Cooling it down: tree rings reveal history of Winter temperatures

Did you know tree rings are like a history book of the Earth's climate?

Tree rings are sensitive to local climate conditions and have been used by scientists to track how summer temperatures have changed over the past millennium, but new research at CQUniversity has uncovered a link to the earth's past winter temperatures for the past 1,700 years.

The study was undertaken by a collaborative group of dendrochronologists, who specialise in the science of tree-ring dating.

The team consisted of CQUniversity Environmental Science lecturer Associate Professor Nathan English and Bachelor of Science (Honours) student Andrew Adriaanse Tucker, alongside University of Tasmania researcher Dr Kathryn Allen.

“Winter temperatures play a role in crop schedules, plant flowering, migratory patterns of animals, and snowpack formation and melting,” Assoc Prof English said.

“Much of our understanding of climate change is based on tree rings that respond to growing season or summer temperatures, so we might have missed a lot of information about changing winter temperatures.

“Currently, there are only two other winter temperature records in the Southern Hemisphere, one in South America and one in Australia but it’s very short -- less than 200 years of record."

To collect the new tree ring samples, the researchers helicoptered into a remote site in the southern range of Tasmania.

Over seven rainy days, they collected 150 cores from dead and living King Billy pine by hand.

Associate Professor English explained that the earlywood, latewood and the whole-width from these tree ring samples provide a valuable temperature proxy in Tasmania and across Australia.

“Trees and tree rings grow faster in the spring. During this period, the ring tends to be very light-coloured and has thinner cell walls. This is referred to as earlywood,” he said.

“At the end of summer, as water is scarce and temperatures are hotter, the trees start to shut down and the cells get smaller, thicker-walled and darker. This is the latewood because it is grown late in the season.

“Unlike other trees in Australia, the width of early wood in King Billy pine tree rings varies with winter temperatures in southern Australia.”

The team transported the core samples back to the lab where they were sanded smooth and measured to within a micron of their width.

“Each ring was assigned a year by comparing them with other cores and correlated with monthly temperature records of the same year,” Assoc Prof English said.

“Combined with other work on King Billy pines, we can use that information to reconstruct winter temperature changes over the past 1700 years.”

Associate Professor English explained that further research is needed to reconstruct cool season temperatures and add context to modern anthropogenic warming.

"Our next step will be to look at other cores from Tasmania, measure their earlywood and then reconstruct winter temperature variability as far back in time as the cores will allow us, hopefully back to about 1200CE,” he said.

“This will provide a good idea of the natural variability of winter temperatures before humans began to impact climate.

“It will also provide important context as to what we might expect winter temperatures in southern Australia to do over the next 100 years.”

The research was funded by CQUniversity, Australian Research Council and the National Geographic Society Committee for Research and Exploration.

Findings have been published as an Open Access article titled "Geographical variation in cool and warm season responses of earlywood and latewood tree-ring chronologies in Athrotaxis selaginoides: https://doi.org/10.1002/jqs.3514