It always seemed odd. A decade ago, NASA satellite data suggested that the canopy of the Amazon rainforest grew faster during a drought. Apparently, this was just an optical illusion.
In 2003, remote sensors showed that the forest canopy reflects more near-infrared light during a drought. Because young leaves are greener – and reflect more infrared light – than old foliage, analysts assumed this was evidence that rainforests grew better during dry years.
A new study suggests otherwise. Douglas Morton of NASA's Goddard Space Flight Center and his colleagues, used alternative sensing methods such as lidar to demonstrate that NASA's sensors were not seeing a greener canopy during droughts at all. It was a trick of the light – perhaps because fresh foliage growth – which actually tends to appear in wet conditions – casts more shadows in the canopy and so absorbs more infrared light. In other words, the forest may reflect more infrared light in times of drought simply because there is no fresh growth to cast shadows.
"Seasonal moisture availability governs the balance between photosynthesis and respiration in Amazon forests," says Morton. In other words, rainforests thrive in the rain.
This conclusion happily coincides with another study also published today. The second study highlights the important link between rainfall and how much carbon the Amazon rainforest can store – and hence whether it contributes to or mitigates global warming.
Luciana Gatti of the City University of São Paulo, Brazil, and her colleagues measured the rise and fall of CO2 levels in the air above the forest during the dry year of 2010 and the wet year of 2011.
They found that the rainforest was carbon neutral during wet 2011 – absorbing as much CO2 from the air as it released. But during dry 2010, the entire basin lost around half a billion tonnes of carbon, mostly as a result of extra fires in the dry vegetation.
The two studies mean ecologists and remote sensors are now in agreement about events in the world's largest rainforest. But the result is possibly bad news for the global climate. The Amazon, just like many other places, is seeing more extreme climatic conditions, says Gatti. If we get more dry years, a proliferation of fires may turn it into an increasingly strong source of CO2, contributing to further dry years and creating a nasty feedback effect to add to our fears of climate change.