Friday, October 20, 2023

Red Alert: Leaf Color Indicates Maple Distress

Sugar maple leaves

The dearth of red fall color in sugar maples, a broad regional trend first noted around 2018, is unrelated to fall weather or to the growing conditions in a given season. It’s a troubling sign that sugar maples as a species may have entered a long-term decline. Although every fall is beautiful, some years are notably vibrant, while others – like 2023 – are more subdued. We know that weather is the main factor that determines the brilliance of the autumn leaf display.

An unusually wet spring /early summer will favour the growth of leaf pathogens like shot-hole fungi, anthracnose, and bacterial leaf spot, all of which cause brown patches on leaves. Conversely, in drought years, trees are starved for water and nutrients, which curbs overall pigment production. Even after a strong growing season, protracted fall rains can tone-down color intensity, and early hard frosts or violent windstorms will truncate the “leaf peeper” season.

Plants make yellow-colored molecules, called xanthophylls, as well as orange ones, known as carotenoids, to aid with photosynthesis and to protect cells from oxidative stress. As we know, these pigments are present in leaves, but are masked by green chlorophyll all summer.

However, the red and purple range, brought to us by a class of chemicals known as anthocyanins, are definitely not cached under chlorophyll’s cloak. These large, polycyclic organic compounds cost plants a lot of energy to make.

While relatively few species color their leaves red in the fall, sugar maples and soft (red) maples are famous for their rosy foliage. Some oaks produce deep scarlets, and dogwood and ash often make red-purple hues. Many genera of shrubs, including Viburnum, Aronia, and
Amelanchier, flaunt their mélange of anthocyanins; salmon, coral, crimson, raspberry, ruby, and burgundy (I may have missed a few variants).

Plants invest in anthocyanins in spring to protect young leaves, as chlorophyll is prone to UV- light damage in cool conditions. Think of red pigment as suntan lotion for baby leaves. As the weather warms and leaves mature and harden-off, plants quit making these pricey compounds.

Early-season red makes sense. But why some trees splurge on anthocyanins in the fall when they should be hoarding energy for next spring is a mystery. As daylight dwindles in autumn, hardwood trees begin to rob their leaves of sugars and other goodies, clawing back roughly half the nitrogen, phosphorous, potassium, iron and other nutrients for safekeeping in trunk and root tissues. While they drain the leaves, they also deposit wax (suberin) between twigs and leaves to plug the vessels that supplied water and nutrients to their leaves all season.

As a result of this self-vandalism, chlorophyll breaks down, which lets the hidden yellow and orange colors to make their debut.

Notoriously frugal and pragmatic, trees don’t dip into their savings accounts – stored starches in woody tissues – without good cause. Few hypotheses exist as to why trees make red. Among those brave or foolish enough to say they know the answer for sure, the “fall suntan lotion” notion is a favourite, since cool nights and sunny days trigger red production. However, it seems odd to claim trees use precious starch reserves to shield chlorophyll from UV rays while they’re hard at work making waxy abscission layers to kill said chlorophyll. By the time a leaf turns red, there’s generally not much left to protect.

Another idea is that when red leaves fall to the ground, they change soil conditions in a way that favours that species and suppresses others. There are myriad plant-made chemicals known to inhibit the growth rates and /or seed germination of competitors, a process known as allelopathy. Typically, roots give off allelopathic chemicals – it’s why you don’t want your garden near a black walnut tree – the juglone released by walnut roots kills tomatoes and potatoes. The problem with this rationale for red is that anthocyanins in fallen leaves have a very weak allelopathic effect.

In the end, though, an explanation for red autumn leaves matters little.

What’s significant to bear in mind is that the making of anthocyanins in fall is optional. In a very real sense, it’s a sign of a tree’s disposable income. When a species once renowned for its ruddy fall foliage suddenly goes on an anthocyanin strike and looks more like silver or Norway maples, it’s a cause for concern.

