‘Tree Story’ explores what tree rings can tell us about the past
Once you look at trees
through the eyes of a dendrochronologist, you never quite see the leafy wonders
the same way again. Peel away the hard, rough bark and there is a living
document, history recorded in rings of wood cells. Each tree ring pattern of
growth is unique, as the width of a ring depends on how much water was
available that year. By comparing and compiling databases of these
“fingerprints” from many different trees in many different parts of the world,
scientists can peer into past climates, past ecosystems and even past
civilizations.
Humans’ and trees’
histories have long been intertwined. In her new book Tree Story,
tree ring researcher Valerie Trouet examines this shared past as she describes
the curious, convoluted history of dendrochronology. It’s a field that was born
a little over a century ago, almost as a hobby for an astronomer at the
University of Arizona.
Andrew Douglass was
interested in tree rings for what they might tell him about how past solar
cycles influenced Earth’s climate. He began amassing a tree ring collection
dating back to the mid-15th century. Then Douglass began examining an even
older source of data: ancient wooden beams from Puebloan ruins in the U.S.
Southwest. By linking the patterns in the beams to his own tree ring samples,
he created a long chronological history for the region — and so the science of
dendrochronology was born. Through this new dating technique, Douglass also
solved a long-standing mystery, calculating ages for the different Puebloan
sites ranging from the 10th to the 14th century.
Trees rings have
documented other pivotal moments in human history, Trouet explains. Unusually
wet years from 1211 to 1225 may have given a boost to grasses in central Asia’s
steppe — fodder for Genghis Khan’s mounted forces and key to the rapid
expansion of the Mongol Empire. The 1986 Chernobyl nuclear power plant accident
left its mark in the strangely aligned wood cells of surviving pine trees. Wood
patterns in a violin crafted by Antonio Stradivari (and worth an estimated $20
million) authenticated not only the violin’s age but its geographic origins.
Tree ring data spanning
over 1,000 years was also instrumental in helping scientists reconstruct the
planet’s recent climate history and in highlighting
the dramatic warming observed in the last century.
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Tree rings preserve clues about past environments. A wide ring, for example, records a rainy year; a thin ring corresponds to a dry one. Even forest fires leave telltale marks.C. CHANG |
Trouet, a member of the
University of Arizona’s Laboratory of Tree-Ring Research in Tucson, is a
dendroclimatologist; she uses tree rings to study Earth’s past climate. She
tells of “the thrill of the chase” to find the oldest, least disturbed trees on
Earth, with circular rings and growth related only to changes in climate. These
trees have helped her identify, for example, periods of medieval drought in
northern Africa that are linked to a large-scale weather pattern known as the
North Atlantic Oscillation — also the probable reason for a historically documented
period of warmth in Europe known as the Medieval Climate Anomaly, she suggests.
Now, she and colleagues
are examining tree rings from Europe to trace how the high-speed jet stream
winds that encircle the Northern Hemisphere have shifted over time. The
waviness of the jet stream — how far south these winds might dip and curl — is
linked to patterns of storms across the northern latitudes. Understanding those
links in the past, Trouet argues, could provide clues to how storminess may
change in the future, as the planet’s climate changes.
Tree Story gives readers a
lively, sometimes visceral feel for Trouet’s work. She describes the beauty of
tiny wood cells smaller in di-ameter than a human hair, and the elbow grease
involved in manually twisting a borer into the heart of a tree to retrieve a
sample. “This requires quite a bit of upper-body strength, especially if you’re
coring dozens of trees a day, and this often comes as a surprise to dendro
newbies.” Trouet’s humor also comes through when she describes how fieldwork is
sometimes driven by testosterone–fueled stubbornness, and how she has had to
convince male colleagues hunting for trees in the mountains that it’s OK to
admit to being tired, hungry or cold. “As a woman scientist, I got 99 problems,
but at least starving or freezing to death to protect my ego ain’t one.”
Peppered throughout the
book are italicized terms and helpful definitions of scientific jargon such as
“crossdating” (matching ring patterns among different trees, whether alive or
dead, to create a consistent chronology). I particularly enjoyed getting a
glimpse into odd tree ring lingo: To “hit the pith” is to core all the way to
the oldest part of a tree; “cookies” are the round cross sections of a fallen
trunk, cut with a chainsaw or an ax.
Trouet loves trees, but
she says she is not a tree-hugger, nor does she believe trees are sentient.
Instead, she is drawn to unlocking the secrets the trees contain. “Wood is
gorgeous,” she writes. “And finding matching tree ring patterns is like solving
a puzzle — it is addictive.”
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