If you were surprised by how late leaves made their autumn transformation this year, don’t feel bad; even experts struggle these days. Susanne Renner, an honorary professor of biology at Washington University, drove to Vermont in early October hoping to see peak color — only to realize she’d arrived too early.
But Renner has become convinced that the current thinking about leaf color is incorrect. The idea that climate change is pushing local leaf color further and further back on the calendar isn’t true, she said.
“This year is not exceptionally late if you compare it over the past 20 years,” she said. “Over the past 20 years in St. Louis, there's no statistical change in the onset of the peak fall foliage.”
And that’s because, she said, it’s not temperatures that affect when leaves turn. It’s hours of daylight.
Despite what you may have heard in elementary school, Renner said that leaves turning bright autumn hues is not merely a byproduct of them converting chlorophyll to the nutrients a tree needs to get through the winter. It’s actually a way of protecting themselves and eking out as much chlorophyll as they possibly can.
“These colors — the yellow, the red — are a kind of sunscreen, and they protect the cells in which chlorophyll is being broken down and the tree is doing everything it can to get the nutrients out before it will shed those leaves,” she explained on Tuesday’s St. Louis on the Air. “And so protecting these cells in a sensitive stage when a lot is already broken down is very important.”
Evolutionarily speaking, Renner said, it would be foolish for leaves to drop based merely on temperature; the occasional icy blast from Canada could wreak havoc on a tree’s ability to harvest what it needs. The amount of daylight is much more reliable, and much less likely to see major swings from year to year. That’s true even as warmer temperatures lead to earlier spring budding, she said.
And that’s not the only reason Renner disputes some models suggesting climate change will wreak havoc on fall’s onset. In a paper published in Science last November, Renner and her co-authors suggested that elevated carbon dioxide levels could actually cause leaves to change colors earlier.
And so while some experts have posited that by the end of this century, autumn leaf colors could show up as many as three weeks later than what we see today, Renner and co-authors take a different view. They believe it will only be a matter of a few days.
The former director of the Munich Botanical Garden and Herbarium, Renner is a true tree enthusiast. She’s recently been studying why American trees are so much more likely to develop vibrant red hues than their European counterparts — a phenomenon botanists have observed for well over a century.
Her research suggests it’s because autumn in the U.S. tends to have much more sunlight than European cities at similar latitudes. Over generations, trees transplanted from Europe to the U.S. will develop crimson colors in the fall as protection, she said.
“I think it has to do with less cloud coverage,” she said. “You have a wonderful bright light here in the fall, brighter than in Europe at the same altitude. If you compare Boston and Rome, the intensity of the radiation coming in September in Boston is significantly higher than it is in Rome — or Madrid, for that matter.”
“St. Louis on the Air” brings you the stories of St. Louis and the people who live, work and create in our region. The show is hosted by Sarah Fenske and produced by Alex Heuer, Emily Woodbury, Evie Hemphill and Lara Hamdan. Jane Mather-Glass is our production assistant. The audio engineer is Aaron Doerr.
