If you haven’t figured this out yet, every year we’re going to have an incredible, knock-your-socks-off, slap-your-mama kind of fall color leaf show, akin to July Fourth fireworks but more vibrant.
At least according to the tourism folks.
“We’re in for another spectacular season of fall color in Asheville, according to Dr. Howie Neufeld, professor of plant ecophysiology at Appalachian State University,” ExploreAsheville touted on its website Sept. 7. “The warm weather could delay peak color but also extend the overall leaf season.”
Neufeld didn’t actually use the word “spectacular,” or anything close to that, in his comments. Neither did the other scientist quoted in the story, Beverly Collins, a retired biology professor at Western Carolina University.
Neufeld noted that NOAA’s long-range forecast predicts slightly warmer weather in the Southeast through November, and warmer temps in late September could delay peak color. Minor warming would delay the peak three to five days.
“If more significant, it could be a week or two, depending on the magnitude of the warming,” Neufeld said in the release. “If it gets warm in the early fall, some trees will delay their color, but those cueing in mainly on day-length will continue to color up on schedule and the color season will become extended.”
That’s a lot of “ifs.” Collins added more.
“If we return to more ‘normal’ cooler temperatures with moderate rainfall, colors should be vibrant, as they were last year,” she said. “If it stays warm, fall color will be later and more spread out. This will result in a longer leaf season.”
Those “ifs” are the crux of my argument today, which is this: Predicting the intensity of fall leaf color is not really all that scientific.
Sure, scientists have a good understanding of why leaves change color, and they can gauge with decent accuracy when it should happen by tracking the shorter days and cooler temperatures. But determining how bright those leaves are going to be, and if all the wide variety of trees — we have about 120 species in the Southern Appalachians — are going to change at something close to the same time, is a dicey scientific business.
Even ExploreAsheville, citing Neufeld, allows that, “In addition to day-length and temperature, factors such as precipitation and insects can influence the quality of fall color.”
I had really nice conversations with Neufeld and Collins, and neither claimed the scientific community has this nailed down, although they did say the science has greatly improved.
That’s primarily because more studies are being done about the leaf season, as it’s affected by climate change and is a huge economic engine for tourism. Also, scientists can study satellite imagery of leaf color changes and plot that against precise meteorological data.
Intriguingly, a fair amount of research also has been done about leaves that turn red, as different factors affect that process. And it’s a controversial topic. (More about that later.)
‘They don’t measure the intensity of the color’
Neufeld said he has a pretty solid success rate of predicting when leaves will change, although it is very weather dependent, but he’s not tracked any kind of intensity success rate. And he’s not aware of anyone who has.
“I’ve got a pile of papers right here on my desk, about 20 or 30 papers, all about what affects fall color — the timing and the duration and all this kind of stuff — but not a single one of them measures the intensity of the color,” Neufeld told me in a phone interview.
This, Neufeld said, “would take quite an effort.”
“It’s very easy from a satellite to look down and see when the color’s changing, but you don’t really get a good measure of the intensity of those red pigments,” Neufeld said. “So that’s still something that people need to do.”
Neufeld said last year he checked his 16 years of predicting fall leaf seasons to assess his accuracy on peak color timing. He predicts for the Boone-Blowing Rock area, which is about 85 miles northeast of Asheville and more than 1,000 feet higher in elevation.
He determined that in eight of the first nine years, he nailed the peak time, usually between Oct. 10-20. But since then, the variability has been about twice as great, meaning the peak time could be right on time, or one or two weeks late.
“I’m 50% less likely to get it right than I did in the first eight years,” he said.
He attributes that to “incipient climate change.”
“The science itself is very good,”Collins said, and “most of the science is carefully done” when it comes to why leaves change color, how stress on trees plays a role and generally when they should change colors. She too noted the science on the color red is quite good.
Generally, too much rain or too much drought is not good for leaf color. Clear September days with cool nights are good, along with a nice cold snap in early to mid-October.
