When all of Belgium tunes in to watch the Tour of Flanders bicycle race, Pieter De Frenne watches the trees.
They’re only on screen for as long as it takes the peloton to whiz by, but it’s enough for De Frenne to recognize them, log them, and gather a tiny insight into how the planet is changing. That’s because De Frenne is a pro cyclist in his dreams and a pro botanist in real life. And along with a team from the University of Ghent, he figured out that somewhere in Belgium’s annual festival of sore legs, cobbled hills and heady beers, there was climate science to be done.
It turns out that three decades’ worth of footage from the Tour of Flanders, whose 105th edition is this Sunday, contained a trove of botany data. By identifying specific trees along key points of the race route, De Frenne’s team of researchers was able to log their leaf cover, April after April, and see how dozens of species were reacting to climate change.
“There are not many professional sports that are displayed on television annually, on the same routes, in the same places,” De Frenne said. “And it’s also exactly the right time, April, when the trees start to flush their leaves.”
If the Tour of Flanders were in June, the bike race might be worthless to botanists. But because it lands at this precise moment of spring, it became clear to his team that plant life was blooming earlier in the year as temperatures crept up. The work was as much about counting leaves as it was about pioneering this innovative approach, which De Frenne and his team laid out in a 2018 paper published in the British Ecological Society’s Methods in Ecology and Evolution.
The idea blossomed in De Frenne’s mind because, like any good Belgian cycling fan, he spends every spring gorging on bike races, bike-race highlights, and bike-race archival footage anyway.
Two episodes stuck out. The first was the 1980 edition of Liege-Bastogne-Liege. The late-spring race through the Ardennes forest turned so snowy that year that winner Bernard Hinault permanently lost feeling in at least one finger. (That edition is still known as Neige-Bastogne-Neige, or “Snow-Bastogne-Snow.”) The second was also a late-spring race, this time in Corsica in 2010, and the flowers bloomed so aggressively that Spanish champion Alberto Contador fell apart with allergies.
Though the incidents were three decades and 600 miles apart, the conditions were so wildly different that De Frenne’s mind started spinning. It was well documented that European springs were trending warmer and impacting plant life. De Frenne wondered if he could study that trend by doing his favorite thing in the world: watching pro cycling from his couch.
He found the answer practically on his doorstep. The Tour of Flanders, which finishes about half an hour from Ghent, turned out to be the ideal race to examine. Not only was there a wealth of archival footage, but the race is always held within the same 7-day window on the calendar along a route that hardly changes. Even in years without full race tape, De Frenne and his team knew they could count on the highlight reels to show the course’s famous cobblestone climbs. All he needed to do was head into the field, choose some trees that had been there for decades—the team settled on 46 different trees and shrubs—and start looking for them in the tape.
For simplicity’s sake, De Frenne started with 1981, which still left him with more than 200 hours of film. In those first few years, he found, the trees by the side of the road were mostly devoid of leaves in early April. “Whereas more recently, almost all the trees have already 100% of them,” he added. “The proportion of trees having flowers or leaves has significantly increased.”
De Frenne attributes the change to consistently milder winters in Northern Belgium and a general temperature increase in the area of 1.5 degrees Celsius on the corresponding dates since 1980. But if the local flora is behaving differently, the local fauna—Flandrien cyclists—hasn’t noticed.
“There have been some really cold years in my career and some real warm ones,” the veteran rider and Flanders native Greg van Avermaet said. “You just hope you get lucky.”
The real breakthrough in the marriage of climate science and pro cycling was the methodology. Most long-term studies of plant behavior are conducted in the carefully tended gardens of monasteries or stately homes—possibly the only locales more stereotypically European than bike races.
“They went out every year and noted the dates of the first leaf, for example, sometimes for centuries,” De Frenne said. “Of course those dates are not available for rarer species or species that were introduced later.”
Others have attempted to track changes in the colors of bears and owls by using geo-tagged photos uploaded by tourists. The trees of Flanders are easier to keep track of than bears and owls. But the truth is that even if the landscape were barren, De Frenne would be watching.
“When I was young, it was not really my dream to be a scientist,” he said. “I wanted to be Lance Armstrong or Miguel Indurain. I became interested in trees later.”
Write to Joshua Robinson at Joshua.Robinson@wsj.com
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