Dr. Magali Nehemy speaks to Cacique Domingos Mundurukú, local elder from Aldeia Bragança, Pará, in the Amazon during a research trip to study how the forest recycles rainfall.
An international research team has found that during the Amazon's dry season, forests rely heavily on recent rainfall stored in shallow soil to keep the region's climate in balance.
Published in Proceedings of the National Academy of Sciences (PNAS) , the UBC Okanagan-led study found that most of the water used by trees in eastern Amazon forests during the dry season comes from the top 50 centimetres of soil-water that fell only weeks or months earlier.
"The Amazon forest produces its own rain by quickly returning water to the atmosphere via transpiration and producing its own rainfall when it needs it the most, during the dry season," says Dr. Magali Nehemy , Assistant Professor of Earth and Environmental Sciences at UBCO.
"Transpiration-water that is returned to the atmosphere by plants-is the largest flux on land. Changing forests changes this process, which in turn impacts rainfall, water availability and the ecosystems that depend on it."
In the Amazon dry season, up to 70 per cent of rainfall can come from this recycled moisture.
Working in Brazil's Tapajós National Forest during the peak of the dry season, Dr. Nehemy's team collected data across two sites: a hilltop forest with a deep water table and a valley forest near a stream where groundwater is shallower.
"The results were surprising," says Dr. Nehemy. "Most of the water used for transpiration in the dry season did not come from deep reserves but from shallow soil. In a year without extreme drought or floods, nearly 70 per cent of transpiration on the hill and nearly half in the valley came from the top 50 centimetres of soil."
The study also shows that a tree's embolism resistance, or how well it moves water through its tissues under drought conditions, explains why some species can keep using this recently fallen rain while others must rely on deeper stores.
"It means the diversity of species and their drought resistance are directly tied to how the forest stabilizes its own climate," says Dr. Nehemy.
For her, the work is not only about physics and roots, but also about people. The field sites sit within the traditional territory of the Munduruku people. Many of the most protected forests left in the Amazon are in Indigenous territories.
The findings have direct implications for climate models and policy.
By linking tree traits, shallow soil water use and dry-season rainfall, the study offers a more mechanistic way to represent Amazon forests in land-surface and climate models, including those used to assess tipping points and water security under continued deforestation and warming.
"In the long term, I would like us to be able to predict how changing vegetation cover shapes water availability via rainfall and climate vulnerability across different areas," says Dr. Nehemy.
"The Amazon really is a rain-making engine. If we weaken the forest's ability to recycle water, we risk weakening the entire hydrological cycle that supports people, ecosystems and agriculture far beyond the forest itself."
