; D001938:Brazil; D003116:Color; D055864:Droughts; D017753:Ecosystem; D005618:Fresh Water / Q000032:analysis; D008954:Models, Biological; D010788:Photosynthesis; D010858:Pigmentation / Q000502:physiology*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D011891:Rain; D063809:Satellite Imagery; D012621:Seasons; D013472:Sunlight; D014197:Trees / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D014329:Tropical Climate