To tell the age of a tree, you count its rings. Most people know that. But few people know that the rings in trees can tell us much more than just a tree’s age. And perhaps even fewer know that there is a field of science dedicated to the study of tree rings. Dendrochronology, from the Greek words for “tree” (dendros) and “time” (chronos), uses tree rings to study the histories of forests, the climate, and human civilizations, as well as the links among them. My upcoming book, Tree Story: The History of the World Written in Rings, explores the insights generated by dendrochronology.
Trees in temperate regions, where growth during summer months is followed by dormancy in winter, form a ring in their stems each and every year. The reliability of the yearly ring formation yields an opportunity to estimate a tree’s age from its number of rings. But not all rings in a tree are equal: some rings are wider than others. This year-to-year variability in the width of rings depends largely on fluctuations in the climate in which the tree grows; in growth-conducive years with lots of rainfall and warm summers, the tree will form a wide ring. In years when there is a drought or a cold spell, the tree grows more slowly and forms a narrow ring. Over time, the tree will form a sequence of wide and narrow rings that is like a long string of secret code. The older the tree, the longer the hidden message.
Dendrochronologists can double-check that they assigned each ring to the correct calendar year by comparing tree-ring sequences of different trees, a process called cross dating. And when studying the climate of the past, researchers can combine the sequences of many trees to construct a robust model of which years were good and bad, wet and dry, over the lifespans of centuries-old trees. In 2015, for instance, my team used tree-ring sequences from more than 1,500 blue oaks (Quercus douglasii) in California’s Central Valley to reconstruct snowpack variability in the Sierra Nevada over the past 500 years. Blue oaks are some of the most faithful recorders of rainfall and snowfall history anywhere on the planet; not a dry winter has happened in California over the past 500 years without producing a narrow ring in blue oak ring sequences.
Luckily, we did not have to cut down a single tree for this project. The standard tool of a dendrochronologist is a hollow corer, about a quarter inch in diameter, which is used to extract tree-ring samples from the tree’s stem. You can compare the process to a biopsy; it does not harm the tree at all. In April 2015, in the fourth year of the 2012–2016 California drought, we heard that the 2015 snowpack was the lowest it had been since they started measuring snow in the Sierra Nevada in 1930. We decided to use the blue oak tree-ring data to put the drought in longer-term context. We found that the 2015 snowpack was not just a record low over the past 85 years; it was a record low over the past 500 years.
This result left me and my coauthors with mixed feelings. We realized that our dedication had paid off—we had learned something that was important to the scientific community, to Californians, and to the state’s policymakers. But we all knew very well that finding a 500-year record low is rarely a good thing, and certainly not when it comes to the natural storage system responsible for providing 30 percent of California’s water supply.
The unprecedented nature of the 2015 snow drought is also a harbinger of things to come: with anthropogenic climate change continuing to accelerate, it is likely that such lows will occur more frequently in the future. As such, tree rings give us not only a window into the past, but also an opportunity to learn from history as we move forward in a changing climate. γ
Valerie Trouet is an associate professor at the Laboratory of Tree-Ring Research at the University of Arizona. Read an excerpt of Tree Story: The History of the World Written in Rings. Follow her on Twitter @epispheric.