On the second crossing of a portage, deep within the wild woods of Northern Ontario, I drink in the last remnants of summer. Having carried the canoe on my first pass, I take my time on the second. In glorious sunshine, with everything in the forest electric with intent, new smells waft past with regularity. From the forest’s white noise my ears untangle both a woodpecker’s distant drilling for insect gold, and the soft, pattering rain of tent-caterpillar poo (I’ll admit that one’s an acquired sound).
Hopscotching through the chiaroscuro of light and shadow in the understory, my eyes catch the vermillion flash of newly fallen leaves dotting the brown litter like small explosions. Ah… fallen leaves. The equinox is coming. The last, lazy exhalations of summer swaying in the canopy above are heralding the sharper inhalations of autumn, prelude to the icy, breath-holding quiet of winter.
Whenever I find myself on the cusp of seasonal change, I’m wont to recall the entirety of this dramatic annual arc. So that day, there wasn’t just the theatre of summer to dwell on, but everything that led into it. And having viewed daily from my canoe the past week evidence of this storyline—the ice marks on shoreline rocks, the lakeside browse-lines of deer, the sap streaks on trees, the falling water levels, the squadrons of geese winging south—it wasn’t hard to imagine the coming lock-down of black-and-white winter, where snow-covered beaver lodges would be one of the few signs offering a reminder of patient life breathing quietly beneath it all.
It was equally easy to imagine the arrival of the vernal maiden—the spring wood after the thaw, before the insects were out or the birds had returned, when regiments of green-and-white flowers sprouted from tan mats, hinting at the life-blood syrup boiling up through the trees. And now, though the forest in general seemed still locked in the industries of summer, the details said otherwise. All was powering-down, the longer, colder nights drawing the leaves into biochemical ballet, a fungible shift from summer’s vibrant green to the red, orange, and yellow palette of fall.
Once upon a time I learned how all this worked in a Plant Physiology course, dooming me to remember it each fall as I track the chromatic shuffle. I’ll share it here, but before considering how the different chemical compounds involved contribute to what we see, I’ll explain how the pigments originate.
The first place to look is at the chemical bonds these molecules contain. These can be either single (one shared pair of electrons between neighbouring atoms), or double (two shared pairs of electrons), but the molecules responsible for colour contain alternating single and double bonds—a system referred to as conjugation. Lots of conjugation going on means a molecule can absorb different wavelengths of light, which, ultimately, leads to the refraction of those colours in the visible spectrum.
Most people can tell you that chlorophyll—the molecule within organelles known as chloroplasts that run a leaf’s metabolic machinery—is responsible for the green colour of plants. An essential component of photosynthesis, the process in which plants convert carbon dioxide and water into sugars using only the energy of the sun, the production of chlorophyll itself requires both warmth and sunlight. As these factors wane with the passing of summer, the days shorten quickly after the September equinox (you may notice sunset is two minutes earlier every night) and production of new chlorophyll slows while decomposition of pre-existing molecules accelerates. Once chlorophyll is out of the picture, other compounds already present in the leaves step up to affect the next phase of colouration.
As chemical families go, both carotenoids and flavonoids are large ones. Lurking right alongside chlorophyll, higher levels of the green stuff during summer mask these pigments. But as chlorophyll begins to degrade, they immediately make themselves visible; both groups of compounds contribute yellows to the game, but it’s carotenoids that are responsible for the vibrant oranges and reds that light up Eastern Canada’s deciduous forests (or Creekside’s transplanted one).
These molecules also degrade as autumn progresses, but much more slowly than the more-fragile chlorophyll. Notable carotenoids include the familiar orange beta-carotene of carrots, the lycopene responsible for tomato red, and lutein, which lends egg yolks their bright yellow.
And then there’s anthocyanins. Unlike carotenoids and fellow flavonoids, you won’t find anthocyanins in leaves year-round. Instead, their synthesis is initiated by an increase in sugars in fall leaves reacting with sunlight. Anthocyanins bring magenta, purple and red hues to leaves, their vividness often tied to the acidity of a tree’s sap (meaning there’s a big environmental component from both soil and weather). Though plant biologists remain uncertain of their role, anthocyanins may help protect leaves from light damage, extending their time on the branch (anthocyanins probably played a large role in what happened in Whistler last fall, when a heat anomaly in September and October caused leaves to stay on most trees well into winter).
Eventually, of course, all leaves are shed, clamouring to the ground in a rain of colour. Mostly they are bound for decay, a restarting of the forest’s nutrient cycle. Some will suffer the ignominy of herding by the Muni’s ridiculous leaf blowers, but others will find their way into the forest, where you, like I, can contemplate the heroic cycle of creation, transformation, and termination they represent.
Leslie Anthony is a biologist, writer and author of several popular books on environmental science.
