by Susan J. Tweit
Say the word “dendrochronology,” and what comes to mind? Perhaps a tree cross-section showing concentric circles of annual rings, with arrows pointing to the rings from years that mark historical human events. But there is so much more to the science of studying “tree time” (dendro + chronos) than just counting rings to calculate a tree’s age.
Dendrochronology began in 1904 when astronomer Andrew Ellicott Douglass sawed rounds from ponderosa pine logs at a Flagstaff, Arizona, log yard to analyze the patterns in their growth rings. Douglass theorized that, since the growth of ponderosa pines in the Southwest is limited largely by available moisture, the width of the rings would reveal past climate patterns with narrow rings indicating drought and wide rings, wet periods.
He was searching for a correlation between climate and sun spots; what he found was a rich climate history stretching back 1,000 years. Douglass’ tree-slices started the world’s largest dendrochonology library at the Laboratory of Tree-Ring Research at the University of Arizona.
NOW AND THEN
Today, researchers usually sample trees with an increment borer laboriously screwed into the tree’s heart, rather than a saw. And what they find in reading cores from around the world (except in the tropics, where trees don’t accumulate tissue in annual rings) reveals trees as super-sensitive recorders of their environment. Whether looked at on the cellular level or the macro level, tree-ring data is yielding fascinating new stories explaining our planet’s history, human events, and future perils.
Take the fall of the Roman Empire. Dendrochronologist Valerie Trouet, professor at the Laboratory of Tree-Ring Research, says that European tree ring data reveals three centuries of climate instability (exceptionally warm and dry periods alternating with cold and wet ones) between A.D. 250 and 550. Those climate fluctuations, she points out, would have depressed farming and food supplies, contributing to the empire’s collapse.
Over in Central Asia, the tree-ring story shows that the period between 1211 and 1225, when Genghis Khan and his cavalry conquered Asia, was wetter than any time in the previous 1,000 years. That bounty of precipitation, Trouet theorizes, likely gave Khan’s cavalry a boost by dramatically increasing the productivity of the region’s semi-arid grasslands.
Here in North America, Park Williams, professor of Geography at UCLA, analyzed the tree-ring evidence of four mega-droughts in the Southwest during the 900s, 1200s, and 1500s, periods that he characterizes as “phenomenally dry conditions lasting for decades or even centuries, unlike anything we have experienced recently.” That is, until the Southwest’s extremely dry years from 2000 through 2018.
When Williams compared the pattern of soil-moisture loss for those recent dry years to the onset of the medieval mega-droughts, the tree-ring data showed a troubling similarity: the sharp decline in soil moisture in the modern data was more severe than that of all but one of the mega-droughts, he says. Hence the shrinking Colorado River and its impacts on power generation and surface-water supplies.
PEERING INTO THE RINGS
Researchers are also looking beneath the surface of tree cores. Trouet and her colleagues examined the density of wood in tree rings to parse the movements of Earth’s jet streams, those waving rivers of wind powered by Earth’s rotation that separate cold arctic air from warm tropical air. First, the researchers trekked to remote forests in the Balkans and Scotland, increment borers in hand, to sample the oldest trees in each area.
After constructing a 1,000-year-long tree-ring chronology, they calculated the thickness of the cell walls to see if it correlated with opposing poles of summer temperatures in the region. It did. Density measurements showed when Scottish summers were anomalously warm and the Balkans shivered because the North Atlantic jet stream had oscillated north, and when the jet stream had strayed south, chilling Scotland’s summer and searing the Balkans. Reading the past in the tree-ring record may help us predict climate change — which regions might be sizzling in heat waves and apocalyptic forest fires or buried in “snowmageddons.”
At the University of Texas, a conversation with a colleague over coffee inspired geologist Dr. Claudia Mora, dean of the College of Geosciences, to examine stable isotopes of oxygen in tree-ring samples as a way of revealing the source of a tree’s drinking water. By analyzing the ratio of O-16 to O-18 in wafer-thin samples of tree cores from the Southeast, Mora showed when trees were slurping water from tropical cyclones (richer in O-16 due to repeated evaporation and condensation) or parched in droughts (when water loses O-16 to soil evaporation).
LIKE A HURRICANE
As befitting a field of tree science invented by an astronomer, dendrochronology research is often interdisciplinary. A recent project to map hurricane patterns before written records involved dendrochronologist Trouet, paleotempestologist Grant Harley, and dendroarcheologist Marta Domínguez-Delmás, with later input from a historian of piracy.
By analyzing Harley’s data correlating ring patterns in slash pine trees on Florida’s Big Pine Key to hurricane occurrence along with Domínguez-Delmás’ work wreck-diving to retrieve preserved timbers for dating, the researchers modeled hurricane activity stretching back to 1495.
A lull in both shipwrecks and hurricane activity between 1645 and 1715 neatly overlapped with the Maunder Minimum, a period known for low sunspot activity and, thus, less solar radiation — which meant cooler sea-surface temperatures, fewer hurricanes, and fewer shipwrecks. That gap also correlated with the Golden Age of Piracy, when pirates such as Anne Bonny and Blackbeard roamed the Caribbean. Without hurricanes, they could freely plunder treasure-laden Spanish ships returning from the Americas.
Tree rings clearly have a lot to teach us. The more carefully we read the rings, the more we learn about our planet’s past and our potential future.
A plant ecologist, Susan J. Tweit began her career studying grizzly bear habitat, mapping historic wildfires, and researching big sagebrush while working for the U.S. Forest Service in Wyoming. She turned to writing after realizing that she loved the stories behind the data more than collecting those data. Tweit has written 13 books on the nature of life and our place in it, along with hundreds of magazine articles, newspaper columns, and essays. Her latest book, Bless the Birds: Living With Love in a Time of Dying, won the Sarton Award for memoir and was a finalist for the Colorado Book Awards.
