Direct evidence that drought-weakened Amazonian forests 'inhale less carbon'

Direct evidence that drought-weakened Amazonian forests 'inhale less carbon'
Researcher on ladder measures tree diameters in Tambopata National Park, Peru, soon after the 2010 drought. This was one of 13 plots across Brazil, Peru and Bolivia that was monitored closely for a week every month in a coordinated effort. Credit: Jake Bryant

For the first time, an international research team has provided direct evidence of the rate at which individual trees in the Amazonian basin 'inhale' carbon from the atmosphere during a severe drought. They measured the growth and photosynthesis rates of trees at 13 rainforest plots across Brazil, Peru and Bolivia, comparing plots that were affected by the strong drought of 2010 with unaffected plots. They found that while growth rates of the trees in drought-affected plots were unchanged, the rate of photosynthesis - by which trees convert carbon into energy to fuel their activities - slowed down by around 10 percent over six months. Their paper, published in the journal, Nature, concludes that trees may be channelling their more limited energy reserves into growth rather than maintaining their own health. Computer simulations of the biosphere have predicted such responses to drought, but these are the first direct observations of this effect across tropical forests.

The three-year international study is the first detailed, large-scale examination of the complete carbon cycle, looking at both the growth and metabolism of forest plots at different sites across the Amazon basin. Each of the plots is one hectare and contains around 400-500 . The rainforest plots chosen were representative of the varying climatic and soil conditions of the Amazon basin. The study is the product of a new effort, co-ordinated by Oxford University, to closely track the functioning of tropical forests across the world: the Global Ecosystems Monitoring network (GEM).

The findings are the result of a huge and challenging field effort: over a three-year period, local students and technicians spent several weeks every month at each site to measure each tree's woody growth rate and the number of small roots that had grown. Researchers weighed monthly leaf fall, using nets, to establish the number of leaves each tree was producing. They also used infra-red gas analysers to track the release of carbon dioxide from living wood, roots, and leaves to estimate the metabolic activity of the forest. By chance, the Amazon drought of 2010 occurred right in the middle of this observation window, but only affected some parts of Amazonia.

Researchers found that while the rate of was constant among trees on plots unaffected by drought, rates on the six drought-affected plots dropped significantly (as compared with before the 2010 drought). They also discovered that while the growth rates of drought-affected plots were unchanged, levels of tissue maintenance and the general health of trees were reduced. The paper reasons that growth rates of drought-affected trees did not change, despite their limited energy supplies, because otherwise they would be at a disadvantage when competing for light, water or nutrients. But it adds that this neglect of maintenance came at a cost, and is likely to have led to the increase (up to a trebling in some sites) in numbers of trees dying in the years after the drought.

Study lead author Dr Christopher Doughty, from the Environmental Change Institute in School of Geography and the Environment at the University of Oxford, said: 'Tropical rainforests have been popularly thought of as the "lungs" of the planet. Here, we show for the first time that during , the rate at which they "inhale" carbon through photosynthesis can decrease. This decreased uptake of carbon does not decrease growth rates but does mean an increase in tree deaths. As trees die and decompose, the concentrations of carbon dioxide in the atmosphere will increase, potentially speeding up during tropical droughts.'

These results support the findings of an earlier paper in Nature, published last year, based on aircraft data. Taken together, the research data provides a picture, from the leaf scale to the global scale, suggesting that droughts in the Amazon basin are affecting levels of in the atmosphere globally, both on a short-term basis though decreasing photosynthesis and on a longer term basis, by increasing tree mortality.

The GEM network plans to continue to closely monitor in the Americas, Africa and Asia over the coming decades, to understand how these forests are affected by changing climate.

'These plots are our canaries in the climate change coal mine,'said co-author and GEM co-ordinator Professor Yadvinder Malhi, also from the Environmental Change Institute in the School of Geography and the Environment. 'As this study demonstrates, they can give us important early insight into the actual mechanisms of how these complex forests are responding to extreme climates. Only through painstaking monitoring, like this, can we hope to understand and realistically model and predict the two-way interactions between climate change and the biosphere.'

Explore further

Vines choke a forest's ability to capture carbon, scientists report

More information: The paper, 'Drought impact on forest carbon dynamics and fluxes in Amazonia', is due to be published by Nature on 5 March 2015. DOI: 10.1038/nature14213
Journal information: Nature

Provided by Oxford University
Citation: Direct evidence that drought-weakened Amazonian forests 'inhale less carbon' (2015, March 4) retrieved 26 June 2019 from
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Feedback to editors

User comments

Mar 04, 2015
Or… you could have asked any farmer from the 1800's. They really spent 3 years on this? I hope that this really isn't the biggest thing they learned from the study.

Mar 04, 2015
Ignoring what the @confusedSoc says; This is a very interesting and important study. With the Amazon in drought, you know that one of the major carbon sinks in the carbon cycle is taken out of commission for the period of the drought. Knowing how the correlation between the Amazon CO2 uptake, the health of the rain forests will allow us to understand the overall impact on climate that could occur from extended droughts.

