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.
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