Scientists in Brazil starve trees of water to test Amazon's limits

Querencia, Brazil: Under Brazil's Amazon rainforest canopy, hundreds of transparent plastic panels hang between tree trunks to starve a hectare of land of half the water it normally receives.

Scientists are creating this artificial drought in Querencia municipality close to the southeastern edge of the world's largest rainforest to better understand the limits of the Amazon's resistance to extreme dry conditions that are predicted by climate change models.

In the experiment, called Seca Limite, or Limit Drought, water is captured by the platforms, then flows through a network of elevated timber gutters before being dumped elsewhere.

Like hospital patients, the vital signs of 61 of the trees are measured, including sap and carbon dioxide flow, respiration and temperature, with solar-powered equipment.

"It's like someone was taking measurements of your pulse and breathing every day," said David Galbraith, a professor of terrestrial ecosystem science at Britain's Leeds University and one of the lead researchers of Limit Drought, as he walked through the wired forest.

The team is also tracking tree trunk size and soil data from six-metre (20-foot) deep holes, while nets capture falling leaves. An artificial intelligence-powered drone has made a 3D model of the forest since the project began late last year.

Across the Amazon, climate change, deforestation and degradation are drying forests, and scientists fear parts of the region are being pushed to a tipping point, after which there is no going back - at least not in human timescales.

This corner of the Amazon, considered a transition zone to the nearby Cerrado tropical savannah, is already drier and warmer and sees more intense human activity than other parts.

"The forest here is ... on the frontline of climate change", said Antonio Carlos Lola da Costa, a geosciences professor at Brazil's Federal University of Para who is co-leading Limit Drought.

"There's the advance of large-scale agriculture, and on the other side (is) the Amazon resisting," he said at a forest station managed by research institute IPAM Amazonia.

Record Wildfires

At November's United Nations COP30 climate summit in Brazil's Amazon city of Belem, much of the debate is expected to centre on how to keep the vast rainforest, 60% of which lies in Brazil, within a margin of safety.

Scientists fear that this deforestation, arson and biodiversity loss could turn parts of the Amazon into degraded forests or even savannas - grassy areas with fewer trees.

A study published in the journal Nature last year concluded that 10% to 47% of the forest is at risk of degrading into other ecosystems by 2050.

Large numbers of rotting and burning trees would dump massive amounts of carbon dioxide into the atmosphere, accelerating climate change further.

Already, there are signs the Amazon's resilience is breaking.

The rainforest suffered four severe droughts in the 21st century, helping to fuel record wildfires last year.

An October report from the World Meteorological Organization showed 2024 saw the highest spike in carbon dioxide concentration in the atmosphere since modern measurements began.

That was likely caused by wildfires and the reduced capacity of drought-ridden forests and warmer oceans to function as carbon sinks amid record global temperatures, it said.

Breaking the forest

For the last 3,000 years, Amazon forests rapidly expanded towards the south, until the 1970s, when Brazil's military regime launched road-building and colonisation programmes, and deforestation rates exploded.

"Anyone could get land in Brazil, as long as they burned the forest down and occupied it," said Ben Hur Marimon Jr., a Mato Grosso State University researcher who is co-leading Limit Drought.

Ben Hur and his wife and fellow ecology professor Beatriz Marimon have been measuring forest plots in this frontier of deforestation for three decades.

"We started measurements in 1994, and 15 years later, we became scared because so many trees started to die," said Beatriz, another lead researcher at Limit Drought.

Data gathered by the couple was published in a 2020 paper in Nature and showed the southeastern region had the highest tree mortality rate in the Amazon.

Drier rainforest?

In the Nova Xavantina municipality, the entrance to one of the forest plots the Marimons have measured since the 1990s is marked with a bullet-riddled plaque warning, "Fishing and hunting are prohibited."

Inside, Ben Hur points to a tree more than 30 metres high whose trunk is starting to rot.

"That one is over 300 years old, and it won't last another century. These are the first ones to die," he said. Nearby, another giant was dead and leafless.

One year of extreme drought can make a tree vulnerable to termites or other maladies, increasing the chances it dies in the next drought, Beatriz said.

The interplay between worsening drought and human-ignited wildfires is also key, including last year when the expanse of burnt Amazon forests was double the previous record in 40 years of data collected by mapping consortium MapBiomas.

But recent research may offer some cause for hope.

Another drying experiment in the north Amazon showed a third of biomass was lost in the first 15 years, but that tree death then stabilised as the forest adapted to the new reality - with less canopy cover and shorter trees - but still a forest.

Limit Drought's results may differ as the experiment is being held in the hotter, drier southeast in an area fragmented by soybean fields and cattle pastures.

Those activities pose a more immediate threat to the area than global climate change, said Beatriz Marimon.

"People are so focused on a climate tipping point that they forget about the human tipping point: the crawler tractor that tears everything down", she said.

"Not in 1,000 years will a tractored forest be the same."