Nature | Editorial

Trick of the light

The Amazon doesn’t absorb extra carbon in the dry season after all. It can become a carbon source.

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Budding biologists learn early the apparently simple holy trinity of ingredients for photosynthesis: carbon dioxide, water and light. In truth, the equation is a little more complicated than that, and when photosynthesis proceeds on a truly massive scale, these complications can have huge implications.

Take, for example, the world’s largest mass of concentrated photosynthesis: the Amazon rainforest of South America. Scientists have long struggled to work out whether the rate of photosynthesis there is controlled by the available amount of water or of sunlight. (Over seasonal timescales, that is — on a 24-hour cycle, it is controlled by the availability of sunlight.)

The uncertainty was triggered by a surprising result from satellite images, which seemed to show that Amazon forests became greener during the dry season, and greenest of all during years of severe drought such as 2005 (S. R. Saleska et al. Science 318, 612; 2007). More green means more photosynthesis, so this result suggested that it was the availability of light, and not water, that was the controlling factor. Clear skies and sunny weather were more important than moisture in the soil.

In a study published on Nature’s website today (D. C. Morton et al. Nature; 2014), researchers show that this is, literally, an illusion. The forest does not become greener during dry periods at all. It just looks that way when the sensor and the Sun are both in the south of the sky. It is not photosynthesis that drives the apparent greening of the forest at such times, but a lack of shadow.

The finding drags attention away from the importance of light in the Amazon’s photosynthesis equation, and towards the need for water. But what of the third point of the triangle, carbon dioxide? There is uncertainty there too: this time over whether in years of drought, the trees will switch from being a net carbon sink to a source, which could worsen global warming. A second study of the Amazon, on page 76, offers the latest data on this debate, and the news is not good. Fire and drought can indeed make the Amazon a net source of atmospheric carbon — whatever colour it is at the time.

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