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Study: Drought impedes tree growth, shuts down Amazon carbon sink

  • The Amazon Basin stores 100 billion tons of carbon, serving as a valuable carbon sink and buffer against climate change.
  • A new study finds that a major drought in 2010 hindered tree growth and caused sufficient tree death to result in the complete shut down of the Amazon carbon sink.
  • That effect was temporary, and in the years between droughts the Amazon returned to being a carbon sink, with growth outstripping mortality.
  • Scientists are concerned that as climate change escalates that the intensity of droughts in the Amazon will continue to increase, stalling tree growth and bringing more carbon sink shut downs.
Thunderheads build up over the Amazon Basin. A 2010 drought caused tree growth to stall across the region, temporarily shutting down the Amazon carbon sink. Photo by Rhett A. Butler

The Amazon rainforest is popularly known as the “the planet’s lungs” — absorbing and storing 100 billion tons of carbon and preventing it from entering the atmosphere. Maintaining that vast carbon sink is seen as vital to limiting climate change impacts.

Now, new research published in the Global Biogeochemical Cycles journal shows that droughts can bring this crucial ecosystem service to a grinding halt.

University of Exeter scientist Dr. Ted Feldpausch and an international team of researchers investigated the impact of droughts on tree growth and carbon storage across the Amazon basin. They found that an Amazon Basin-wide drought in 2010 significantly affected carbon storage throughout the basin, killing many trees and slowing the growth of those that survived.

“Prior to this drought the Amazon forest had been gaining biomass”, reported Feldpausch, but the 2010 drought completely “shut down the biomass sink for the basin”. The research points to the possibility that the current drought underway in the Amazon, along with future extreme droughts, could have similar effects.

Extreme drought can stall tree growth and cause tree death. Photo by Rhett A. Butler.jpg

This was the first basin-wide study to investigate how multiple droughts interact to affect tree growth and tree death in the Amazon. Making use of a 15-year dataset collected by the Amazon Forest Inventory Network, known as RAINFOR, the research team was able to disentangle the effects of localized droughts in 2005 and 2007, and the more widespread drought of 2010.

“The first large-scale, direct demonstration of tropical drought slowing tree growth is extremely important,” said lead author Feldpausch. “It tells us that climate changes not only increase the rate of loss of carbon dioxide [from the forest] to the atmosphere, by killing trees, but also slows down the rate of uptake. And yet, the Amazon clearly has resilience, because in the years between the droughts the whole system returned to being a carbon sink, with growth outstripping mortality.”

RAINFOR is “a long-term project that works with [researchers] from more than 30 different countries to essentially understand how the Amazon forest works”, explained Oliver Phillips, a University of Leeds professor and study co-author.

The network collaborates with local researchers to monitor over 300 one-hectare plots across the Amazon Basin, looking for large-scale, long-term changes in the ecosystem. “Within these [plots] we can track the life and death of individual trees”, said Phillips. These two factors are key to the forest’s role as a carbon sink: tree growth means more atmospheric CO2 absorbed and stored; while tree death releases carbon back into the atmosphere.

Drought stresses tropical forests by reducing their ability to photosynthesize. When water is short, air bubbles can appear in the xylem, the tree structure that carries water from root to leaf. Those bubbles can disrupt the continuous column of water, an impact that is fatal. Taller trees, with a longer xylem, tend to be more at risk.

During dry times, plants can protect themselves, preventing air bubbles from forming by closing small holes in their leaves called stomata; but with their stomata closed, they cannot absorb CO2 from the atmosphere, or conduct photosynthesis to produce energy for growth and repair.

A blue-and-yellow macaw. Scientists don’t yet know how drought and stalled tree growth will impact Amazon wildlife, or which species might be affected. Photo by Rhett A. Butler

Yet, research shows that trees try to maintain growth, even during drought. A thirteen-year study of the effects of artificially reduced rainfall on plots in Brazil found that drought-stressed trees continue to store energy and grow as normal, right up until their death. This growth comes at a cost, however. During the 2010 drought, trees in Brazil, Peru and Bolivia were found to divert resources away from preserving health in favor of continued growth. Feldpausch’s study provides further evidence that surviving Amazon trees are not escaping drought unscathed.

Although scientists long hypothesized that mature forests like those in the Amazon would stay in equilibrium — with growth and death balanced over the long term — data has now proven this is not the case. “Perhaps the most central discovery of the RAINFOR network is that those processes are not in balance”, said Phillips, providing key evidence that the Amazon as a whole is responding to large-scale environmental change.

Increasing CO2 levels in the atmosphere have been stimulating plant growth in the Amazon for decades. More carbon dioxide means more photosynthesis and more efficient water use by trees — so initially the effects of climate change on growth in the Amazon may have been positive. But in recent years things have changed. “We’ve seen the growth rate saturate, at least partly due to the increase in droughts”, explained Phillips.

Last year, Phillips and colleagues at the University of Leeds published a study combining historic data on carbon storage in the Amazon, spanning the last 30-years, along with recent data from the RAINFOR plot network. Their study showed that net carbon absorption in the Amazon has been steadily declining, and tree mortality across the basin has increased by more than a third since the 1980s.

Despite increases in rainfall both globally and across the Amazon due to climate change, droughts are expected to increase in frequency as annual rainfall becomes more concentrated. Put simply, global warming is resulting in long periods of drought, punctuated with short, very hard rainstorms. “It’s getting wetter, and yet when the plants need the water, towards the end of the dry season, they’re more likely to be under [drought] stress”, explained Phillips.

The just-ending strong global El Niño event resulted in serious drought and wildfires over much of the Amazon during the 2015 June to December dry season, and the 2016 Amazon fire forecast predicts extremely high fire risk for this coming dry season. “Previous research has shown an interaction between drought and fire”, said Feldpausch; “so it’s cause for concern”. Burning forests rapidly add stored carbon to the atmosphere.

Professor Phillips highlighted a related threat: “These climate changes are also affecting the exceptional plant and animal diversity of Amazonia. A big challenge now is to discover which species are at risk.”

Scientists are concerned that escalating climate change and deepening drought will result in more shut downs of the Amazon carbon sink. Photo by Rhett A. Butler.

Feldpausch noted that the 2016 El Niño and the Amazon drought it brought will offer the researchers the “opportunity to probe this [phenomena] in more detail”, to determine whether tree growth and carbon sinks there have been suppressed as they were in 2010.

With dry season temperatures projected to soar in coming decades, the Amazon may see an even greater impact on the growth and survival of trees — along with repeated shut downs of its carbon sink. Phillips said that the rainforest is now entering a “new thermal regime” which “the plants haven’t seen in their recent evolutionary history”. This doesn’t necessarily spell doom for the rainforest, he cautioned, noting that scientists don’t yet know how well Amazonian plants might adapt to climate change and this new environment.

Still there is reason for concern, said the researchers. Ongoing drought in Amazonia, coupled with deforestation and wildfires, threatens to suffocate one of our most precious natural assets at a time when we urgently need the ecosystem services provided by “the planet’s lungs” to help us curb rising CO2 levels.

 

Citation:

Feldpausch, T.R., Phillips, O. L., et al. (2016) Amazon forest response to repeated droughts. Global Biogeochemical Cycles; DOI: 10.1002/2015GB005133