The feed back mechanisms are vital to understand how to regulate CO2 emissions from fossil fuel consumption of the populous and corporations.

Mar 05, 2015
Actually the study only showed a 10 % lower uptake of CO2 in drought. The study failed to note that by continuing their normal growth rates that the trees were necessarily sequestering the same amounts of carbon in their tissues to make their cellulose as they normally would. Consequently whether the carbon in CO2 was used directly or not, the up take of carbon continued from other sources at the same rate in order to maintain growth. While the study is certainly contributory, "huge" is a bit over the top. The effects of moisture level on growth, photosynthesis and CO2 metabolism has been documented for a very long time. Additionally, it should be noted that the Amazon Basin and the African savannahs are the largest CO2 producing areas of the world - far outstripping human production. The real problem here is human overpopulation and resulting deforestation. Not addressing human overpopulation makes our hand wringing and token efforts at otherwise reducing CO2 levels - absurd.

Mar 05, 2015
Additionally, the study didn't record the net uptake CO2 of the forest around the trees and whether other plants that are more adapted to dry season climates and other organisms don't increase their use of CO2 and carbon - and what the overall net CO2 might be. The study quite literally couldn't see the forest for the trees it was focused on. It also didn't note the normal seasonal variation of CO2 uptake over long period to compare the 10% drought reduction of CO2 uptake by large trees - with their range of CO2 metabolism rates. Contributory, not "huge."

Mar 05, 2015
The Brazilian rain-forest is commonly called the lungs of the planet and should be treated as suck, a vital organ of vital ecological importance. I agree with most of you analysis Dug except for a couple of points. First the Amazon basin traps 5000 kg of carbon per hectare per year. The total area of forest is 400 million hectares so the whole forest could be absorbing 2 billion tons of carbon per year. Deforestation reduces that and drought does as well indirectly by forest fires and reduced plant size. Your claims that the Amazon Basin and the African svannahs are the largest CO2 producers in the world is just flat out wrong. This video from NASA shows just how strongly the CO2 uptake is in the rain forests. (compared it with the massive production of CO2 from the industrial nations).
In that video, you will never see the Brazilian rain forests ever "turn orange".

Mar 05, 2015
Hmm... so plants have evolved to grow just as well in extreme drought. Only in AGW Cult "science" that's a bad thing because they breathe less of the evil CO2.

Mar 05, 2015
Hmm... so plants have evolved to grow just as well in extreme drought. Only in AGW Cult "science" that's a bad thing because they breathe less of the evil CO2.
CO2 last week was 401.10 ppm. Last year 2014 for same period it was 397.89 ppm. For the previous 1 million years earth has never peaked above 273ppm of atmospheric CO2. Of course everyone knows the CO2 is a heat trapping green house gas and in the excessive amounts mankind has unleashed into his nest, will damage eco-systems and life beyond any capability to repair. ie. A Planetary Deathwish.

On you other nerd point @goricle; ignoring the fact that plants have not evolved to grow just as well in extreme drought, your skipping 99.9% of the rest of the plant kingdom. Even this report show there was a 10% decline in CO2 uptake as @Dug points out. Here is why that is important to know;

Mar 07, 2015
Dug claimed
Additionally, it should be noted that the Amazon Basin and the African savannahs are the largest CO2 producing areas of the world - far outstripping human production
Actual evidence for your claim please ?

Human production significant at ~ 230,000 Litres of petrol/second (24/7), this staggering amount of CO2 & sizable amount of heat.

We had a fake scientist claim to have 4 technical degrees incl of physical chemistry but, couldn't establish CO2's addition of thermal resistivity re infra-red (IR) to space.

Best but obtuse logic he failed at, pathetic as it is was:-
"Water Vapour is a stronger greenhouse gas than CO2.. therefore CO2 is a red-herring"

Nothing based on CO2's physical properties in relation to IR absorbancy/emission eg:-

as above link shows, the IR bands are comparable & weighted by frequency ie energy !

ie. Mature scientists making "red herring" claims must deal in W/m^2

Mar 07, 2015
ConfoundedSociety claimed
Or… you could have asked any farmer from the 1800's. They really spent 3 years on this? I hope that this really isn't the biggest thing they learned from the study
So you are claiming the top-soil nutrients & biodome & water flow mechanisms in tropical environments are somehow identical to farmland - where in the USA ?

Care to respond to those other strange claims u make at the drop of a hat, which only seem to obfuscate & not actually contribute anything useful to a dialectic, eg such as your obtuse comments here, which don't seem to go anywhere helpful to anyone:-

What is your motivation for trying to marginalise the basis of any sort of scientific position ?

What do U think this does re your credibility ?

Mar 08, 2015
Well it is a good thing that the CO2 levels are up since it is a well known fact that plants require less water with higher CO2 levels. Otherwise the drought inflicted damage to the Amazon would be worse.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more