But science on color intensity is lacking.
“Even if there were absolutely research showing that the brightness of yellow, the brightness of orange and the brightness of red were related to certain degrees of cold, or certain percentage of drought — even if we had that research — we couldn’t predict for a given place or year,” Collins said. “Because how cold it gets or how dry it is is weather dependent and we can’t predict the weather. So even if the science were perfectly in place, the underlying variables are unpredictable.”
So this leaf prediction business, Collins says, is about 80-90 percent science-based.
“That 10-20 percent is how unpredictable the weather is going to be,” Collins said.
I’ll also note that Collins gave it straight in Western Carolina University’s news release about this fall’s potential leaf color.
“It pains me to say this, but I think that last year was an anomaly in our colors, which were so bright, because we had perfect conditions,” Collins said.
In Franklin where Collins lives, and over here in Buncombe County, which is about the same elevation (2,000 feet or so), we historically get a good cold snap around Columbus Day, and the leaves really start changing, with peak season typically about a week later, Collins said.
“Because it has been dry, we might see some leaves turning earlier, which sounds incongruous, but it’s because the dryness also brings on stress and it’s basically end of season stress that causes the leaves to change color,” Collins said in that release. “The second factor is how cold it gets at night. The thing that brings on color really quickly is having it get cold and down into the lower 40s with bright sunny days.”
So, as both Collins and Neufeld told me, we could get a weird warm spell in October that slows everything down. Or unusually windy weather from hurricane remnants that pulls peaking leaves from the trees.
In short, it’s a crapshoot. Neufeld said predicting the weather more than seven days out with real accuracy is tough.
A forester’s perspective
I reached out to Jordan Luff, the management forester at DuPont State Forest, part of the N.C. Forest Service. He said the ecophysiologists “who are actually recording and tracking seasonal weather patterns to predict autumn leaf colors” are relying on science in their predictions pretty heavily.
“I’d have to say the rest of us 99% are riding fully on the intuition/educated guesswork train!” Luff said via email.
“Predicting the timing and quality of leaf color change is about as tough as predicting the weather, and that is primarily because summer and early autumn weather patterns are some of the main drivers of leaf color change in deciduous trees,” Luff said. “An abnormally dry summer or drought causes trees to become stressed, which can cause leaves to color change earlier than normal — normal being when rain and temperature patterns are optimal for tree health.”
Luff gave a nice rundown of the basic science behind leaf change, noting that well-hydrated trees hold their leaves longer “and provide for better autumn colors.” Cool, dry days with sunny skies also help.
“The other important driver of changing leaf colors is the number of daylight hours, which is a bit more consistent,” Luff said.
Collins said three factors influence leaf color: leaf pigments, length of night, and the weather. Three types of pigments are involved in autumn color:
- Carotenoids: These produce yellow, orange, and brown colors (think corn, carrots, daffodils, etc.)
- Anthocyanin: Think red, mostly, as in cranberries and apples.
- Chlorophyll: This gives leaves a basic green color and is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for food.
The hotly debated redness of leaves
Let’s address Neufeld’s notions about those red colors, because this is a hotly contested issue in the scientific world. I kid you not.
Trees known for turning red in the fall around here include dogwoods, sourwoods, sugar maples, and black gums.
Neufeld co-authored a paper on why trees produce the red pigment, anthocyanin.
“There’s been a great controversy in the literature about why trees that turn red do so, because the orange and the yellow pigments are in the leaves all summer,” Neufeld said. “But the red pigment is not there in the summer — it’s made in the fall.”
“That has brought up the hypothesis that if the plant is going to take the energy and the time to make this compound right before the leaves fall off, there must be a reason for them to do it or there otherwise they wouldn’t do it,” Neufeld said. “So what would be the adaptive value of it?”
Hey, I just wanted to write a column on leaf color. I didn’t know I was going to be called on in class.
