‘False’ tree rings could provide a new record of long-ago hurricanes | Science

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 to our modern, warming world.

Clay Tucker, a geographer at the University of Alabama, 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.”