This phenomenon hit me right between the eyes starting in the fall of 2018. Not surprisingly, it came on the heels of unprecedented (in terms of low soil moisture) droughts in 2012, 2016, and 2018. Cornell University Plant Pathologist Dr. George Hudler, now retired, has always said it can take 2-3 years of normal soil moisture for a tree to recover from moderate dry periods, let alone these mega-droughts. Furthermore, in 2013 and 2017, sugar maples used large amounts of energy to put out massive distress crops of samaras after the two worst droughts.

Waves of tent caterpillars, which do not touch red maples, stripped sugar maples of their leaves for two or more consecutive years from 2015 through 2019. This was followed by outbreaks of spongy-moth (formerly gypsy or LDD moth) caterpillars that swept across southern Canada and parts of New England between 2019 and 2022. Among all my forester and arborist contacts in northern New York State and southeastern Canada, as well as colleagues from Cornell Cooperative Extension, not one could find a red sugar maple in the fall of 2018. The same went for 2019 through 2022.

This fall, I have noticed a proportion of sugar maples, all of which appear to be less than 30 cm in diameter, showing a minimal amount of anthocyanin in their leaves. This gives me hope some smaller age-class sugar maples might be recovering after 3 years without severe drought, and 2-5 years with no major defoliations. The issue, of course, is that we expect drought years to become more frequent and intense.

Yet even prior to the change in sugar maple’s color regime, scientists were concerned about this species. In October 2015, the alarming results of a study which documented forty years of maple growth rings in the NY’s Adirondack region came out of the State University of New York’s Environmental Science and Forestry College in Syracuse. One of the study’s co-authors, Dr. Neil Pederson, an expert on tree rings and climate change, wrote these chilling lines in the report:
“Outside of studies of red spruce in the 1970s, I have never seen anything quite like this. Most tree-ring studies of canopy trees in the region do not show a decline like what we see in these sugar maple. Combined with evidence of reduced natural regeneration of sugar maple in the region, it is a concern.”

Sugar maples are in uncharted territory. Scientists at the US Forest Service believe that by the end of the century, sugar maple will exist almost exclusively in Canada. But in what condition?

It’s possible that enclaves of sugar maple will survive in isolated nooks and crannies in the Adirondack, Green, and White Mountains and other similar terrain. Variation of slope and aspect in the mountains creates “climate refugia,” micro-habitats favourable to sensitive species. These refugia resist change, but are not immune to it.

Luckily, we do have agency in determining our future. According to the Canadian Association for Educational Resources, “By 2100 the atmospheric CO 2 concentration (the gas responsible for most temperature change) will be between 540 and 970 ppm,” depending how much carbon dioxide we pump into the air.

The huge discrepancy between those numbers offers us a chance to slow the rate at which tree species’ ranges march northward. It’s hard to feel motivated when we know our decisions are a drop in the pool. Well, drops matter. It takes something like 50 billion drops to fill an Olympic- size pool. If each Earthling coughed up (figuratively, please) 6 or 7 drops, it would be full.

No matter where we live, everyone has access to a dropper of some sort. Maybe it’s planting a tree. Perhaps we could bike to work, switch to LED bulbs, or trade-in our pickup truck if we’re not a farmer or a contractor. Every drop makes it less likely the next generation will ask “Hey Grandma and Grandpa, did sugar maples really used to grow here?

Photo at top: Sugar Maple Leaves

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

5 Responses

  1. Irving M. Baxter says:

    Thank you for the in depth story of what is taking place behind the seasons for our Red and Sugar Maple Trees! 🙂

  2. upstater says:

    In Central NY, southeast of Syracuse we note the dearth of red sugar maples for the past several years. We had a drier summer than the Adirondacks and leaf dropping has occurred earlier than past years. The sugar maples are bare 2 weeks sooner than the past. Our valley is a ribbon of dead ash. None of this bodes well.

  3. Jim Fox says:

    Great article … interesting, informative, mind-stretching. Your articles are turning me into a real geek, Paul. Keep ’em coming!

  4. Lawrence says:

    Noticed a complete lack of red and orange colors in my Sugar Maple this season. Last year my maple was spectacular. This year…. Not so much!

  5. Thomas Barry Wolfe says:

    Our sugar maples had an exceptional seed production this year. Could this be responsible to some degree for the small and muted color leaves this year?

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