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03.2020 | THE SCIENTIST 59

READING FRAMES

T

o 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 d r y, 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. Yo u 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.g

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 at the-scientist.com.

Johns Hopkins University Press,

April 2020

Counting growth rings in trees can tell us about

a lot more than the plant’s age.

BY VALERIE TROUET

The Wisdom of Rings