The Great Ginkgo Leaf Dump Is Here
Thanks to climate change, it’s getting a little later every year.
A girl plays in the aftermath of a giant ginkgo dump in Beijing in 2006.Jason Lee / Reuters
ROBINSON MEYER NOV 10, 2017 SCIENCE
The day is finally here. From the northern reaches of New England to the southernmost stretch of the Chesapeake Bay, one of autumn’s most famous performances will take place this weekend.
On Friday or Saturday night, the first hard frost will likely sweep down the coast. Ginkgo trees—known for their fractured, twisted branches and broad, fan-shaped leaves—will react like a surprised burglar and drop all their leaves to the ground at the same time. On city streets lined with ginkgo trees—like my old block in northwest Washington, D.C.— a soft yellow padding will cover everything, erasing the distinction between sidewalk and street. Only the occasional fire hydrant hints at the synthetic understory.
Early on Thursday morning, the ginkgo tree in front of James Hall at the University of New Hampshire got an early start on the act, sloughing its leaves to form a small circle of gold. The event is affectionately anticipated by the school’s department of natural resources, which is housed inside the red brick hall. Since 1977, students and faculty in the department have played a guessing game, trying to anticipate when the tree’s leaves will dump. A box in the building’s foyer entreats students to place their guesses.
“I’ve been in the department for 15 years, and I knew this was going on but didn’t pay too much attention,” says Serita Frey, a soil microbiologist and a professor at the university.“It didn’t have anything to do with climate change at first. Everyone just knew that the ginkgo tree dumps its leaves in one day.”
Why the single-day drop? In the autumn, deciduous trees form a scar between their leaves and stems to protect themselves from diseases and winter’s coming chill. Most flowering trees, like oaks and maples, form the scar at different rates, in different parts of the tree, over the course of weeks. Their leaves then fall off individually. But ginkgoes form the scar across all their stems at once. The first hard frost finishes severing every leaf, and they rain to the ground in unison.
A few years ago, Frey became curious about whether there was data documenting the ginkgo-dump day over the years. According to the National Climatic Data Center, fall temperatures in New Hampshire are now more than 2 degrees Fahrenheit warmer than they were in the late 1970s. Did that affect the flagship ginkgo tree? Had someone even kept records about the contest that far back?
The aftermath of a ginkgo dump in Shanghai (Aly Song / Reuters)
For the first few days of her search, she couldn’t locate the record. “And then I found a piece of paper with handwriting on it that some secretary back in 1977 started, and that someone from the department had been adding to every year,” she said. “I put all that information in a spreadsheet, and I’ve been updating that graph every year since.”
The graph revealed that the ginkgo-dump day had been sliding forward over the ensuing decades. Every decade, the ginkgo tree loses its leaves an average of three days later than it had 10 years prior. When the James Hall ginkgo dumped its leaves this Thursday, November 9, it was the second-latest that the tree had ever hit the autumn milestone.
When Did the James Hall Ginkgo Lose Its Leaves?
Data was collected by the University of New Hampshire’s students and staff since 1977. (Courtesy of Serita Frey)
“It’s our poster tree—our local example of climate change,” she told me. Frey uses the slide in her classes about global warming.
The James Hall ginkgo is not, of course, the only organism subject to the creeping relocation of the seasons. In 2016, the U.S. National Park Service examined when the first leaf or first bloom of spring arrived in 276 of its parks. In three-quarters of parks, spring was arriving earlier than it had in the past; in half of the parks, spring now arrives earlier than it did in 95 percent of the years since 1901.
Nor is the story of the James Hall ginkgo data trove unique. About two decades ago, climate scientists realized that springtime bloom records were some of the longest-running observations of the climate system available. By examining and digitizing old records, they have found:
that a certain type of oak in Surrey, U.K., blooms a month earlier now than it did in the 1950s;
that European birds now begin their southerly migration later than they did 40 years ago;
that American aspens bloomed 26 days earlier at the end of the 20th century than they did at the beginning;
and that Swiss horse-chestnut trees bloom 40 days earlier than they did 200 years ago.
The most impressive of these seasonal records exists in Japan, where an environmental scientist has compiled the date of the first cherry-blossom bloom in the city of Kyoto, going back to 800 BCE. That study found that cherry blossoms in that city now flower earlier than they have in almost 1,000 years.
But all these measurements examine the first (and often advancing) appearance of spring. Frey’s ginkgo-tree record looks at the often-regressing signature of autumn. The ginkgo-leaf rain is also a seasonal symbol that appears across the East Coast: I have seen great-leafed ginkgoes sprout on small farms, on suburban streets, and in tiny downtown Manhattan parks. Urban arborists often plant ginkgoes because they are more resistant to pollutants and pesticides than other trees. And why not? Ginkgoes have persevered in something close to their current form since dinosaurs walked the Earth.
Not that everyone loves the tree’s annual autumnal transformation. Along with their leaves, ginkgoes also dump their big pungent berries, which split and rot on the pavement. A frustrated New Yorker once complained that they reeked of “boiled egg ****s.”
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