A tree’s annual growth rings reveal how it has
flourished—or floundered—over time, with the size of
the rings indicating years of health or hardship. But
sometimes nature throws a wrench into the works, and a
tree will form more than one growth ring in a year.
Now, such “false rings,” found in trees along the U.S.
Gulf Coast, have been linked to hurricanes, researchers
report today at a meeting of the American Geophysical
Union. With tree ring records stretching back more than
1000 years, the team is preparing to examine how the
frequency of historic storms compares with our modern,
warming world.
Clay Tucker, a geographer at the University of Alabama,
Tuscaloosa, and his colleagues spent much of 2020 and
this year wading and canoeing through stands of bald
cypress trees across three river basins in coastal
Mississippi, Alabama, and Florida. The team extracted
pencil-width cores of wood from roughly 120 trees—a way
to exhume a tree ring record without hurting the trees,
Tucker says. “A woodpecker would do more damage.”
Back in the laboratory, the researchers examined the
cores under a microscope to look for false rings, which
form when a tree that’s stopped growing for the season
is suddenly kick-started out of dormancy. One common
trigger for a secondary growth spurt is flooding,
Tucker says. “The tree doesn’t know it’s not spring.”
Next, Tucker and his colleagues linked 20 instances of
false rings since 1932 with big floods, as recorded by
stream gauges. Roughly 80% of those “flood years” had
also experienced an accompanying tropical storm or
hurricane, the team reports. That makes sense, Tucker
says, because streamflow is strongly linked to storm-
related rainfall. “Water resources … in the
southeastern United States depend on hurricanes.”
Dave Stahle, a geoscientist at the University of
Arkansas, Fayetteville, who was not involved in the
find, says the strong association between false rings
and storms will help scientists solve a fundamental
question: Has the frequency of hurricanes making
landfall gone up or down over time? Some research
suggests we might see more hurricanes as the climate
warms. But firming up that hypothesis will require a
hurricane record that goes further back in time, Stahle
says.
The new bald cypress record should offer exactly that,
Tucker says. The team is about to start analyzing its
core samples, some of which contain wood more than 1000
years old. The researchers are also looking forward to
combining their measurements with another proxy record
of hurricanes: storm-tossed sediments. Tree rings have
the advantage of being annual, but sediment records
stretch further into the past, Tucker says. “Maybe we
can marry the two.”
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