“One hypothesis was that it was a warning to insects, kind of saying, ‘Don’t lay your eggs on me. I’m healthy. I’m making this pigment, so go somewhere else,’” Neufeld said. “But the physiologists think, ‘OK, these red pigments act as a sun shield and maybe they’re protecting the leaf from too much radiation.”
That would give the leaf more time to withdraw nutrients and store them in the twig.
“And if it pulls those nutrients out of the leaves and stores them in the twig, it can use them next spring to jumpstart the production of new leaves before the roots get a chance to start taking up nutrients,” Neufeld said.
Great! All settled then, right?
“So that begs the question, though, why don’t all trees turn red, if that’s what they’re doing?” Neufeld continued. “And so the idea is that somehow the trees that are orange and yellow have some other mechanism to keep their leaves intact to get those nutrients back, but the trees that turned red have to rely on this process. So that’s been a discussion in the literature. And it’s been quite a vigorous discussion back and forth, as to, ‘Is this really what’s going on?’”
You know, I actually suggested this column to my editor.
Hey, Harvard is on the job!
Luff and Neufeld mentioned the Harvard Forest, a 4,000-acre long-term research forest owned and maintained by Harvard University that in part looks at plant phenology, the study of seasonal, natural phenomena.
It also has a webpage dedicated to “Autumn Foliage Color,” as Luff pointed out, and that “lists a bibliography of related studies dating back to the early 1900s, primarily in the scientific fields of ecophysiology or tree physiology.”
“One study from the Harvard Forest developed a model for predicting changes in autumn color out to 2099, and suggested that projected climate change will lead to an overall increase in the amount of autumn colors for most species, though individual species will see changes to their timing and duration of color change,” Luff said. “This could lead to peak colors for each species occurring at times that no longer overlap, leading to a less brilliant landscape during autumn.”
Another study, he noted, included an experiment that showed increasing carbon dioxide causes “delays in leaf color change and leaf fall in both U.S. and European aspens/poplars, independent of temperature.”
Neufeld noted that scientific studies have been conducted that “looked at the timing of when leaves fall off in drought years, versus non-drought years.” In drought years, premature leaf fall is a problem, especially with tulip poplars, a common tree in the mountains that are also very sensitive to water stress. That can make their leaves turn brown instead of a bright yellow.
In short, plenty of science about leaf color is happening. Neufeld read me some of the titles of all those papers sitting on his desk, all from the last three or four years. You’ll need to know that “senescence” means the condition or process of deterioration with age. A few study titles:
- “Larger temperature range undermines later autumn leaf senescence in Europe.”
- “Variation in leaf senescence for 1,360 woody plants in the fall.”
- “Metabolic insights into anthocyanin biosynthesis.”
- “Autumn senescence is not triggered by day length.”
- “Leaf senescence exhibits a stronger response during warm than cold autumns.”
“There are just dozens of papers coming out, and autumn is the least understood time of the year,” Neufeld said.
Here’s the upshot: Let’s not pretend the matter is scientifically settled.
Before you call me an Autumn-hating sourpuss, know this: I love fall. It’s my favorite season, and I wish it would last right up until Christmas.
But I’m also realistic. In 28 years in the mountains, I feel like the leaves actually have been spectacular about five or seven times. The other years were merely beautiful.
Collins, who was a professor of plant ecology, agreed.
“You’re exactly right — again, just think of the weather,” she said, noting that we get great snow years at about the same interval. “Otherwise, it’s just blah.”
Of course, no tourism agency is going to trumpet the upcoming leaf season as “just blah.” So look for more “spectacular” seasons.
In fact, every year.
Asheville Watchdog is a nonprofit news team producing stories that matter to Asheville and Buncombe County. John Boyle has been covering Asheville and surrounding communities since the 20th century. You can reach him at (828) 337-0941, or via email at jboyle@avlwatchdog.org. To show your support for this vital public service go to avlwatchdog.org/donate.