The science of fall foliage and climate change, among Vermont’s maple trees and beyond

COLCHESTER — Fall foliage flickers bright in Vermont most years, but external stressors like floods, droughts or excessive heat can mute the fiery autumn leaves we all know and love. 

Though the season is still off to an exciting start, especially in the highlands, all of the above environmental stressors have played a role and had an impact in the time leading up to this fall. 

Mark Lubkowitz, plant biology professor at Saint Michael’s College, spoke with the Sun to share how deciduous plants shut down shop for the winter, and what ingredients they use to do it — as well as the implications of climate change on Vermont’s most iconic trees. 

How and why leaves turn during fall 

What you see in the fall is really a developmental process, consisting largely of recycling efforts, intended to reabsorb and stash nutrients, to be used again after a harsh winter, Lubkowitz said. 

Deciduous trees in regions with very defined seasons, like New England, have evolved with very thoughtful age biology — old root systems that have been mining the soil for useful minerals for decades, many of which can be reused year after year. 

“Just like you’d, ideally, accumulate wealth over time, these trees are accumulating wealth in the form of minerals they've gotten out of the soil,” Lubkowitz said. “And one of the most important ones is magnesium. And here's why: Magnesium is a critical component of chlorophyll.” 

Chlorophyll is the chemical that allows plants to act like their own mini solar panels, harvesting sunlight to create their own energy. Chlorophyll also contains the pigment which makes leaves appear green before they turn. The process of reusing precious magnesium enables the abundance and success of next spring’s canopy. 

This annual reabsorption of magnesium is what pulls that green pigment away, often leaving behind yellow hues. In addition to chlorophyll, plants have accessory light capturing pigments, made in smaller quantities — these small batch chemicals are overshadowed and masked by the strong green pigment of abundant chlorophyll during summertime, but reappear in fall. 

These accessory pigments function less so to capture light as energy, and more so to act as a buffer or capacitor when chlorophyll is fully saturated and can’t take any more sun. These other compounds, called carotenoids and xanthophylls, work as a sponge to prevent photo damage. 

Trees try to maximize photosynthesis in the summer while minimizing photodamage, always. 

“There's a lot of energy in sunlight, and they can also get DNA damage from it — that's what a sunburn is,” Lubkowitz said. “When we get too much sun, and when they get too much sun, they also get cellular damage. They manifest and manage it differently, but they have the same challenge.” 

One of the many challenges of being a plant is being non-modal, or unable to move from one general spot to another, he said, which accounts for many of these unique adaptive mechanisms. 

Unlike animals, plants can’t run or hide if they feel threatened by a predator or environmental stressor — so they have evolved with other “behaviors” instead to cope with many of those same challenges. 

But what about those bright reds that stand out from the yellowy-browns of fall in Vermont? 

The classic maple, known for producing vibrant leaves and syrupy sweets all integral to Vermont’s cultural tradition as well as a multimillion dollar tourist and tasting industry for the Green Mountain State, is the crown jewel of autumn goodness. 

Maples’ fall reds can be attributed to the production of anthocyanin in these plants. Anthocyanin is a compound that further protects from photodamage beyond the extent of carotenoids alone. These trees produce their own “sunscreen,” so to speak, when their chlorophyll is recycled. 

The anthocyanins that contribute to the reds and oranges seen in maple trees are also what make red wine red, and what make blueberries purple. Anthocyanins have a few functions in plants, one of which being to absorb light energy at a very close wavelength to chlorophyll. 

Because energy cannot be created or destroyed, anthocyanins dissipate the light energy they receive back out to the environment — they quasi-vibrate, Lubkowitz said. 

“You can't really hear the vibration, but vibration is synonymous with sound,” he said. “I always joke that if you could listen really carefully, you can hear the sound of the trees as they're changing colors, which isn't quite accurate, but they give the energy back off their vibration.” 

So, as chlorophyll is broken down, the anthocyanin absorbs the excess solar energy to prevent its living machinery from getting damaged over time. Although it’s a powerful compound, it, too, takes energy to produce, and not all plants function with the same mechanisms or trade-offs. 

Anthocyanins are really large and really expensive molecules. They are necessarily nitrogen-rich and it takes a lot of resources to build them. So from an evolutionary perspective, there’s a reason why not all plants do it, and why it happens only in colder climates. 

“The beauty of different types of life is we have done things differently,” Lubkowitz said. “You don't have to necessarily do everything perfectly. You just have to do it good enough. So it's not like every molecule or every atom of nitrogen, phosphorus and magnesium needs to be recovered, but you need to get enough of it.” 

