Amazon forests maintain consistent canopy structure and greenness during the dry season

Lidar and optical satellite observations of Amazon forests indicate consistent canopy structure and reflectance during the dry season, challenging the paradigm of light-limited tropical forest productivity. Recent remote-sensing data from the Amazon suggested that there is a 'green up' of vegetation during dry seasons, implying that light rather than water is the major limiting factor for forest productivity. Douglas Morton and colleagues have now reanalysed the evidence and show that the green up is in fact an optical artefact of the observation method, the result of changes in the relative azimuth angle of satellite observations between the June solstice and September equinox. Correcting for this removes the green-up phenomenon, adding support to other studies that indicate that water availability, rather than light, is the main driver of plant productivity in Amazon forests. The seasonality of sunlight and rainfall regulates net primary production in tropical forests1. Previous studies have suggested that light is more limiting than water for tropical forest productivity2, consistent with greening of Amazon forests during the dry season in satellite data3,4,5,6,7. We evaluated four potential mechanisms for the seasonal green-up phenomenon, including increases in leaf area5,6,7 or leaf reflectance3,4,6, using a sophisticated radiative transfer model8 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.