The samples from lowland tropical cedars provided a natural archive of rainfall data scientists called a key tool to plotting the natural variation in the region's climate system, the BBC reported Tuesday.
The cedar species examined has shallow roots and is more dependent on water gathered from rainfall that seeps into topsoil, making the trees a good indicator of rainfall, the researchers said.
Before now rainfall data in the region was spotty and only went back over the past 50-60 years, they said.
"Climate models vary widely in their predictions for the Amazon, and we still do not know whether the Amazon will become wetter or dryer in a warmer world," co-author Manuel Gloor from the University of Leeds in Britain said.
The eight trees from one site revealed not just how much it rained at that site but over the entire Amazon catchment, researchers said.
"The record is so sensitive, we can say what year we are looking at," Roel Brienen from the university's school of geography said.
The tree rings are very closely related to annual variation in the river levels of the Amazon and thus the amount of rainfall that flowed into the oceans, he said.
"For example, the extreme El Nino year of 1925-26, which caused very low river levels, clearly stands out in the record."