Sun seekers: how the dry season turns the Amazon green

Photo: Flickr/Amauri Aguiar

Photo: Flickr/Amauri Aguiar

In summary: 
  • Old-growth trees in the Amazon rainforest are never short of water – with root systems stretching 20 metres and deeper underground, they tap a vast reservoir.
  • Satellite remote sensing and ground-based observations show the trees’ productivity is limited by access to sunlight – when the rain stops, growth is immediate and energetic.

The arrival of the dry season in the Amazon is the rainforest’s cue to go green.

According to satellite data, the green tinge is unmistakeable. Now, an analysis of ground data, cross-matched with the view from space, confirms the phenomenon.

“The rainforest – the lungs of the earth – is more limited by sunlight than it is by water,” says Professor Alfredo Huete, of the Plant Functional Biology & Climate Change (C3) cluster at UTS.

The findings are included in the paper Dry-season greening of Amazon forests, just published in Nature by Professor Huete and his collaborators.

“Both satellite remote sensing and ground-based observations support the conclusion that Amazon forests green up with sunlight in the dry season,” they write.

Professor Huete says the paper settles a long-running academic debate and answers a “bold” assertion made by other scientists in 2014: that rainforest activity is constant throughout the year, neither excited nor depressed by extended sunlight.

Professor Huete has been working on this issue for several years and first published findings on dry-season greening of the rainforest in 2006.

“In using satellite sensors to study the forests we saw that as soon as the dry season appeared and the sun came out, everything seemed to become more productive and energetic,” he says.

“New leaves came out almost exactly as you would expect in dry areas when the rains come – just substituting sunlight for water.

“This contradicted people’s intuition and models – everyone seemed to think that when you turn off the water everything stops. But in the tropical rainforest everything is living in water because it’s nearly always raining. What trees need more than anything else is sunlight.”

The 2014 challenge to these findings, based on the sun’s position in the sky distorting satellite data, took Professor Huete and his colleagues back to basics. They recognised they had not allowed for any impact from the sun – ultimately they concluded its effect was demonstrable – and conceded they may over slightly overestimated the greening.

Enhanced vegetation index for the Amazon region, as captured by NASA’s MODIS satellite. Enhanced vegetation index for the Amazon region, as captured by NASA’s MODIS satellite.

But however they looked at their data – collected from the NASA MODIS sensor and from an 80-metre tower above the Amazon forest canopy – they reached the same conclusion.

“This study is about doing a very careful job to connect the ground sources of information – including a direct measurement of carbon and oxygen exchange as well as leaf data – with the satellite which collects data via 36 spectral bands such as infrared, thermal and so on.”

Professor Huete says the interest in this field of research is understandable: massive-scale biomes and ecosystems are behaving in a way that is contrary to what you might expect.

“But if you think about it, it makes sense. You have these gigantic tree forest systems with very deep roots. Those root systems are 20 metres and deeper underground with an unlimited water supply because even in the dry season it rains every day.”

Professor Huete says some good things did come from the 2014 challenge.

“From a scientist’s point of view, this experience stresses the importance of connecting what you see on the ground with what you see from space. It’s a basic principle, we believe – never rely on the satellite data alone, as was done in the 2014 challenge. We just don’t agree with their conclusion.”

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