Amazon Could Withstand Prolonged Drought, But Only at Great Ecological Expense

The Amazon rainforest, often regarded as the lungs of the planet due to its vast capacity to absorb carbon dioxide, faces an uncertain future under the mounting pressure of climate change. Recent research conducted over more than two decades in the northeastern region of the Amazon offers critical insights into how this sprawling biome might respond to prolonged drought conditions that climate science predicts will become increasingly common. This groundbreaking long-term experiment reveals the complex interplay between tree mortality, biomass loss, and ecosystem resilience in a warming and drying Amazon, with profound implications for the global carbon cycle.

Spanning over a hectare in size, equivalent to a small city square, the experimental plot in northeastern Brazil has been subjected to artificially induced drought since 2002. Thousands of transparent panels were installed above the forest floor to divert about half of the natural rainfall away from the trees, thereby simulating an extended period of soil moisture deficit. This setup, unique in its length and realism, has allowed researchers to meticulously monitor physiological stress, tree mortality, and changes in biomass over 22 years, enabling unprecedented observations into how tropical rainforests adapt, suffer, and possibly recover under drought stress.

One of the most striking findings is the catastrophic mortality of large trees during the first 15 years of the experiment. These towering individuals, which constitute the bulk of above-ground biomass and carbon storage, were highly vulnerable to the prolonged water scarcity. Their widespread death caused the plot to lose over one-third of its total biomass, which includes trunks, branches, stems, and roots. The ramifications of such biomass loss extend beyond the study site itself; if replicated at the Amazon scale, this would mean releasing an immense amount of stored carbon back into the atmosphere, thus accelerating global warming and diminishing the rainforest’s capacity as a significant carbon sink.

Following this initial wave of die-off, researchers observed a surprising shift in ecosystem dynamics. The mortality rate declined sharply, and the surviving trees exhibited less drought stress during the subsequent seven years despite the persistent soil dryness. This suggests that the forest adjusted to the new environmental baseline, where reduced competition for the limited water resources allowed remaining individuals to better sustain themselves. The forest may thus demonstrate a degree of resilience, albeit with a markedly altered structure and function.

Nonetheless, the altered forest is not a return to its previous state. The biomass remains substantially lower than in typical undisturbed Amazonian rainforest, though still higher than in many dry forests or savannas. This durable state of reduced biomass indicates a novel equilibrium that may persist as long as drought conditions continue, signaling a “new normal” for the region’s carbon dynamics. This long-term stability masks the initial ecological cost—the loss of large carbon-dense trees—which the forest may never fully recover from within human timescales.

It is important to note that the experimental drought primarily targeted soil moisture reductions; atmospheric factors such as humidity, air temperature, and their interaction with plant physiological processes were not manipulated. As such, the study’s findings likely underestimate the full suite of climate change effects that the Amazon will endure. Increasing temperatures and vapor pressure deficits can exacerbate drought stress by intensifying evapotranspiration demands, potentially driving tree mortality beyond the levels observed in the study.

Add to this the increasing prevalence of compounding extremes, such as severe storms, wildfires, and land-use pressures, and the outlook becomes yet more complex and concerning. These factors can act synergistically with climate-induced drought, amplifying forest degradation and impairing regeneration pathways. The loss of canopy cover from dead trees may also alter microclimates beneath, influencing species composition and ecosystem functions in ways that remain poorly understood.

From a carbon cycling perspective, the transition toward a lower-biomass ecosystem carries worrying consequences. With fewer large trees to sequester atmospheric CO2, the Amazon’s role as a global carbon sink diminishes, potentially converting it into a carbon source under sustained drought and warming scenarios. This shift could accelerate the pace of climate change by feeding back into atmospheric greenhouse gas concentrations. Considering the vast area covered by Amazon rainforests—more than two million square miles—such changes bear enormous implications for global climate regulation.

While the survival of some trees and the stabilization phase post-mortality highlight an unexpected element of resilience, it is far from a cause for complacency. The biome’s reduced capacity to store carbon puts increasing importance on protecting currently intact forests and addressing anthropogenic pressures such as deforestation and fragmentation. Long-term and large-scale monitoring, coupled with mechanistic ecosystem models, are critical for forecasting future trajectories and informing adaptive conservation efforts.

The ongoing study represents a collaboration between leading ecological and climate research institutions, including the University of Edinburgh and the Federal University of Pará, among others. Supported by major research bodies such as the Natural Environment Research Council and the UK Met Office Newton Fund, this research underscores the necessity of long-term ecological experiments to unravel the complexities of climate change impacts on vital ecosystems like the Amazon rainforest.

In summarizing, the findings elucidate that while the Amazon’s rainforests may survive prolonged drought, the environmental toll is immense. The mortality of large trees not only recycles billions of tons of carbon into the atmosphere but also transforms the forest’s structure and function for decades or longer. The apparent resilience lies in a compromised state, one that demands urgent attention from the global community to mitigate further destabilization and preserve the climate regulation services that the Amazon uniquely offers.

This research highlights a critical need to integrate studies of soil moisture deficits with atmospheric stressors, landscape disturbance regimes, and biotic feedbacks to fully grasp the Amazon’s future. Only through such comprehensive understanding can policymakers and conservationists devise strategies to support forest health in a rapidly changing world, ensuring that this iconic biome continues to play its pivotal role within the Earth system.

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References: Study published in Nature Ecology & Evolution, University of Edinburgh and Federal University of Pará-led research