Scientists working in the central Brazilian Amazon have conducted a large-scale field experiment, installing 6,000 plastic panels over a one-hectare plot to simulate future drought conditions. The long-term study, part of the wider AmazonFACE (Free-Air CO2 Enrichment) project, aimed to reduce natural rainfall by 50% and has revealed significant impacts on the forest, including a one-third reduction in total living plant biomass.

The experiment, known as the AmazonFACE throughfall exclusion experiment, commenced with data collection in 2013 and saw the panels fully installed by 2015. Located within the Cuieiras Reserve, approximately 60 kilometers north of Manaus, Brazil, the project is led by researchers including Patrick Meir from the University of Edinburgh, Carlos Quesada from the National Institute of Amazonian Research (INPA), and Lucy Rowland from the University of Exeter. Their objective was to understand how mature Amazonian forests might respond to more frequent and intense droughts, a scenario projected by climate models for the region.

Findings from the experimental plot indicate a substantial ecological shift. Over several years, the total living plant material, or biomass, within the panel-covered area declined by 33% compared to adjacent control plots that received natural rainfall. A critical factor in this reduction was the disproportionate mortality of large, older trees, which died at twice the rate observed in control sections of the forest. While some growth was noted among saplings and smaller trees, this new growth was insufficient to offset the biomass lost from the larger, established canopy trees. Researchers attribute these changes primarily to a significant reduction in soil moisture levels under the panels, mimicking prolonged drought stress.

Key details from the experiment:

  • Location: Cuieiras Reserve, 60 km north of Manaus, Brazilian Amazon.
  • Methodology: 6,000 plastic panels installed at canopy height over a 1-hectare plot.
  • Rainfall Reduction Target: Approximately 50% of natural rainfall blocked.
  • Experiment Duration: Panels installed by 2015, with data collected over multiple years.
  • Biomass Impact: Total forest biomass decreased by 33%.
  • Tree Mortality: Large, older trees died at twice the rate of those in control plots.
  • Lead Institutions: University of Edinburgh, National Institute of Amazonian Research (INPA), University of Exeter.

These findings carry significant implications for the Amazon rainforest's long-term health and its role in global climate regulation. The Amazon currently acts as a substantial carbon sink, absorbing vast amounts of carbon dioxide from the atmosphere. The observed reduction in biomass and increased mortality of large trees suggests that future drought conditions could diminish the forest's capacity to store carbon, potentially transforming parts of the biome into a carbon source. This research provides crucial empirical data for refining climate models and informing conservation strategies aimed at protecting the Amazon.

Ongoing monitoring of the experimental site will continue to track the long-term ecological responses and the specific mechanisms driving tree mortality and biomass changes. The broader AmazonFACE project is also simultaneously investigating the combined effects of drought and elevated atmospheric carbon dioxide, aiming to provide a more comprehensive understanding of the Amazon's future under a changing climate. The insights gained from these experiments are considered vital for policymakers and environmental scientists working to mitigate the impacts of climate change on critical ecosystems worldwide.