Bolivian Tree Rings Provide Archive of Amazon Rainfall

Rings from just eight lowland cedar trees in Bolivia have produced a century-long record of rainfall patterns across the entire Amazon basin.

Led by the University of Leeds in England, an international team of researchers measured the variation in two oxygen isotopes trapped in the wood’s rings—oxygen-16 and oxygen-18—to find out how much it had rained in the Amazon during the last 100 years.

“That is an area about 25 times the size of the United Kingdom,” said study lead author Roel Brienen in a press release.

“The isotope values recorded in tree rings were very closely related to annual variation in the river levels of the Amazon, and thus of the amount of rainfall that flows into the ocean.”

The Amazon basin is one of the wettest places on Earth, taking up a huge area along the equator. The response of its hydrological cycle to climate change is therefore relevant to the rest of the world.

“In a similar way that annual layers in polar ice sheets have been used to study past temperatures, we are now able to use tree rings of this species as a natural archive for precipitation over the Amazon basin,” said study co-author Manuel Gloor in the release.

“If we find older trees with similar signal strength then this will greatly help us to advance our knowledge of the system.”

Various trends can be seen in the record provided by the trees, and it is so sensitive that actual years can be pinpointed.

“For example, the extreme El Niño year of 1925-26 which caused very low river levels, clearly stands out in the record,” Brienen said.

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