For the oldest samples, Hessl and lead author Neil Pederson, a tree-ring scientist at Columbia University's Lamont-Doherty Earth Observatory, focused on an unusual clutch of trees found while researching wildfires in Mongolia.
The strand of gnarled, stunted Siberian pines were emerging from cracks in an old solid-rock lava flow in the Khangai Mountains, according to a statement from Columbia.
Trees living in such conditions grow slowly and are particularly sensitive to changes in weather, so they provided an abundance of data to study.
Some of the trees had lived for more than 1,100 years. One piece of wood they found had rings going back to about 650 B.C.
Researchers compared those samples to younger fallen trees and some pieces bored from living trees.
"Through a careful analysis of tree-ring records spanning eleven centuries, the researchers have provided valuable information about a period of great significance," said Tom Baerwald, a program director for the National Science Foundation, which funded the research.