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Revealing Historic Winter Climate Using Tree Rings

Fallen tree with tree rings on a bed of grass and leaves.

Counting the number of tree rings tells us how old the tree is but new research reveals how these rings can tell us more about historic winter climates. 

"Dendrochronology is just the study of tree rings. The conventional way, you would look at it as just how wide or narrow the rings are. And that would tell you what happened for most trees - what happened during the growing season," said Steve Voelker, professor in the department of plant, soils and climate at Utah State University.

He said that up until recently we could not use tree rings to understand winter climate.

"The really new thing that we found is that these trees are recording what happens in winter, which usually doesn't happen in trees," Voelker said. "Most trees are just dormant when it's really cold out in the winter, and so that's why they don't record the winter. The unique thing that we found is that by sampling next to Lake Superior that provided a unique climate where the spring and early summer conditions largely reflect the previous winter."

Lake Superior holds 600 times more water than the Great Salt Lake. This large body of water takes a long time to heat up, so in the spring the lake is the same temperature as the previous winter. According to Voelker trees that grow along the edge of Lake Superior in the spring develop spring tree rings that are actually responding to winter temperatures.

"Basically we used stable isotopes of carbon and oxygen," he said. "So these are chemical signatures imprinted on the tree rings and these signatures, arise because heavy stable isotopes of an element basically have one or two extra neutrons. And so there, are still the same element but they respond differently to the environment. And that allows us to look at the ratio of the heavy to light stable isotopes and understand what environmental cues that is a recording back in time in the tree rings."

According to Voelker, by using tree ring signatures we can expand our winter climate history back to when we experienced colder winters 500 years ago. Climatologists predict that due to climate change, the record-breaking polar vortex the Midwest experienced this past winter will become more common. And tree ring signatures of historic winters could help governments better plan for colder winters in the future.