The biological reactions that go into leaf changes are temperature dependent, so the lower the temperatures go, and the higher the light intensity is during the day, the more energy is coming in without the warm-temperature-dependent conditions for trees to manage the amount of light energy they are getting come fall — and this, Lubkowitz said, is where the magic happens. 

Leaves pop out most vibrantly in bluebird-bright skies and crispy-cool air. These conditions constitute the prime tipping point where making anthocyanins is worth the return on investment, and explain why such imagery isn’t witnessed anymore if you were to go to the southeast. 

This is why New England is known as an ultimate destination for leaf peeping. 

Changing of the leaves, changing with the climate 

The primary factor determining when leaves make the decision to turn is primarily photoperiod, but it can also be mitigated by temperature and stress. 

The hot, nearly five-week-long drought Vermont just experienced, which followed the flooding event of summer, caused leaves to start wilting up a little early this year, and perhaps a little drier or duller. Trees check out early after a bad year, in hopes they’ll save up better in the next one. 

When tree leaves skip the warm, colorful stage — a controlled genetic process of programmed organ death scientifically known as senescence — and instead go straight to looking like brown shriveled husks of their former selves this time of year, it is likely due to a premature, uncontrolled organ death constituting a stress response. 

Lubkowitz has noticed this phenomenon happening a lot this year in his backyard. 

“I think that we have stress in our forest right now because of too much water, followed by not enough water, and it's also been warm — we haven't had the cool days to really trip that anthocyanin production,” he said. 

And for trees that can’t recover all their magnesium in the fall, they might not be able to mine enough the following year to build back a full canopy, and if this happens too much over time, plants can’t get the energy they need, which starts ultimately doing damage to the cells. 

Like with any living thing, if the overarching climate patterns alter enough that the habitat is no longer suitable to a given species, the species will become less likely to survive, more vulnerable to displacement, and over time, will fade from that particular landscape altogether. 

Maple trees, being best suited to temperate climates with distinct, cool winter seasons, will become less common in Vermont’s lowlands over time, and eventually move northward where conditions are more tailored to the plants’ biological needs. 

“One of the climate adaptation movements that's occurring right now that's really interesting and really sad, is there's scientists who are collecting seeds of more southern species, and they're bringing them northward and sort of pre-seeding,” Lubkowitz said. “Creating a seed bank in the soil so that they're helping the plants migrate north faster than they could on their own.” 

“One of the things that is really so depressing is this thermal inertia,” he added. “We're going to have thermal inertia for a while, decades, so our ability to adapt is going to be critical. We're going to have to be proactive in helping nature adapt, because I don't know any species, or very few species, that evolved to deal with climates that change this quickly.” 

Thermal inertia is the phenomenon by which, even if we were to stop emitting carbon dioxide now, the planet would continue to warm and eventually plateau before coming back down. Heat banks built up more rapidly than Earth is used to dealing with will take a long time to dissipate. 

“One of my favorite expressions is: Geography is destiny,” Lubkowitz said. “It just really resonates with me and at so many levels, human culture is based upon our geographical environment. If you look at our touchstones, fall foliage is an enormous touchstone culturally.” 

Maple is economically important to Vermont with the maple syrup and bright leafy colors it produces, but it’s also been an integral part of Vermont’s cultural identity, Lubkowitz said. As our climate changes, the geographically-dependent things that characterize us will, too. 

“Loss is just that — loss,” he said. “You know, life is incredibly adaptive. Humans are incredibly adaptive, and we will adapt, but loss is loss… It’s a big problem… You can see why people feel so unempowered, when the system that got us here is changing, and it’s so hard to pivot.” 

“But I actually am an optimist,” he added. “Since I teach at a college, and I interact with Gen Z quite a bit. Gen Z is very much like, ‘We're gonna fix this because we have to.’ So I'm sort of like, ‘Yeah, humans are innovative. They will.’ I'm not saying there won't be a lot of suffering, and that there won't be loss in there. It's not going to be easy. But they're solution-oriented.” 

Folks coping with solastalgia or place-based climate anxiety may find peace in meaningfully honoring the memories of what they grew up with, leaning into the support communities can foster via connectedness with one another, engaging in mindfulness practices and by honoring current valued traditions that still have a presence here for the time being. 

Some of Lubkowitz’s favorite leaf peeping spots, still glimmering with life and leaves, are in the Huntington region near where he lives, as well as in wetlands near the Ethan Allen Homestead with a raised biking path, and on another set of dirt biking roads stretching from Lincoln to Ripton on a trail that delves through national forests and is framed by two cute country stores — where you can still, to this day, get a classic maple creemee.