Amazon forests can survive drought - but it comes at a high cost

The Amazon rainforest appears able to endure decades of intense drought, yet doing so would mean losing many of its tallest, most carbon-laden trees and releasing vast amounts of stored carbon into the air.

Scientists from the University of Edinburgh, the Federal University of Pará, and partner institutes reached this conclusion after running the world’s longest rainforest drought experiment in a 22-year study.

Tree mortality and carbon storage

Starting in 2002, researchers covered a one-hectare plot in north-eastern Brazil with clear plastic panels that diverted roughly half the rainfall into gutters.

The site, about the size of London’s Trafalgar Square, was left otherwise undisturbed so the team could watch how a mature forest coped with a sharply reduced water budget.

During the first fifteen years, the canopy thinned dramatically as many of the plot’s largest trees died. Their deaths removed more than one-third of the stand’s total woody biomass – the trunks, branches, and roots where carbon is locked away.

Such a mass drought across the two-million-square-mile Amazon would send a pulse of carbon skyward and hobble the region’s role as a global carbon sink.

A fragile new equilibrium

After the initial crash, conditions began to stabilize. With fewer giants competing for resources, surviving trees had greater access to soil moisture. Measurements showed they were no more drought-stressed than trees in a nearby control plot receiving normal rainfall.

The rejuvenated survivors even began making modest carbon gains, though the plot’s total biomass remains far below that of a typical intact rainforest.

“Our findings suggest that while some rainforests may be able to survive prolonged droughts brought on by climate change, their capacity to act as both a vital carbon store and carbon sink could be greatly diminished,” said lead author Pablo Sanchez Martinez, a postdoctoral researcher at the University of Edinburgh.

The experiment demonstrates that the Amazon forests possess a kind of resilience despite drought. However, the path to that resilience is paved with lost giants, reduced biomass, and the sudden release of long-sequestered carbon.

Additional climate change factors

The study isolated one stress: reduced rainfall reaching the soil. Real climate change, however, entails hotter air, shifting humidity, fiercer storms, and more fires.

The authors note that their biomass-loss figures may be conservative once those additional factors are folded in. They also point out a geographical wrinkle.

The experimental site lies in the Amazon’s drier northeast, a region projected to grow even warmer and less rainy. Central and western Amazonia may respond differently. Long-term plots in multiple zones will be needed to paint a complete picture.

Future of drought in the Amazon

Global carbon-cycle models routinely assume that tropical forests will keep soaking up CO2 at current rates. This experiment shows that assumption may be risky. If drought becomes chronic, forests could flip from a net sink to a source until a new, lower-biomass equilibrium is achieved.

“Ecological responses to climate can have very large impacts on our environment, locally and globally; we cannot understand and predict them without long-term collaborative research of this sort,” said senior author Patrick Meir, a professor at the University of Edinburgh.

Climate negotiators often view the Amazon as a safety valve that buys time for emissions cuts. The results here suggest the valve’s capacity is not limitless – and severe drought could crank it closed for decades.

Studying drought in the Amazon

Running a two-decade field manipulation in dense rainforest required physical endurance, a stable funding stream, and local partnerships.

Crews had to keep thousands of plastic panels clear of leaves and fallen branches while monitoring soil moisture, sap flow, canopy structure, and tree deaths year after year.

Researchers plan to layer additional stressors onto similar plots – raising air temperature, altering humidity, or exposing trees to smoke – to mimic the full suite of climate impacts. Remote-sensing satellites and airborne laser scans will help scale plot-level findings to the basin as a whole.

Adaptation comes with costs

In the meantime, the current results deliver a stark message. The Amazon can adapt to a drier future, but the adaptation comes with heavy carbon costs.

Protecting the forest from deforestation and fire remains essential; so does limiting global warming to prevent droughts from growing even more severe.

Only by combining local conservation with rapid emissions cuts, scientists argue, can the world keep the Amazon in its role as a living, breathing ally against climate change – even if that ally is thinner and lighter than before.

Funding came from the UK’s Natural Environment Research Council, the Royal Society, and the UK Met Office. Collaborating institutions included the Universities of Exeter and Cardiff and Spain’s CREAF.

The late professor Antonio Carlos Lôla Da Costa, who co-led the project on the Brazilian side, provided crucial continuity and local knowledge.

The research is published in the journal Nature Ecology and Evolution.

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